LCD Display

Introduction To LCD Display

LCD display consists of an array of tiny segments known as pixels that can be manipulated to present information. As a result of this technology, many types of this displays are used in applications like calculator, watch, messaging boards, clock, equipments, machines and a host of other devices that one can think of. Most of the Display types are reflective, meaning that they use only ambient light to illuminate the display. Even displays that do require an external light source consume much less power than CRT devices.

An LCD basically consists of two glass plates with some liquid crystal material between them. The small size compared to CRT makes it practical for applications where size, current consumption and weight are the main consideration in electronics design.

A liquid crystal display is a thin, lightweight display device with no moving parts. It consists of an electrically controlled light polarizing liquid trapped in cells between two transparent polarising sheets. The polarizing axes of the two sheets are aligned perpendicular to each other. Each cell is supplied with electrical contacts that allow an electric field to be applied to the liquid inside. Figure below shows the typical LCD modules which will be able to display graphics or characters when interface with a micro controller or microprocessor.

Before an electric field is applied, the long, thin molecules in the liquid are in a relaxed state. Ridges in the top and bottom sheet encourage polarization of the molecules parallel to the light polarization direction of the sheets. Between the sheets, the polarization of the molecules twists naturally between the two perpendicular extremes. Light is polarized by one sheet, rotated through the smooth twisting of the crystal molecules, then passes through the second sheet. The whole assembly looks nearly transparent. A slight darkening will be evident because of light losses in the original polarizing sheet.

When an electric field is applied, the molecules in the liquid align themselves with the field, inhibiting rotation of the polarized light. As the light hits the polarizing sheet perpendicular to the direction of polarization, all the light is absorbed and the cell appears dark.

Transmissive and reflective LCD display

LCDs can be used in transmissive or reflective modes. A transmissive LCD is illuminated from one side and viewed from the opposite side. Activated cells therefore appear dark while inactive cells appear bright. The lamp used to illuminate the LCD in such a product usually consumes more power than consumed by the LCD itself.

A reflective LCD, as used in pocket calculators and digital watches, is viewed by ambient light reflected in a mirror behind the display. This type has lower contrast than the transmissive type, because the ambient light passes twice through the display before reaching the viewer. The advantage of this type is that there is no lamp to consume power, so the battery life is long.

Customized or standard LCD design

Figure below shows an example of a LCD display which has segments that can be controlled to show +/- signs, readings from the 7 segment. It is interfaced to the PCB by using a rubber connector with carbon that connects the segments and COMs of the LCD to the ports of a micro controller. Most manufacturers have some standard design of LCD to choose from but if a customized design is needed, one have to engage the LCD maker to make a customized tooling for the LCD. In this case, the design will be unique and will only be sold to the dedicated designer.

In the application of the LCD, one needs to know the viewing angle of the users. The design have to be specified upfront whether the viewing angle is 6 o'clock, 9 o'clock, 12 o'clock or 3 o'clock as this will affect the viewing angle of the product that has been designed.

If cost is a major constraint in the electronics project that uses LCD display, it is normally advisable for the designer to choose from the standard catalogue of the manufacturer and purchase it.

INFRA RED VISION SYSTEM for a Toy Car

This is the project of infrared vision system for a toy car. It will project 2 modulated IR beams ahead and detect any reflection of these beams on any obstacle ahead of the car. The circuit will then invert the car's motor for a given time thus changing the direction of advance as it goes in reverse. No special control for steering is necessary as the car has the front wheels' shaft in an eccentric support: when going forward it auto aligns itself, when running backwards the shaft turns and the car describes a curve.

CONSTRUCTION, CIRCUIT DESCRIPTION and PHOTOGRAPHS

The main circuit given above is mounted on a PC board that will be fixed over the car. Ahead and on the sides we'll put the IR emitting diodes (2), in the center, the IR reception phototransistor FT. Also the capture frequency potentiometer for easy adjust is located above, on the PC board.

Principal details

The original toy had a wireless remote control unit transmitter with only 1 channel, backwards. The car normally runs forward but upon signal reception the car's circuit inverts the motor polarity and the car runs backwards. As shown in Fig 1, the front wheel shaft is fixed at an eccentric point S in relation with car's centerline. When the car is reversing, the front shaft also turns due to the different friction torque acting on point S, therefore turning the vehicle.

This mechanical construction is particularly practical when we have only one radio channel available to control de car.

In our project we will cannibalize the car: the reception circuit won't be used, only the 4x1.5 volt AA cell holder and the motor (3vdc) will remain. Also, the wireless 1 channel remote control won't be used.

The car's battery holder will be modify in order to have: +6 volt, +3 volt and ground. A switch may be used to prevent the motor from running: this is an important feature when calibrating and alineating the sensors...

The circuit will be mounted according to the following photographs.

Note how nicely the PC board fits over the car.

The PC board must have a length slightly less than the car.

The frontal sensors must have a clear view of what lies ahead...

Proper sensor alignment must be done with a static object at a certain distance from sensors.

SOME TIPS ABOUT SENSOR ALINEMENT AND CALIBRATION

* ALINEMENT

IR emitters leds irradiate a cone of infra red light therefore some experimenting is necessary to determine position and best shrouding in order to prevent that the FT receptor located in the middle receives direct IR emissions.

In Fig 2 are shown the main beams of the IR emitters (in red) and the lateral beams (in orange). The FT phototransistor has a reception cone depicted in sky-blue. Note that direct lateral emissions of infra red light reach the FT. We don't want this direct type of illumination, what we want are the reflections of the main IR beams (in red) produced by an object ahead.

Fig 3 depicts how we may prevent lateral emission beams from reaching the FT. If we put some kind of sleeve or shroud around the emitter leds we may control how much this lateral beam will expand, i.e.: the angle of aperture.
Remember the four photographs of the car shown above? If you click on the "front view" photograph you'll see a close-up of the shrouded emitters. Certainly you may also shroud the FT.

All we have discussed relates to the horizontal plane, but similar considerations can be made considering the vertical plane. Precautions against ground reflection should be consider, i.e.: the angle from horizontal plane the IRs aim for.

* CALIBRATION

It's not very difficult to calibrate the PLL reception module, just follow the instructions:
According to Fig 4a the circuit is energized but the motor is turned off. Put an obstacle at about 4 inches ahead of IR emitters and slowly turn the PLL pot until the capture led turns on. Now the PLL is tuned; mark the pot value at which the capture was achieved.
As there may be harmonics of the main capture frequency, continue marking different positions at which the capture led turns on. Later, when we move farther away the obstacle, this points will disappear.

Now according to Fig 4b we repeat the process of Fig 4a but with the obstacle at position #2. Repeat the process at farther distances: i.e.: position #3.
You'll see that at greater distances from the sensors, the other marks in PLL pot disappear, just leaving only the main frequency f0.

ABOUT THE CIRCUIT

* The transmitting Module

Here's there's no big deal. while you continue reading this description (you may call the new window every time it's needed).

Let's start from the left: a NE 555 is configured as a astable oscillator running at 18 KHz. Its output (pin 3) goes into a transistor which drives the two IR leds. You may notice a 100 ohm trimmer to adjust IR intensity. This is useful if you want your batteries to run longer or if there's a lot of bouncing reflections from around that confuses the FT. A 220 uF electrolytic cap and a 1N4001 diode serve as decoupler to isolate the transmitting module, thus preventing the 18 KHz signal to propagate by the main +6 volt bus. You may select another value or even insert in parallel a 100 nF ceramic disc cap.

* The Reception Module

Here you have the phototransistor FT and an amplifier stage with some selectivity around 18 KHz, the received signal continues and enters the NE567 PLL (Phase Locked Loop). This integrated circuit works as follows: when the arriving signal has a frequency equal to the one programmed into the 567, pin 8 goes to ground. The central capture frequency f0 is programmed by the RC line (30 Kohm pot, 43 K resistor and 1 nF ceramic cap) between pins 5,6 and ground. Capacitors between pins 1 and 2 set the width of the frequency capture window, which means that the 567 will "capture" any frequency between f1 and f2.

The reception of a signal with different frequency than the 18 KHz selected in the PLL, keeps pin 8 at high (+6v). Upon reception and capture of a 18 kHz signal, pin 8 goes to ground thus turning the capture led on. The junction between the two 1 kohm resistors, initially at +6 volts now goes to approximately +4 volts (remember the 1.6 to 2 volt drop in the led when conducting). Now, with +4 volt at the base, the PNP transistor will be turned on (we need the base at least 0.7 volt below the +6 v line), enabling us to use this PNP transistor as a switch.
* The Timing Module

With the PNP transistor turned on, we have now an instant charge on the 10 uF electrolytic capacitor connected to the PNP collector. But this charge is applied to a 100 kohm resistor, thus generating a timing constant equal to 2xRxC. In our case about 2 seconds: This means the collector of the PNP will decay to ground in about 2 seconds. The next NPN transistor has its base connected to the PNP collector, so, when the PNP's collector suddenly displays +V, the NPN is turned on and will be on for the time selected in the RC constant.
This NPN drives a relay with 2 normal-close contacts. This relay will be energized for the time the RC and the NPN transistor timing circuit is programmed.

* The Relay-Motor Module
In this module the motor's supply is taken from +6 and +3 volt points. The relay contacts are connected in such a way as to invert the motor supply polarity, thus enabling motor reversion upon signal capture.

The relay is a PC-Mounted electromechanical relay, with a coil of 5 or 6vdc and contacts with a rating of less than 1 Amper (the motor draws less than 1 amper for sure). Is DPDT type (double pole, double throw). Radio Shack has many type of these, catalog number #900-2334, OEG brand, model OMI-SS-2-05D, about $2.50 each. See the following picture for details:

I used a PC mounted relay (miniature type also) Metaltex, model ML2RC1, 6 vdc coil (made in Japan).
Don't forget to identify and check the relay's contacts before wiring the system!

DC13.8V to DC250V easy circuit

I have done a lot of work with valves in recent years. For me valves have many advantages, least of all the price; since they are now "obsolete" it is quite easy to get hold of them for next to nothing at rally's and junk sales. I recently purchased a couple of hundred battery valves for less than SEK1 (US$ 0.15) each.

The biggest problem with valves is the PSU needed to provide +250 vDC and 6.3 vAC for the fillaments. The transformers are no-longer available at a reasonable price, but a pair of 12v-6v-0v-6v-12v mains transformers will do the job just as well. For portable use only one transformer is required together with a pair of power transistors such as 2N3055 etc.

Above is the circuit of a converter that will generate the required volrages from a 12 volt source.

Above is the circuit of a converter that will generate the required volrages from 220 volt mains.

Just one word of warning: Dont forget to put a "bleeder" 100K - 270K ohm resistor accross the 250vDC output if your equipment doesn't already have one built in. Failure to do this can be fatal because the PSU smoothing capacitors can store a lethal charge for days.

Finally if you think that there is no place for valves in QRP or portable work then think again. An "EL84" will deliver 10 watts on the lower HF bands. It draws 300mA @ 6.3v (2.1 watts) for the heater as well as + 53mA at 250vDC (about 15 watts) for the anode and screen. The DC-PSU.GIF will provide this and draw less than 2 amperes from a car battery. A 48A/H car battery will therefore last over 24 hours NONSTOP TRANSMIT !!! If you also think that valves are difficult to build with then try one. You will be surprised.

HA1377 Bridge Amplifier BCL cap 17W (Car audio)

This is circuit Car audio Amplifier(Bridge Amplifier BCL) , It use IC HA1377, Supply Volt 12V-13V. Speaker 4 OHM. Nice Circuit and Easy to Build.

Circuit HA1377 Bridge Amplifier BCL cap 17W

PCB HA1377 Bridge Amplifier BCL cap 17W

TDA2030 amp OTL 15W

Circuit TDA2030 amp OTL 15W

PCB TDA2030 amp OTL 15W

Loudspeaker Protection with Soft Start

This is a small protection circuit from loudspeakers, from DC voltage that likely to exist after some damage in the power amplifier. If a DC voltage is presented in the exit of amplifier, RL1 it interrupts immediately the line of loudspeakers preventing thus to reach in he. Parallel it provides a delay time of 3 seconds from the moment where the power supply will be applied. This delay protects the loudspeakers from undesirable bangs that are observed when open the supply switch. The Leds D 4-5 provide a optical indication for the circuit operation [D4 (green)=OK and D5(red)= delay or presence DC voltage]. The supply of circuit becomes from a symmetrical ±12Volts, which we can take from small independent power supply or from afterwards suitable demotion of main power supply. It will be supposed you make a circuit of protection for each final amplifier that you dispose. Proportional attention it should you show for the quality of RL1 that the contacts of will be supposed to bear the current that passes from he.

Circuit Tone control adjustable bass-treble Stereo by IC LM348

Circuit Tone control adjustable bass-treble Stereo by IC LM348

PCB Tone control adjustable bass-treble Stereo by IC LM348

Power Amp OCL by 741+2N3055+MJ2955

Circuit Power Amp OCL by 741+2N3055+MJ2955

This is old circuit Power amp OCL, But easy circuit and very nice. To use for play music in your home. It low cost too. It use IC 741 or LF351(good) and Transistor x 4 (2N3055+MJ2955+BD139+BD140) and little component. Power supply volt +35V/-35V and 3A for Mono, 5A for Stereo.

PCB Power Amp OCL by 741+2N3055+MJ2955

Power Amp OCL 100W by Transister MJ15003,MJ15004

This Circuit Power Amp OCL 100W by Transister.It old circuit,but nice circuit amplifier.
Use transister MJ15003 and MJ15004, power supply +38V,-38V 3A.
Output power 100W at Speaker 8OHM.

Circuit Power Amp OCL 100W by Transister MJ15003,MJ15004

PCB Power Amp OCL 100W by Transister MJ15003,MJ15004

Inverter 12V to 220V 100W by Transistor

This circuit power Inverter 100W, it easy and good ideas.

Circuit Inverter 12V to 220V 100W by Transistor


PCB Inverter 12V to 220V 100W by Transistor

12 Volt to 220 Volt Inverter 500W

This is circuit Inverter 12VDC to 220V 50Hz 500W. It easy to make and Low cost.

Automotive 12V to +-20V converter (for audio amplifier)

The limitation of car supply voltage (12V) forces to convert the voltages to higher in order to power audio amplifiers.

In fact the max audio power x speaker (with 4 ohm impedance) using 12V is (Vsupply+ - Vsupply-)^2/(8*impedance) 12^2/32 = 4.5Watts per channel, that is laughable...

For powering correctly an amplifier the best is to use a symmetric supply with a high voltage differential. for example +20 - -20 = 40Volts in fact 40^2/32 = 50 Watts per channel that is respectable.

This supply is intended for two channels with 50W max each (of course it depends on the amplifier used). Though it can be easily scaled up or the voltages changed to obtain different values.

click on image for higher resolution schematic

Overview - How it works

It is a classic push-pull design , taking care to obtain best symmetry (to avoid flux walking). Keep in mind that this circuit will adsorb many amperes (around 10A) so take care to reinforce power tracks with lots of solder and use heavy wires from the battery or the voltage will drop too much at the input.

The transformer must be designed to reduce skin effect, it can be done using several insulated magnet wire single wires soldered together but conducting separately. The regulation is done both by the transformer turn ratio and varying the duty cycle. In my case i used 5+5 , 10+10 turns obtaining a step up ratio of 2 (12->24) and down regulating the voltage to 20 via duty cycle dynamic adjust performed by the PWM controller TL494.

The step-up ratio has to be a little higher to overcome diode losses, winding resistance and so on and input voltage drop due to wire resistance from battery to converter.

Transformer design

The transformer must be of correct size in order to carry the power needed, on the net there are many charts showing the power in function of frequency and core size for a given topology. My transformer size is 33.5 mm lenght, 30.0 height and 13mm width with a cross section area of 1,25cm^2, good for powers around 150W at 50khz.

The windings , especially the primary must be heavy gauged, but instead of using a single wire it is better to use multiple wires in parallel each insulated from the other except at the ends. This will reduce resistance increase due to skin effect. The primary and secondary windings are center tapped, this means that you have to wind 5 turns, center tap and 5 windings again. The same goes for the secondary, 10 turns, center tap and 10 turns again.

The important thing is that the transformer MUST not have air gaps or the leakage inductance will throw spikes on the switches overheating them and giving a voltage higher than expected by turn ratio prediction, so if your voltage output (at fully duty cycle) is higher than Vin*N2/N1 - Vdrop diode, your transformer has gap (of course permit me saying you that you are BLIND if you miss it), and this is accompanied with a drastical efficiency reduction. Use non-gapped E cores or toroids (ferrite).

Output diodes, capacitors and filter inductor

For rectification i preferred to use shottky diodes since they have low forward voltage drop, and are incredibly fast. I used the cheap 1N5822, the best alternative for low voltage converters (3A for current capability).

The output capacitors are 4700uF 25V, not very big, since at high frequency the voltage ripple is most due to internal cap ESR fortunately general purpose lytics have enough low esr for a small ripple (some tens of millivolts). Also at high duty cycle they are feed almost with pure DC, giving small ripple. The filter inductor on the secondary centertap furter increases the ripple and helps the regulation in asymmetrical transients

Power switch and driving

I used d2pak 70V 80A 0.004 ohms ultrafets (Fairchind semiconductor), very expensive and hard to find. In principle any fet will work, but the lower the on-resistance, the lower the on-state conduction losses, the lower the heat produced on the fets, the higher efficiency and smaller the heatsinks needed. With this fets i am able to run the fets with small heatsinks and without fan at full rated power (100W) with an efficiency of 82% and perceptible heating and with small heating at 120W (some degrees) (the core starts to saturate and the efficiency is a bit lower, around 75%)

Try to use the lowest resistance mosfet you can put your dirty hand :-) on or the efficiency will be lower than rated and you will need even a small fan. The fet driver i used is the TPS2811P, from Texas instruments, rated for 2A peak and 200ns. Is important that the gate drive is optimized for minimal inductance or the switching losses will be higher and you risk noise coupling from other sources. Personally i think that twisted pair wires (gate and ground/source) are the best to keep the inductance small. Place the gate drive resistor near the Mosfet, not near the IC.

Controller

I used the trusty TL494 PWM controller with frequency set at around 40-60 Khz adjustable with a potentiometer. I also implemented the soft start (to reduce powerup transients). The adjust potentiometer (feedback) must be set to obtain the desired voltage. The output signals is designed with two pull-up resistors on the collector of the PWM chip output transistor pulling them to ground each cycle alternatively. This signal is sent to the dual inverting MOSFET driver (TPS2811P) obtaining the correct waveform.

Power and filtering

How i said before the power tracks must be heavy gauged or you will scarify regulation (since it depends of transformer step up ratio and input voltage) and efficiency too. Don't forget to place a 10A (or 15A) fuse on the input because the car batteries can supply very high currents in case of shorts and this will save you face from a mosfet explosion in case of failture or short, remember to place a fuse also on the battery side to increase the safety (accidental shorts->fire, battery explosion, firemen, police and lawyers around). Input filtering is important, use at least 20000uF 16V in capacitors, a filter inductor would be useful too (heavygauged) but i decided to leave it..

Final considerations

This supply given me up to 85% efficiency (sometimes even 90% at some loads) with an input of 12V because i observed all these tricks to keep it functional and efficient. An o-scope would be useful, to watch the ripple and gate signals (watching for overshoots), but if you follow these guidelines you will avoid these problems.

The cross regulation is good but keep in mind that only the positive output is fully regulated, and the negative only follows it. Place a small load between the negative rail and ground (a 3mm led with a 4.7Kohm resistor) to avoid the negative rail getting lower then -20V. If the load is asymmetric you can have two cases:

-More load on positive rail-> no problems, the negative rail can go lower than -20V, but it is not a real issue for an audio amplifier.

-More load on negative rail-> voltage drop on negative rail (to ground) especially if the load is only on the negative rail.

Fortunately audio amplifiers are quite symmetrical as a load, and the output filter inductor/capacitors helps to maintain the regulation good during asymmetrical transients (Basses)

ATTENTION

Keep in mind that THIS IS NOT A PROJECT FOR A BEGINNER, IT CAN BE VERY DANGEROUS IN CASE OF PROBLEMS, NEVER BRIDGE, BYPASS OR AVOID FUSES THESE WILL SAVE YOUR BACK FROM FIRE RISK.

FOR FIRST TESTING USE A SMALL 12V power supply and use resistors as load monitoring switches heat and current consumption (and output) and try to determine efficiency, if it is higher then 70-75% you are set, it is enough. Adjust the frequency for best compromise between power and switching losses, skin effect and hysteresis losses

Bill Of Materials
=================
Design: 12V to 20V 100W DC-DC conv
Doc. no.: 1
Revision: 3
Author: Jonathan Filippi
Created: 29/04/05
Modified: 18/05/05

Parts List
--- --------- -----
Resistors
---------
2 R1,R2 = 10
4 R3,R4,R6,R7 = 1k
1 R5 = 22k
1 R8 = 4.7k
1 R9 = 100k

Capacitors
----------
2 C1,C2 = 10000uF
2 C3,C6 = 47u
1 C4 = 10u
3 C5,C7,C14 = 100n
2 C8,C9 = 4700u
1 C12 = 1n
1 C13 = 2.2u

Integrated Circuits
-------------------
1 U1 = TL494
1 U2 = TPS2811P

Transistors
-----------
2 Q1,Q2 = FDB045AN

Diodes
------
4 D1-D4 = 1N5822
1 D5 = 1N4148

Miscellaneous
-------------
1 FU1 = 10A
1 L1 = 10u
1 L2 = FERRITE BEAD
1 RV1 = 2.2k
1 RV2 = 24k
1 T1 = TRAN-3P3S

Inverter 100W by IC 4047 + 2N3055

This circuit Inverter input 12V (battery 12V) to out 220V 50HZ, Easy circuit because less component to use. It use IC 4047 (oscillator 50HZ) and Power Transistor 2N3055 x 2 ,OUTPUT Power 100W.

Circuit Inverter 100W by IC 4047 + 2N3055

PCB Inverter 100W by IC 4047 + 2N3055

DC12V to DC50V converter (for car)

This circuit for Car audio input battery 12V to 50VDC, It use transistor and IC TL072.

Circuit DC12V to DC50V converter (for car)

PCB DC12V to DC50V converter (for car)

Dooebuzzer with 555

555 Table Lamp Circuit

Door Buzzer by NE555 + LM386

Egg Timer By 555

555 Timer 5 to 30 Minute

Description:
A switched timer for intervals of 5 to 30 minutes incremented in 5 minute steps.

Notes:
Simple to build, simple to make, nothing too complicated here. However you must use the CMOS type 555 timer designated the 7555, a normal 555 timer will not work here due to the resistor values. Also a low leakage type capacitor must be used for C1, and I would strongly suggest a Tantalum Bead type. Switch 3 adds an extra resistor in series to the timing chain with each rotation, the timing period us defined as :-

Timing = 1.1 C₁ x R₁

Note that R₁ has a value of 8.2M with S3 at position "a" and 49.2M at position "f". This equates to just short of 300 seconds for each position of S3. C₁ and R₁ through R₆ may be changed for different timing periods. The output current from Pin 3 of the timer, is amplified by Q1 and used to drive a relay.

Parts List:
Relay 9 volt coil with c/o contact (1)
S1: On/Off (1)
S2: Start (1)
S3: Range (1)
IC1: 7555 (1)
B1: 9V (1)
C1: 33uF CAP (1)
Q1: BC109C NPN (1)
D1: 1N4004 DIODE (1)
C2: 100n CAP (1)
R6,R5,R4,R3,R2,R1: 8.2M RESISTOR (6)
R8: 100k RESISTOR (1)
R7: 4.7k RESISTOR (1)

Pre Tone Control Stereo (bass-mid range-treble) by IC NE5532

This circuit Tone Control Stereo you can ajustable bass , mid range and treble. It use IC NE5532.
Supply Volt min 12V 80mA .
Easy to build, PCB small.
Circuit Pre Tone Control Stereo (bass-mid range-treble) by IC NE5532


PCB Pre Tone Control Stereo (bass-mid range-treble) by IC NE5532

Tone Control(bass-treble) Stereo by IC NE5532

Circuit Pre Tone Control Stereo (bass-treble) by IC NE5532 x2.
It nice circuit,Volt supply +12V,-12V.

Note : IC NE5532 or LF353 or 4558

PCB Pre Tone Control Stereo (bass-treble) by IC NE5532 x2.

LED TORCH

Conventional torches come in all shapes and sizes.From a single AAA cell to 4, 5 and 6 "D" cells, as well as "lantern" and "fisherman's." This project uses a white LED to produce illumination equal to a small torch.

White LEDs have different "characteristic voltages." A 1,000mcd white LED used in this project had a characteristic voltage of 3.5v and a 3,000mcd white LED had a characteristic voltage of 3.2v. Both LEDs were driven at 20mA and the 3candala LED produced a brighter, whiter light while the 1candella LED had a yellowish ring around the edge of the illumination.

A LED torch is one of the simplest projects you can build and it's very interesting as it uses a super-bright white LED. In the history of LED production, red LEDs were the first to be invented and their output was so dim you could barely see if they were illuminated. You needed a darkened room to see them at all.Then came green, yellow and orange LEDs. As time went by, the brightness improved and it came to a point where the output would shine into the surrounding air. These were called Super-bright LEDs.Then came the blue LED. At first it was dull, but gradually the output increased to a dazzling glare.With the combination of red, green and blue, manufacturers had the potential of producing a white LED. This was the dream of all LED manufacturers.
Since the illumination produced by a LED comes from a crystal, it is not possible to produce white light from a single crystal or "chip." The only way is to combine red, green and blue. As soon as the output of blue came up to the quality of the other colors, a white LED was a marketable product.White LEDs are now with us and their output makes them a viable alternative to the globe. There is an enormous array of LED torches on the market, from $2.00 "give-aways" to $200 "rip-offs." Although a LED torch is passable for illuminating an area, it certainly does not have the illuminating capability of a $10 lantern, using a 6v battery.
A LED torch is more of a "fun-thing" to see how far LEDs have come in the past few years and see what can be done with a single cell and an handful of components. When we first decided to produce a LED torch project, we wanted to fit the circuit into a 2-cell torch but a white LED requires about 3.4v to operate, and two cells produce only 3v. So we had to think of a number of ways around the problem. That's why we have produced a number of circuits.
As you know, a LED will not operate on a voltage below its characteristic voltage. It simply will not operate AT ALL.
This characteristic voltage depends on the type of LED and is about 1.7v for a normal red LED, while a super-bright LED is about 3.1v - 4v.
The exact characteristic voltage varies with the colour, the intensity of the LED, the current flowing and the way it is manufactured. This feature cannot be altered after it is manufactured and the EXACT voltage must be delivered, otherwise the LED will be not work or if the voltage is higher, it will be destroyed. This is the cold, hard fact. The supply voltage must exactly match the characteristic voltage. This sounds a difficult thing to do, but a simple solution is to add a resistor in series and the voltage across the LED will sit at the exact value required by the LED, while the extra voltage will appear across the resistor. According to Ohm's Law, a current will flow though the resistor and this will also flow though the LED. This applies when the circuit is supplied with a DC voltage.
All we have to do is create a voltage higher than 3.4v and we can drive one of the latest SUPER-HIGH-BRIGHT white LEDs with a single cell, using a step-up-voltage circuit. This will produce a series of pulses to the LED and the brightness will be slightly higher than if a steady DC voltage is applied. These are the things we will be covering in this project.

This project explains the operation of a "transformer" in flyback mode. A transformer is one of the most complex items in electronics. Even a simple hand-made "transformer" requires a lot of understanding to see how it works. This project will demystify some of the features.

CIRCUIT A

The first circuit in this discussion is the simplest design. It consists of a transistor, resistor and transformer, with almost any type of LED. The circuit will drive a red LED, HIGH BRIGHT LED, or white LED. The circuit produces high voltage pulses of about 40v p-p at a frequency of 200kHz. Normally you cannot supply a LED with a voltage higher than its characteristic voltage, but if the pulses are very short, the LED will absorb the energy and convert it to light. This is the case with this circuit. The characteristic voltage of the LED we used was very nearly 4v and this means the voltage across it for a very short period of time was 4v. The details of the transformer are shown in the photo. The core was a 2.6mm diameter "slug" 6mm long and the wire was 0.95mm diam. In fact any core could be used and the diameter of the wire is not important. The number of turns are not important however if the secondary winding does not have enough turns, the circuit will not start-up.

HOW THE CIRCUIT WORKS
The transformer is configured as a BLOCKING OSCILLATOR and the cycle starts by the transistor turning on via the 2k7 base resistor. This causes current to flow in the 60-turn main winding. The other winding is called the feedback winding and is connected so that it produces a voltage to turn the transistor on MORE during this part of the cycle. This winding should really be called a "feed-forward" winding as the signal it supplies to the transistor is a positive signal to increase the operation of the circuit. This is discussed in more detail in Circuit Tricks. This voltage allows a higher current to flow in the transistor and it keeps turning on until it is saturated.
At this point the magnetic flux produced by the main winding is a maximum but it is not expanding flux and thus it ceases to produce a voltage in the feedback winding. This causes less current to flow into the base of the transistor and the transistor turns off slightly. The flux produced by the main winding is now called collapsing flux and it produces a voltage in the feedback winding of opposite polarity. This causes the transistor to turn off and this action occurs until it is completely off.
The magnetic flux continues to collapse and cuts the turns of the main winding to produce a very high voltage of opposite polarity.
However this voltage is prevented from rising to a high value by the presence of the LED and thus the energy produced by the collapsing magnetic flux is converted to light by the LED. The circuit operates at approx 200kHz, depending on the value of the base resistor and physical dimensions of the transformer. The circuit draws 85mA from the 1.5v cell and the brightness of the LED was equivalent to it being powered from a DC supply delivering 10 - 15mA.

Before we go any further, there are a number of interesting circuits on the web. The following two circuits need explaining. The first circuit is identical to our "Circuit A" except the design engineer did not do his homework. He only added 8 turns to the 100uH inductor and found the circuit did not start-up. His solution was to add another transistor and tie the base to the collector. What a waste of a transistor!
The second circuit is a very inefficient design. The second transistor is being turned on via a 1k resistor on the collector of the first transistor and when this "turn-on" current is not required, it is being shunted to "deck." Our circuit uses the "oomph" of the secondary winding to saturate the transistor and this produces the highest efficiency.
Here is a circuit from one of the major chip manufacturers:
Apart from the circuit being enormously complex and expensive, 62mA is too high for many white LEDs. The maximum current must be kept to 20 - 25mA.

The first "poor design" got me thinking. Maybe the signal at the transformer end of the 220R needs to be stabilised to improve the performance of the circuit. I tried a transistor and it did not work.
But I actually thought of placing a small capacitor at the join and taking the other end to the 0v rail. This will allow rail voltage to enter the feedback winding of the transformer but prevent the signal generated by the winding being lost through the 2k7 resistor.
The following circuit is the result:

The brightness of the LED did not alter but the current changed from 85mA to 28mA. The circuit instantly became 300% more efficient. I could not believe it.
When I put the CRO across the LED, I realised why. The frequency of the circuit changed from 200kHz to 500kHz. The LED was getting more than twice the number of pulses per second.
That's why you cannot trust anything or anyone. This improvement has never been presented in any circuit on the web. Obviously no-one has done any experimenting at all. If the brightness of the LED is equal to a DC voltage of 4v and a current of 10mA, the circuit we have produced is slightly more efficient than delivering a DC voltage to the LED, even though there are some losses in the transformer and transistor. This proves the fact that LEDs driven with a pulse, are more efficient than being driven by a DC supply. With this we turn to a surface-mount chip that has been designed to carry out the exact same task as circuit B. The chip is called PR4401. The following is the promotion advert for the chip:




I could not find any sales literature on the internet, but the manufacturer requires 9,000 pieces to be bought at a cost of 36 cents per piece. This comes to $3,240 if you want to incorporate it into your project. I have described the pro's and con's of this chip in another article "Circuit Tricks" and you should read the features and work out what they really mean. When you build circuit "B," you will realise the specifications given in the .pdf for the chip, could be improved. We have achieved a supply current of 18mA for an equivalent brightness of 10mA. The chip requires 25mA. So, all the technology in the world has not surpassed a hand-made circuit.
The advantage of our design is the ready availability of components and you can change them to suit your own application. If you want to increase the brightness, the 2k7 can be reduced to 1k5.
If you want to drive 2 LEDs, they can be added in series:

Adding a 100u across the battery will increase the current by 4mA and the brightness will increase slightly. When 2 LEDs are placed in series, the current drops from 28mA to 23mA and the brightness from each LED is slightly less. This circuit is operating at about the maximum capability of the transformer. The actual limiting factor is the size of the "core." It can only "hold" a certain amount of magnetic flux and return it to the windings during the collapsing part of the cycle. A larger core will allow three or more LEDs to be illuminated.
The "high efficiency" of this circuit is due to the "pulsing of the LED." When a LED is pulsed with a high current for a short period of time, the brightness is equivalent to a lower, steady, current. That's why a current of 23mA from the battery will illuminate 2 LEDs with an equivalent brightness of about 8mA of steady current. It is very difficult to compare the brightness of one LED against another and these results are the best you can make by visual inspection. We are not driving the LEDs to their maximum but the output is very impressive.

THE TRANSFORMER
The secret of this circuit is the transformer. We normally think of a transformer as a device with an input and output, with the voltage on the input and output being connected by a term called "turns ratio." If the output has more turns than the input, the output voltage will be higher. This is called a step-up transformer. If the output has less turns than the input, the output voltage will be lower.
This applies to "normal" transformers where the voltage is rising and falling at a regular rate, commonly called a "sinewave." But the transformer in this circuit is different. The voltage applied to it is not rising and falling smoothly, and thus it does not work in normal "transformer mode."
The voltage is being applied and then turned off. When the voltage is applied, the primary winding (the 60 turn winding) produces magnetic flux. When the voltage is turned off, the magnetic flux collapses and produces a VERY HIGH voltage (in the REVERSE DIRECTION), in all the windings.
Our transformer is really a coil in flyback mode with a feedback winding.
The feedback winding delivers a voltage to the transistor to turn it on HARDER. If the winding is connected around the wrong way, the circuit will not work. The other important factor about the transformer is the core material. There are many different types of ferrite. Ferrite is a type of iron which is powdered very finely so that the magnetic lines that pass through the particles do not create eddy-currents. These eddy currents absorb the magnetic flux. The material we have used is F29 and this is suitable for high frequency applications.
The circuit also employs a term called RE-GENERATION. This is the effect where a circuit is turned on slightly by a component (the 2k7 base resistor in this example) and then the transistor turns itself on more and more until it is fully turned on. The feedback winding is configured so that the voltage it produces (actually the current it produces) is fed into the base to turn the transistor on.
Thus the feedback winding is very clever. It produces energy and is delivered in a particular direction - in other words it can be a positive or negative energy. In this case it produces positive energy, to turn the transistor on harder.
This is called POSITIVE FEEDBACK as it turns the transistor ON during the active part of the cycle. Now we come to the MAIN, PRIMARY or FLYBACK winding.
This winding produces a high voltage during part of the cycle (the FLYBACK part of the cycle) and this is passed to the LED. If the LED is removed, the transformer produces a high voltage with a low current, but when the LED is inserted, an amazing thing happens. The energy from the transformer is converted to a lower voltage with a higher current. What actually happens is the LED absorbs the energy and turns it to light as soon as the voltage rises to 3.6v.
We could achieve the achieve the same low-voltage, high current requirement, with less turns, but the number of turns has actually been determined so the core does not saturate.
The voltage for the LED is produced when the transistor is switched off and the magnetic flux in the ferrite core collapses.
The speed of the collapse produces a very high voltage in the OPPOSITE DIRECTION and that's why a positive voltage emerges from the end connected to the LED. These two facts are important to remember. The other important fact is called "transformer action." This is the action of magnetic flux.
When a voltage is applied to a winding of a transformer or a coil of wire, a current will flow and this will produce magnetic flux. If another winding is present, the magnetic flux will cut the turns of this extra coil and produce a voltage in it. However, there is a very important point to remember. The magnetic flux can be: EXPANDING, STATIONARY or CONTRACTING. When the magnetic flux is expanding, a voltage will appear in the second winding mentioned above.
When the magnetic flux is stationary, NO VOLTAGE will appear in the second winding.
When the magnetic flux is contracting a voltage will appear in the second winding with REVERSE POLARITY. The size (the amplitude or "value") of the reverse voltage will depend on the speed of the collapsing magnetic flux. If the flux collapses quickly, the amplitude will be very high.
That's how the transistor turns itself on and on until it is fully turned on. At this point the current flowing through the circuit is a maximum but the flux is not expanding so the base of the transistor does not see the high "turn-on" energy and thus the transistor suddenly turns off.
The magnetic flux collapses and the transistor sees a reverse voltage on the base to keep it turned off until the flux is fully collapsed. The current through the 2k7 enters the base to start the cycle again. From this you will be able to see how the transistor and transformer work.

THE FLASHER
Now we come to the problem of flashing a white LED, using a 1.5v supply. The following circuit performs this task:
The oscillator charges the 100u via the 1N 4148 diode and when the voltage reaches about 10v, the BC 547 transistors "zeners" (breaks down) and conducts. Energy in the 100u is then dumped into the LED to make it illuminate. This causes the voltage across the 100u to drop and the transistor comes out of conduction. The oscillator then continues to charge the 100u to repeat the cycle.
The zener voltage of the transistor is not 10v as approx 4v is dropped across the LED. This conforms with an article on the web that said the emitter-collector junction is equal to a 6v2 zener. The 330R charging resistor produces a fast flash and the 1k produces a slow flash. The current for the circuit is approx 22mA and any type of LED can be fitted.
Measuring the current-consumption of a circuit is a very difficult thing to do. When you insert a a meter into the positive line (or negative line) of a circuit, you introduce extra resistance and the operation of the circuit will alter. You may think the low resistance of an ammeter will not affect the performance, but quite often the "ammeter " is really a "milli-amp meter" and the "shunt resistance" on the 200mA scale can be 4 - 7 ohms. This is quite considerable when a circuit is operating on 1.5v and drawing 30mA. This can be a loss of 100mV to 200mV and the current taken by the circuit will alter considerably.
That's why the best approach is to place a 1 ohm resistor in line with the positive of the battery and measure the millivolt drop across the resistor. Each millivolt drop will correspond to 1mA flow and this will change the circuit conditions as little as possible. The following circuit shows how this is done:

A 100u electrolytic across the circuit will reduce the impedance of the supply and keep the circuit working as normal as possible.

As a point to note: The White LED Flasher circuit did not start-up on a flat AAA cell. Solution: take two flat cells and connect them in series and see how long the LED will flash. You will be very surprised. The circuit will draw about 30mA and the LED will flash very quickly. The circuit will continue to work on two very flat cells until the flash rate drops to one flash per second.
This type of circuit puts a very heavy "strain" or "noise" on the power supply. In other words it puts a heavy demand on the battery for a short period of time. This is not a problem if the only item connected to the battery is the flasher circuit. But if the battery is also driving a circuit such as an mp3 player or microcontroller, the high-frequency noise may upset the operation of the electronics.

THE OSCILLATOR TRANSISTOR
The oscillator transistor needs to sink a very high current for a very short period of time (as mentioned above) and thus it must be a "high-current" type. A "high-current" type improves the efficiency of the circuit. If the transistor cannot sink the transformer to the 0v rail, it effectively becomes a "resistance" in the network. Suppose the supply is 1500mV (1.5v , 1v5) and the transistor can sink to 500mV, 30% of the voltage is dropped across the transistor and thus the circuit is using only 66% of the incoming energy. If the transistor can only sink to 0.75v, the circuit is using 50% of the incoming energy. Some transistors can sink to 0.3v and thus the circuit is more efficient.

STABILITY
Now we come to the stability of the circuit. The circuit is very unstable and very unreliable. Touching the components with a finger changes the frequency of the flash-rate and connecting CRO to the collector of the oscillator transistor inhibits the flashing. The oscillator keeps working but the zener transistor fails to operate.
This circuit is totally unsuitable for a commercial design and it reminds me of some of the original transistor flasher circuits. They required precise values of resistance and did not work when the supply voltage dropped.
Fortunately someone came up with the flip-flop flasher and changed everything. It is totally reliable and operates under all sorts of conditions.
Now we come to the design of a higher output circuit, to satisfy those who want to use a larger cell and drive 2 or 3 LEDs to maximum brightness.

HIGHER OUTPUT
To drive more LEDs, a higher output is needed. We have already mentioned, the limiting factor with the circuits above is the transformer. To achieve a higher output, the size needs to be increased. This is quite easily done by getting a larger core. It is the core that determines the amount of flux that can be stored. When turns are wound on a core, the result is called an inductor and when a second winding is added, the result is called a transformer.
Most of the inductors and transformers we use in the circuits in this article have an open magnetic circuit. This means the flux escapes out one end of the core and in general the result is not very efficient. But it has proved to be satisfactory.
An improved core is called a "pot core" and consists of two halves as shown in the diagram below:

The magnetic lines go around the "magnetic circuit" as shown in the diagram above and pass through an air gap. The air gap is to compensate for the DC across the coil (transformer). If the air gap is closed up, the inductor will saturate before the circuit is fully conducting and this may make the inductor less effective. All this theory is very complex and you really have to try the component to see the effect.
Our circuits use a simple "in line" inductor as shown above or a "bobbin" as shown below in the third item. The photo below shows the "slug" transformer used in circuits A, B, and C and the "bobbin" transformer used in circuit D. The size of each transformer gives some idea of the relative output. The centre inductor is a 10mH choke. This is unwound to get the bobbin for the transformer.

The bobbin is re-wound with 35 turns of 0.5mm wire for the primary and 20 turns for the feedback winding. The two pins connect to the primary and the 20 turn-winding is wound on top, with flying leads. The gauge of the wire is chosen so that the windings completely fill the bobbin. The feedback winding can be a thinner gauge, without any detriment to the operation of the circuit. By the appearance, you could expect up to 5-10 times more output from the bobbin.
But with a higher output, you need to provide some form of energy-limiting circuit to prevent damaging the LED. The following circuit provides current limiting so that the LED will produce maximum brightness for the voltage range 1.5v to 0.9v.

This gives a choice to suit a variety of torches. The smallest penlight torch will only have enough room to drive a single LED while the larger "C" and "D" cell torches will drive two or three LEDs.
There are some slight differences between each of the circuits and you need to read the article if you want to deviate from any of the layouts we have given. For instance, the 2SC 3279 transistor is capable of sinking 2 amps and this makes it a better driver for circuit-2 but its collector-emitter voltage is only 10v and it may zener in circuit 3, where the voltage is very near this value.

Circuit-1 drives one LED from a single cell

Circuit-2 drives two LEDs from a single cell

Circuit-3 drives three LEDs from two cells

CURRENT REGULATION
The circuit includes a feature called "current regulation." You can also call the feature "voltage regulation" as both have the same effect of controlling the brightness of the LED. It can also be called a "constant brightness" arrangement. It's a feedback arrangement consisting of a BC 547 connected to the base of the main transistor. When the voltage across the "detector resistor" rises above 0.7v, the BC 547 turns ON and prevents the main transistor operating. This allows the LED to produce a constant brightness over a wide supply voltage. The circuit will theoretically work to 0.8v. Do not remove the current regulating transistor as the circuit will over-drive the LED when the supply is 1.5v. The excess current will instantly destroy the LEDs.

The actual operation of the circuit can be explained in a little more detail. When the circuit is turned on, the oscillator transistor produces a high voltage from the inductor and this is rectified by a diode to charge a 100u electrolytic. When the voltage rises to over the total characteristic voltage of the LED or LEDs, they turn on and current flows though the 39R "detector resistor." The voltage across the 100u will continue to rise and since the characteristic voltage of the LEDs has been reached, any further voltage rise will appear across the resistor. As soon as this voltage reaches 0.7v, the feedback transistor begins to turn on. The feedback transistor acts like a variable resistor as shown in the diagram below and some of the current from the feedback winding is passed to the 0v rail, through the transistor. The oscillator transistor sees a reduced "turn-on" effect and the output of the stage is reduced.

In this way the brightness of the LEDs can be kept constant throughout the life of the battery.

The circuit is actually being "pulled back" when a fresh cell is connected, by the action of the feedback transistor. As the voltage from the cell reduces, the oscillator circuit will not be able to produce a high output and the action of the feedback section will not be needed. Eventually the voltage of the cell will be so low that the LED will start to dim. This is the end of the life of the cell.

Caution: Do not allow more than 25mA to flow though a white LED (unless it is being pulsed) as it will be instantly DESTROYED. Other LEDs (such as low-brightness red LEDs) are much more tolerant - but white LEDs are easily damaged.

CIRCUIT SIMULATION
A number of circuits similar to this project have been presented on the internet. One circuit had twice the number of components and used 4 transistors. The art of designing a circuit is to make it as simple as possible, while providing all the needed features. It is pointless making a circuit complex, as it simply adds to the cost and makes fault-finding more difficult.
But a note near one of the circuits was really annoying. It said the circuit "had not been tested, only a simulation was run." While these simulation programs work in a number of applications, they certainly cannot take into account the characteristics of an inductor. This is one item that no-one can predict. It's performance depends on so many variables. If you think you can design a circuit such as this on a simulator, and it will work, you are kidding yourself. Electronics is not that simple.
Transistors exhibit different characteristics according to the current flowing though them and a circuit such as ours requires the main transistor to pass a very high current for a short period of time.
Fortunately, Japanese transistors are capable of passing a high current while some Philips transistors will fail to pass the test. The gain of a transistor under these stressful conditions cannot be determined from a data-sheet. Circuits should never be presented in an article unless they have been tried and tested. A simulation program cannot possibly take into account the effectiveness of an inductor in any particular situation, even though the inductance is known. There are hundreds of ways to produce a 10uH inductor, or any inductor for that matter. It can be air-cored or ferrite cored. The windings can be thick or thin wire. The core can be made of several different materials. On top of this it will depend on the frequency of the circuit. The output voltage of an inductor that has been specially designed for a particular circuit can be 100 times higher than an incorrectly designed item. That's why it takes a considerable amount of "trial-and-error" to produce an ideal inductor or transformer.
The output voltage has a lot to do with the "Q-factor" or quality factor and this is a value that is associated with the way the inductor or transformer has been designed. The "Q value" is basically the ratio of the supply voltage compared to the output voltage. No simulation program can "guess" the value of "Q" and since the operation of the circuit is entirely dependent on this value, it has to be constructed. would not even attempt to put this type of circuit on a simulator.

DESIGNING AN INDUCTOR
There are many ways to go about designing an inductor or transformer. You can sit down and study the theory of inductance, the effectiveness of ferrite material at different frequencies, the use of different wire gauges and the associated inductance formulae. If you think you will be able to produce an inductor for this circuit entirely from theory, (with the first prototype working perfectly), you are kidding yourself. There are a number of parameters you cannot specify in the formulae.
Even if you did come up with an answer, no electronics-designer would be satisfied with the first result. He would need to see the prototype and add or remove turns to see the effect. He would use thicker or thinner wire and note the effect. He would carry out all sorts of experimentation, including monitoring the battery current while noting the current though the LEDs to work out the efficiency of the circuit. It could take 50 or more prototypes to arrive at the best design. So, where do you start when designing a transformer or inductor? No-one really knows where to start. It all comes from trial-and-error and guessing a starting-point. The easiest way is to copy an existing design.
But if you don't have something to copy, you can begin with say 10 turns. Note the output voltage and current taken by the circuit. Increase the winding to 20 turns. Again note all details. From the figures you can work out if you are going in the right direction. Continue collecting data with both additional turns and reduced turns as, sometimes, an unusual feature suddenly arises. Keep working until you are satisfied with the results.
Even if you have studied inductor theory, you will still have to carry out the practical side of things.
Nothing takes the place of actually "doing-it."

DESIGNING OUR TRANSFORMER
In our 3 circuits, there are many different combinations of windings that will work. The reason is the circuit is non-critical. You have to understand the operation of an inductor in an entirely different way to the theoretical model to see how it operates. This is called a "loose" circuit and a wide range of primary windings will produce the same result. For example, a primary winding of 35 turns will produce the same LED brightness as 55 turns and the current from the supply will be the same.

The output of the transformer (on no-load) will be more than 200v and thus the circuit must not be operated on no-load as the voltage may damage the transistor. If the LEDs are removed, the circuit will charge the capacitor to more than 45v and this is above the operating voltage for a 100u/25v electrolytic. If you remove the LEDs and turn the circuit on, then re-solder the LEDs, they will be damaged. This is because the electrolytic will have charged to 45v. Thus it is very difficult to experiment with the circuit to see how the transformer charges the electrolytic.
You will have to follow our explanation:

HOW DOES THE ELECTROLYTIC CHARGE?
The electrolytic is charged by pulses from the inductor. In circuit-3 the voltage across the electrolytic is 10v and it is delivering current to the three LEDs at a constant rate of 17mA.

CRO waveform - output of inductor

In the CRO diagram above, the pulses (or spikes) occupy about 10% of the total time.
The area under the graph (under each spike) is shown in orange and this represents the energy supplied to the electrolytic.
The inductor is capable of producing a very high spike when in flyback and this voltage allows a burst of current to pass though the diode and charge the electrolytic. When the inductor is operating under no-load, it is capable of producing a spike of more than 200v, but this voltage is not allowed to be produced when the load is connected. The voltage-spike is limited to the characteristic voltage across the LED or LEDs, plus the voltage drop across the diode and minus the battery voltage. The voltage will be about 9v. If we are drawing 17mA for 100% of the time, we must deliver 10 times 17mA for 10% of the time to keep the electrolytic charged. Thus a current of about 17 x 10 = 170mA is needed to pass through the diode to charge the electrolytic.
The other feature of the diode is it prevents the voltage on the electrolytic being discharged to the 0v rail via the transistor when it is turned on.

The frequency at which the circuit operates is determined by the inductance of the inductor. The cycle start when the power is applied and the transistor turns on to allow current to flow though the main winding. This produces magnetic flux in the feedback winding to turn the transistor on harder. This continues until the transistor is turned on fully and maximum flux is produced. But the flux is not expanding flux and thus it does not cut the turns of the feedback winding and the transistor does not get the full turn on current into the base. The transistor is turned off and this causes the magnetic flux to collapse. This flux is in the opposite direction and it produces a reverse voltage in the feedback winding to keep the transistor fully turned off. The main winding also produces a voltage in the opposite direction and it delivers a pulse of energy to the electrolytic via the high-speed diode. As soon as the magnetic flux is spent, (converted to electrical energy) the cycle starts again.
The combination of these two operations creates the length of time for one cycle.
In our case the circuit operates at approx 90kHz.

LED OUTPUT
There is a lot of hype and confusion about the light output of some super-bright LEDs. Sometimes there is very little difference when you compare the output of 1cd, 3cd and 6cd (6,000mcd) LEDs, when supplied from wholesalers. One of the reasons is the difficulty in identifying each LED. They have no markings and if they are not kept in their correct bag, they can get mixed up! There are literally dozens of different types. Secondly, the difference in brightness is due to the angle at which the light-beam emerges from the LED. This is due to the lens inside the LED and/or the way the LED is potted, producing a divergent beam or a narrow beam. Almost all LEDs have a different illumination intensity, color and spot-size, depending on the manufacturer, beam angle and quality of the chip producing each color (efficiency). Some have a blue appearance in the centre of the spot white light while others have a noticeable green fringe. This project is an ideal way to test 2 or 3 LEDs at the same time. Since they are in series, they pass the same current and the intensity control will allow you to vary the brightness and compare the outputs. When experimenting, keep a record of the type of LED by paining it with red or while nail polish. Keep the same reference on the bag from which they came. This will prevent them getting mixed up.


CONSTRUCTION
Firstly you need to decide on the type of housing you want to use. This will determine the circuit you will use, the number of LEDs and the shape of the PC board. It's best to get a kit of components as the core for the inductor is supplied with winding wire and these are normally difficult items to get. If you want to use the project for experimentation, circuit-3 has an adjustable brightness control. The only extra components you will need are red LEDs to take the place of the white LEDs, when you are setting up the circuit.

IF IT DOESN'T WORK
If the circuit does not work, you have two choices. You can buy another kit or carefully work though the assembly and see where you made the mistake. Things like the orientation of the transistor, diode and LED need to be checked but the general reason for the project not working is the connection of the transformer. Simply reverse one of the windings. It does not matter which way the windings are wound on the ferrite core. By simply reversing one of the windings, the transformer will work. Do no reverse BOTH windings as this will not solve the problem.

EXPERIMENTING
Before experimenting with any of the circuits, there are a number of things you must be aware of. The inductor is capable of producing a very high voltage when no load is connected and this can cause damage to the oscillator transistor, the electrolytic and/or the LEDs. We have already mentioned some of the ways the components can be damaged and the most critical component is a white LED. It will not tolerate excess current, even for a fraction of a second. Ordinary red LEDs are very tolerant and this gives you a false sense of robustness. The circuit is capable of charging the electrolytic to more than 45v and if a white LED is connected when the electrolytic is fully charged, it will illuminate very brightly and die.
The situation does not occur when the circuit is operated normally and this means experimenting with the circuit is risky if you don't know what you are doing. One solution is to use 2 red LEDs to take the place of each white LED. You can take all current, voltage and efficiency measurements with the red LEDs and when the circuit is operating as required, the LEDs can be replaced with a white LED.
Don't let the sensitive nature of a white LED deter you from experimenting - simply substitute them.
This project has been specially designed for experimenting. The main reason for using a hand-made inductor is to allow different arrangements to be tried. One point you will have to remember:
The energy from an inductor in flyback mode depends on the amount of ferrite in the core. The core supplied in the kit can only supply enough energy to fully illuminate 2 white LEDs. When 3 LEDs are used, the maximum current it can supply is about 15mA.

PARTS LIST
8 - 330R all 1/4 watt 5% resistors
3 - 2k2
2 - 10k
1 - 47k
1 - 100n surface-mount capacitor
1 - 100u 16vw electrolytic
1 - 1N 4004 power diode
3 - BC 547 transistors or similar
9 - 3mm bi-colored LEDs
1 - 3mm red LED
1 - 3mm green LED
2 - tactile switches
1 - 18pin IC socket
1 - PIC16F628 Tic Tac Toe microcontroller IC
1 - SPDT slide switch
5cm very fine tinned copper wire
50cm - very fine solder
1 - 3cm double-sided tape for battery box
1 - 4-AAA cell battery holder
4 - AAA cells
1 - TIPC board