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Thursday, December 31, 2009

Radio Remote Control using DTMF

Here is a circuit of a remote control unit which makes use of the radio frequency signals to control various electrical appliances. This remote control unit has 4 channels which can be easily extended to 12. This circuit differs from similar circuits in view of its simplicity and a totally different concept of generating the control signals. Usually remote control circuits make use of infrared light to transmit control signals. Their use is thus limited to a very confined area and line-of-sight. However, this circuit makes use of radio frequency to transmit the control signals and hence it can be used for control from almost anywhere in the house. Here we make use of DTMF (dual-tone multi frequency) signals (used in telephones to dial the digits) as the control codes. The DTMF tones are used for frequency modulation of the carrier. At the receiver unit, these frequency modulated signals are intercepted to obtain DTMF tones at the speaker terminals. This DTMF signal is connected to a DTMF-to-BCD converter whose BCD output is used to switch-on and switch-off various electrical appliances (4 in this case).

Updated Circuit

The remote control transmitter consists of DTMF generator and an FM transmitter circuit. For generating the DTMF frequencies, a dedicated IC UM91214B (which is used as a dialer IC in telephone instruments) is used here. This IC requires 3 volts for its operation. This is provided by a simple zener diode voltage regulator which converts 9 volts into 3 volts for use by this IC. For its time base, it requires a quartz crystal of 3.58 MHz which is easily available from electronic component shops. Pins 1 and 2 are used as chip select and DTMF mode select pins respectively. When the row and column pins (12 and 15) are shorted to each other, DTMF tones corresponding to digit 1 are output from its pin 7. Similarly, pins 13, 16 and 17 are additionally required to dial digits 2, 4 and 8. Rest of the pins of this IC may be left as they are. The output of IC1 is given to the input of this transmitter circuit which effectively frequency modulates the carrier and transmits it in the air. The carrier frequency is determined by coil L1 and trimmer capacitor VC1 (which may be adjusted for around 100MHz operation). An antenna of 10 to 15 cms (4 to 6 inches) length will be sufficient to provide adequate range. The antenna is also necessary because the transmitter unit has to be housed in a metallic cabinet to protect the frequency drift caused due to stray EM fields. Four key switches (DPST push-to-on spring loaded) are required to transmit the desired DTMF tones. The switches when pressed generate the specific tone pairs as well as provide power to the transmitter circuit simultaneously. This way when the transmitter unit is not in use it consumes no power at all and the battery lasts much longer. Updated Circuit -(Bug Free)

The receiver unit consists of an FM receiver (these days simple and inexpensive FM kits are readily available in the market which work exceptionally well), a DTMF-to-BCD converter and a flip-flop toggling latch section. The frequency modulated DTMF signals are received by the FM receiver and the output (DTMF tones) are fed to the dedicated IC KT3170 which is a DTMF-to-BCD converter. This IC when fed with the DTMF tones gives corresponding BCD output; for example, when digit 1 is pressed, the output is 0001 and when digit 4 is pressed the output is 0100. This IC also requires a 3.58MHz crystal for its operation. The tone input is connected to its pin 2 and the BCD outputs are taken from pins 11 to 14 respectively. These outputs are fed to 4 individual ?D? flip-flop latches which have been converted into toggle flip-flops built around two CD4013B ICs. Whenever a digit is pressed, the receiver decodes it and gives a clock pulse which is used to toggle the corresponding flip-flop to the alternate state. The flip-flop output is used to drive a relay which in turn can latch or unlatch any electrical appliance. We can upgrade the circuit to control as many as 12 channels since IC UM91214B can generates 12 DTMF tones. For this purpose some modification has to be done in receiver unit and also in between IC2 and toggle flip-flop section in the receiver. A 4-to-16 lines demultiplexer (IC 74154) has to be used and the number of toggle flip-flops have also to be increased to 12 from the existing 4


1) Transmitter Section - download-it
2) Receiver Section - download-it
3) Front panel (for cabin) - download-it

4) Data flow Diagram (PPT) - download-it

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Monday, November 30, 2009

MAINS operate Christmas Star

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Here is a low-cost circuit of Christmas star that can be easily constructed even by a novice. The main advantage of this circuit is that it doesn’t require any step-down transformer or ICs. Components like resistors R1 and R2.capacitors C1, C2, and C3, diodes D1 and D2, and zener ZD1 are used to develop a fairly steady 5V DC supply voltage that provides the required current to operate the multivibrator circuit and trigger triac BT136 via LED1.
The multivibrator circuit is constructed using two BC548 transistors (T1 and T2) and some passive components. The frequency of the multivibrator circuit is controlled by capacitors C4 and C5 and resistors R3 through R7. The output of the multivibrator circuit is connected to transistor T3, which, in turn, drives the triac via LED1. During positive half cycles of the multivibrator’s output, transistor T3 energizes triac BT136 and the lamp glows.

LED Lighting For Christmas

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Using light effects for decoration on festive occasions is a normal practice. Designers are coming up with varieties of electronic circuits to fill the imagination of users. Here is an easy-to-assemble circuit for christmas decoration as shown in Fig.1. It comprises four transistors, eighteen LEDs, a few resistors and two capacitors. Transistors T1 and T2 are configured as an astable multivibrator, which means one of the two transistors is always conducting. Thus the combination produces clock pulses. The values of time-constants formed with R6-C2 and R8-C1 pairs have been selected to produce a lowfrequency clock that is visible to human eye. The collectors of transistors T1 and T2 are connected to driver transistors T3 and T4. These are used to light up two rows of LEDs connected in parallel with alternate clock pulses. The frequency at which LED1 through LED9, and LED10 through LED18, alternately light up is about 2 Hz. You can easily change this frequency by changing the values of capacitors C2 and C1.
Resistors R2 and R4 are used to set the current through the LEDs. Red (LED1 through LED9) and green LEDs (LED10 through LED18) are used for simulating christmas decoration effects. For the brightness variation, you can change the values of resitors R2 and R4. Take any general-purpose PCB and cut it into a star shape. Thereafter, assemble the circuit and solder the colour LEDs onto it such that it looks like a christmas star.
Alternatively, you can design the PCB in circular shape with a festive white lacquer finish on component side and conductor tracks on the other. Place the control circuit at the centre of the PCB board, with LEDs mounted along the outer edge as shown in Fig. 2. Along this edge, there are three circular tracks:
The middle one is the positive supply, which goes to the anodes of all LEDs. The outer track is connected to the cathodes of the red LEDs and the inner tracks are connected to the cathodes of the green LEDs.

To obtain the best effect with the combination of red and green LEDs, mount them alternately on the PCB board. Exercise care so that you do not accidentally connect the red and green LEDs in parallel. The forward voltage drops of red and green LEDs are different. The circuit works off a 3V-9V battery. It consumes little current, so two/ four AA cells or a 9V battery can easily power the electronic star. You can also use a stabilised 3V-9V DC mains adap-
Fig.1: LED lighting circuit for Christmas tor in place of the battery.

Reference : EFY

Wednesday, October 28, 2009

Long Range FM Transmitter Circuit

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This circuit is a circuit diagram fm transmitter. This circuit is somewhat different from the previous fm transmitter circuit. Transmitter circuit described here has the additional RF power amplifier stage, after the oscillator stage, to increase the power output of 200-250 milliwatts. With a good matching 50-ohm ground plane antenna or multi-element yagi antenna, this transmitter can provide a good enough signal strength to a distance of about 2 kilometers. The circuit built around transistor T1 (BF494) is the basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts.
Transistor T2 (2N3866) forms a VHF-class power amplifier. This increases the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power produced at the collector of transistor T2. For better results, assemble the circuit on a good quality glass epoxy board and house the transmitter in the case of aluminum. Shield the oscillator stage using aluminum sheets. Transistor T2 must be mounted on the heat sink. Do not switch on the transmitter without a matching antenna. Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

Coil winding details are given below:
L1 – 4 changes of 20 SWG wire close wound over 8mm diameter plastic former.
L2 – 2 changes of 24 SWG wire near top end of L1.
(Note: There is no core (ie air core) is used to coil on top)
L3 – 7 changed from 24 SWG wire close wound with 4mm diameter air core.
L4 – 7 changed from 24 SWG wire-wound on ferrite beads (choking)
Potentiometer VR1 is used to change the fundamental frequency whereas potentiometer VR2 is used as power control.

Simple DC to AC Inverter

This DC to AC inverter circuit work based on unstable multi vibrator does. In this circuit, IC CD4047 is chosen as a heart of unstable multi vibrator, because this IC type gives a complementary output that has opposite phase to another ( pin 10 and 11 as seen in Figure 1), and has 50 % duty cycle that satisfy to generate a pulse for inverter.

In order to increase the current out of multi
vibrator so enough to generate a higher AC power too, then we must use MOSFET IRFZ44. IRFZ44 gives out high current to drive a step-up transformer, so AC power is available at the high voltage side of transformer.
This circuit is called as simple DC to AC inverter because of the output haven't a sinusoidal signal

yet, so there are many harmonic signal at the output. To suppress this signal we must use a filter such as a capacitor C. Because of this simplicity this circuit is suitable only for lighting demand. To build a sinusoidal DC to AC inverter, we can use a PWM signal for driving a step-up transformer, such as at page of DC to AC inverter using AT89C2051.

Friday, October 23, 2009

Parrot Sounding AC door Bell

Here is a mains-operated doorbell that produces parrot-likesweet voice without requiring any musical IC. The circuit is cheap and easy to construct. The AC mains is fed to the circuit without using any step-down transformer.

The complete circuit is shown in Fig. 1. The main components of the circuit are a resistor-capacitor network, transistor BC337 and audio output transformer X1. The oscillation frequency depends on the combination of resistors R4 and R5 and capacitors C3, C4 and C5. When switch S1 is closed, the audio signal generated due to oscillations is amplified by transistor BC337 and parrot-like sound is reproduced from loudspeaker LS1 connected across the secondary of transformer X1. Here we have used an 8-ohm, 0.5W loudspeaker. The audio output transformer (X1) is normally used in transistor radio. The function of the audio output transformer is to transform the high impedance of the output amplifier to match the much lower impedance of the speaker. This is necessary to get an efficient transfer of the audio signal to the speaker. If a wrong audio transformer is used, the result can be low output and loss of tone quality.

The audio frequency tone across the speaker terminal is about 3 kHz. The dimensions of the audio transformer used in the experimental setup are shown in Fig. 2. The circuit is powered directly from 220V AC mains. The operating DC voltage obtained at the cathode of diode D1 is about 6V. However, if you press switch S1 continuously for a few seconds, the maximum voltage developed at this point may go up to 20 volts, which must be avoided to prolong the life of the circuit. R1 limits surge current in the circuit. The parallel combination of resistor R1 and capacitor C1 limits the circuit current to a safe level for circuit operation. R2 across C1 provides DC path for the current as well as a discharge path when the circuit is switched off. This is to prevent a possible shock to the operator by charged capacitor C1.

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Tuesday, September 29, 2009

20 Watts RMS Amplifier Using TDA2004

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The Circuit present here is a 20Watts Car Stereo Amplifier. The main features of this powerful MULTIWATT® package (a trademark of SGS-THOMSON Microelectronics), a power amplifier IC chips designed specifically for car radio application, are the high current capability (3.5A) and the capability to drive a very low impedance (down to 1.6R). Here is the schematic diagram of the standard circuit as shown in its data sheet.

Technical Specification of TDA2004

The TDA2004A is a class B dual audio power amplifier in MULTIWATT[ package specifically designed for car radio applications


VS Opearting Supply Voltage 18 V
VS DC Supply Voltage 28 V
VS Peak Supply Voltage (for 50ms) 40 V
IO (*) Output Peak Current (non repetitive t = 0.1ms) 4.5 A
IO (*) Output Peak Current (repetitive f . 10Hz) 3.5 A
Ptot Power Dissipation at Tcase = 60°C 30 W
Tj, Tstg Storage and Junction Temperature –40 to 150 °C

(*) The max. output current is internally limited.

TDA2004 has low noise, low distortion, and robust. The robustness is supported by its operation safety protection features: very inductive loads, load dump voltage surge, overheating, output AC-ground short, fortuitous open ground. Other important things is space and cost saving : very low external components counts, and very simple mounting system with no need for electrical isolation between the package and the heat sink because the heat contact metal of the package is already connected to ground. [The above circuit's schematic diagram is taken from: SGS-THOMSON Microelectronic Data Sheet]
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Wednesday, September 16, 2009

Strobe Light

Strobe lights are widely used by disco lovers to create wonderful visual effects in disco halls and auditoriam. The circuit of a battery operated portable miniature strobe light, which can be constructed using readily available inexpensive components, is described here. For convenience and simplicity, an ordinary neon lamp is used here in place of the conventional Xenon tube. The whole gadget can thus be easily accommodated in a small cabinet, such as a mains adaptor cover, with a suitable reflector for neon lamp to give a proper look. Since current requirement of this circuit is very small, it may be powered by two medium-size dry cells (3V) or Ni-Cd cells (2.4V). Transistors T1 and T2 in the circuit form a complimentary-pair amplifier. When switch S1 is momentarily depressed, the circuit oscillates because of the positive feedback provided via resistor R2 and capacitor C1 to the base of transistor T1. The sharp pulses in the secondary winding induce a high voltage in primary winding of transformer X1, which in fact is a line driver transformer (used in reverse) which is generally used in 36cm TV sets. High voltage pulses induced in primary side are rectified by diode D1 and rapidly charge reservoir capacitor C2 to nearly 300V DC. When switch S1 is released, capacitor C2 holds the voltage level for a finite period while capacitor C3 charges slowly through resistor R3. When voltage across capacitor C3 becomes high enough, neon strikes and the capacitor rapidly discharges through the lamp. When voltage across capacitor C3 falls below the extinguishing potential of neon lamp, it goes off and capacitor C3 starts charging again. This cycle keeps on repeating for a short time, based on the reservoir capacitor C2’s value. Precautions. The neon lamp flasher section of this circuit carries dangerously high voltages. All precautions should therefore be taken for protection. Before any repair work, discharge capacitor C2 using a short length of wire with a 100k resistor connected in series.

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Monday, August 31, 2009

SG3525 DC Converter 12V to +35V,-35V

The selected switching topology is called a "push-pull" converter, because the transformer has a double primary (or a "centre-tapped" one, if your prefer). The centre tap is permanently connected to the car battery (via an LC filter to avoid creating peaks in the battery lines, which could affect other electronic equipment in the car). The two ends of the primary are connected to a pair of paralleled MOSFETs each that tie them to ground in each conduction cycle (Vgs of the corresponding MOSFET high).

These MOSFETs should be fast, able to withstand high currents (in excess of 30A each if possible) and have the lowest possible Rds(on). The proposed On-Semiconductor�s MTP75N06 can withstand 75Amp and has a Rds(on) below 10 milliohm. This is important, because the lower this resistance is, the less power they are going to dissipate when switching with a square waveform. Another alternatives are MTP60N06, or the more popular BUZ11 and IRF540.

Although the schematics show a previous bipolar push-pull stage, you can also connect the gate resistor directly to the output of the controlling IC, leaving out the transistors, as the SG3525 is capable to drive up to 500 mA (theoretically), more than enough to switch the MOSFETs fast.

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Thursday, July 30, 2009

Analog Audio Delay Line(3d Sound)

It is fun to make a variable space in your small room, but it’s hard to make the actuator to move your wall or room partition. Using analog audio line delay, you can adjust your room virtually. Just turn a knob in your audio set and you can adjust your room size. The circuit described here will make your dream come true, giving a feel that your speaker is located 15 meters behind you, even though your room is actually 3 meters wide. Here is the circuit’s schematic diagram.The core of this circuit is SAD512D integrated circuit, an analog audio delay. The chip uses 512 capacitors memory to hold 512 sampled analog signal. The delay can be adjusted from about 5,1 ms to 51 ms by R12 pot. Feed the input of this analog delay circuit with a mixed right and left audio signals from your stereo system. The output of this circuit then fed to a small power amplifier and place the output speaker behind you. Now you can perceive like your speaker is 15 m away behind (with maximum delay setting). If you build two unit the cascading the circuit will result in 30 meter expansion of your virtual room.

The circuit consist three main block. The first block (U1A, U1B) is a fourth order low pass filter (-24dB roll-off per octave) with 2.5kHz cut off frequency. The second block is the adjustable analog delay integrated circuit (IC SAD512D). The delay is controlled by the oscillator around U2 which is adjustable from 5KHz to 50Khz. The last block is similar to the first block, a low-pass filter with 2.5KHz cut off frequency.

A variable resistor R9 is provided to adjust the input offset, avoiding signal clipping and maximizing the audio range. For easy adjustment, feed the input with high level audio signal until the output is distorted, then adjust R9 until the distortion is minimum, or if an oscilloscope is available, adjust the R9 until the clipping is equal for both positive and negative cycle. Finally, adjust R28 to give minimum sampling clock noise.


Reference Part
C1,C4,C10,C12 10n
C11,C2 1n5
C3 4.7uF/50V
C13,C5 1n9
C6 390p
C8,C7 1u/25V
C9 1uF/25V
C14,C15,C16 100n
J1 input connector
J2 out connector
R1,R2,R3,R5,R6,R7,R11,R20,R21,R22,R24,R25,R26 10k
R4,R23,R27 4k7
R8,R15 15k
R9 2k2 POT
R10 2k7
R12 220k POT
R13 10R
R18,R14 1k
R17,R16 330R
R19 100k
R28 250R trimpot
U1 TL084
U2 4011

Thursday, July 2, 2009

Temperature Monitor

Using a thermistor in the position shown makes a heat activated sensor. A change in temperature will alter the output of the op amp and energize the relay and light the LED. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.

Sound Operated Switch

This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electric (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms. The two BC109C transistors form an audio preamp, the gain of which is controlled by the 10k preset. The output is further amplified by a BC182B transistor. To prevent instability the preamp is decoupled with a 100u capacitor and 1k resistor. The audio voltage at the collector of the BC182B is rectified by the two 1N4148 diodes and 4.7u capacitor. This dc voltage will directly drive the BC212B transistor and operate the relay and LED. It should be noted that this circuit does not "latch". The relay and LED operate momentarily in response to audio peaks.

5 to 30 Minute Timer

A switched timer for intervals of 5 to 30 minutes incremented in 5 minute steps. 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 is 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)

6 Input Mixer

The mixer circuit below has 3 line inputs and 3 mic inputs. The mic inputs are suitable for low impedance 200-1000R dynamic microphones. An ECM or condenser mic can also be used, but must have bias applied via a series resistor. As with any mixer circuit, a slight loss is always introduced. The final summing amplifier has a gain of 2 or 6dB to overcome this. The Input line level should be around 200mV RMS. The mic inputs are amplified about 100 times or by 40dB, the total gain with the mixer is 46dB. The mic input is designed for microphones with outputs of about 2mV RMS at 1 meter. Most microphones meet this standard. The choice of op-amp is not critical in this circuit. Bipolar, FET input or MOS type op-amps can therefore be used; i.e 741, LF351, TL061, TL071, CA3140 etc.

Tuesday, June 30, 2009

VHF Video Transmitter 60-200 MHz

Here's a simple video transmitter for VHF TV channel will accept baseband video input, hence it can be driven by most CCD cameras and VCR video outputs. It ouputs roughly 80mW and when used with a 40cm telescopic antenna over 100 metres range is possible.
The transistor of the video transmitter can be a BC108, BC546, BC337 or a 2N2222. L1 is wound on a 10 mm air former. Use 6 turns 24 SWG for frequency 60-80 MHz, 4 turns for 150-180 MHz, and 2 turns for 180-200 MHz

You can use this with a monochrome or color video signal. To transmit sound just build the wide band FM transmitter and tune it to the audio channel.

Thursday, May 28, 2009

1W Mono Amplifier with IC TDA7052

This circuit is a 1 watt mono amplifier using the TDA 7052 from Philips. It is designed to be used as a building block in other projects where a battery powered audio amplifier is required to drive a small speaker. It will operate best from 6 – 12 V DC and requires no heatsink for normal use.

Circuit Description :
There are only 5 external components. C1 is the input coupling capacitor, which blocks any DC that might be present on the input. C2 and C3 provide power supply decoupling, and R2 provides adjustable input level. This can be used as a volume control.

Components :
C1 : 2.2uF electrolytic capacitor
C2 : 100nF ceramic/mono
C3 : 100uF electrolytic
R1 : 1K ohm resistor
R2 : 10K ohm log potentiometer
Spindle for potentiometer
TDA7052 Integrated Circuit
8 pin IC socket
Kit 27 Printed Circuit Board


Friday, April 24, 2009

Electronic mosquito repeller

Here is the circuit diagram of an ultrasonic mosquito repeller.The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.

  • Assemble the circuit on a general purpose PCB.
  • The circuit can be powered from 12V DC.
  • The buzzer can be any general purpose piezo buzzer.
  • The IC1 must be mounted on a holder.

Tuesday, March 31, 2009

5V power supply with overvoltage protection.


For circuits using TTL ICs the supply voltage is a great concern and a slight increase in supply from the rated 5V may damage the IC. Using fuses alone does not solve the problem because a fuse may take several milliseconds to blow off and that’s enough time for the IC to get damaged.

In this circuit a crowbar scheme is used in which a triac short circuits the power supply and burns the fuse. The burning time of the fuse is not a concern because the power supply is already shorted by the triac and the output voltage will be zero. When the output voltage exceeds 5.6 volts the zener diode D2 conducts and switches ON the triac T1.Now T1 acts as a closed switch, shorting the circuit. The output voltage drops to zero and fuse gets burned off. Since the switching of triac takes place within few micro seconds there will be no damage to the TTL ICs or any other such voltage sensitive components in the load circuit.

Circuit diagram with Parts list.


  • Assemble the circuit on a general purpose PCB.
  • If 1A Bridge is not available, make one using four 1N4007 diodes.
  • The trip voltage can be varied by varying the values of D2 and R2.
  • All capacitors must be rated at least 25V.
  • The transformer T1 can be a 230 V AC primary, 12v secondary, 2A step-down transformer.

Lead acid battery charger circuit


Here is a lead acid battery charger circuit using IC LM317.The IC here provides the correct charging voltage for the battery.A battery must be charged with 1/10 its Ah value.This charging circuit is designed based on this fact.The charging current for the battery is controlled by Q1 ,R1,R4 and R5. Potentiometer R5 can be used to set the charging current.As the battery gets charged the the current through R1 increases .This changes the conduction of Q1.Since collector of Q1 is connected to adjust pin of IC LM317 the voltage at the output of of LM 317 increases.When battery is fully charged charger circuit reduces the charging current and this mode is called trickle charging mode.

Circuit Diagram with Parts List.

Notes .

  • Connect a battery to the circuit in series with a ammeter.Now adjust R5 to get the required charging current. Charging current = (1/10)*Ah value of battery.
  • Input to the IC must be minimum 15V to get 12 V for charging the battery .Take a look at the data sheet of LM 317 for better understanding.
  • Fix LM317 with a heat sink.

LED torch using MAX660


This is a simple LED torch circuit based on IC MAX660 from MAXIM semiconductors. The MAX 660 is a CMOS type monolithic type voltage converter IC. The IC can easily drive three extra bright white LEDs.The LED's are connected in parallel to the output pin 8 of the IC. The circuit has good battery life. The switch S1 can be a push to ON switch.

Circuit diagram with Parts list.


  • Assemble the circuit on a general purpose PCB.
  • The IC must be mounted on a holder.
  • The circuit can be powered from two torch cells connected in series.
  • The capacitors C1 and C2 must be Tantalum type.
  • The diodes D1 to D3 must be of 1N4148.

Bidirectional H-Bridge DC-Motor Motion Controller

In applications requiring absolute accuracy in the speed control of dc servo motors, there’s no substitute for the traditional tachometer-based feedback loop. But for somewhat less demanding situations, adequate accuracy often can be achieved without the complication and expense of a tach. This can be done by taking advantage of the built-in electromechanical constants of the motor itself.

For example, the fact that every permanent-magnet dc motor exhibits a stable relationship between rpm and armature back-EMF implies that a reasonable job of constant-speed operation can be accomplished merely by driving the motor from a well-regulated voltage supply.

Even better speed regulation, sometimes rivaling tachometer feedback, can be achieved by adding a armature current, to the motor drive voltage. If this term is trimmed to accurately cancel armature resistance equal to motor-rated-voltage/lockedrotor-stall-current, the motor rpm will remain nearly constant over a wide range of loads. Although armature resistance cancellation via positive current feedback is hardly a new idea, the circuit described here gives it a novel twist by combining this trick with a motion-reversing H-bridge circuit topology.

The circuit works as follows: A speed-set point control voltage is produced by multi-turn precision potentiometer R3, acting in concert with VR1’s 1.25-V reference voltage. The resulting 0-0.75 V is scaled by a factor of 16 by op-amp A1 to produce a 0-12 V no-load M1 target armature voltage.

Speed-stabilizing, current-proportional positive feedback comes from current-sensing R1, is attenuated by R2, and summed by A1 with the speed-set point voltage. Optimum adjustment of R2 can produce almost perfect cancellation of M1’s parasitic resistance, resulting in a very “stiff” torque-versus-rpm characteristic. Motor speed-control performance will therefore be nearly independent of mechanical loading up to the voltage limit of the drive circuit.

The regulation of the motor drive in response to the composite control signal output by A1 (speed-setpoint plus current-feedback) is the job of either differential amplifier A3 or A4. It depends on the desired motor drive polarity and consequent direction of rotation as indicated by the state of direction-control flip-flop A2. For positive (clockwise) rotation, A2’s output is low and A4 is in control. This occurs because A3’s low output turns on Q3, which pulls Q5’s gate high, grounding the negative M1 connection via R1. Meanwhile, the same Q3 voltage applied to A3’s positive input causes A3 to rail Q6’s gate positive, holding the p-channel FET off. This prevents the possibility of “shoot-through” conduction between Q5 and Q6. A4 then can accurately sense, via the R5-R6-R7-R8-R9 differential network, the voltage applied to M1 and regulate it via power MOSFET Q4.

Thus, M1 is forced to run at the speed set by R2 until optical retro sensor E2/Q2 senses the arrival of the mechanical load at its clockwise limit. Light reflected into Q2 results in conduction, which overcomes the detection threshold set by feedback pot R4.This pulls A2’s positive input high and toggles the state of the direction control flip-flop. The resulting positive excursion of A2’s output turns on Q7 and forward-biases D1, disabling the A4/Q4 control loop.

Meanwhile, Q3 turns off, releasing Q5 and A3. This results in reversal the motor drive polarity and the initiation of speed-regulated counterclockwise motion. The motor will now continue to run counterclockwise until retro-sensor E1/Q1 senses the arrival of the mechanical load at the counterclockwise limit. Bridge polarity will consequently toggle again, causing the motor to reverse again and so forth ad infinitum (or at least until power is removed!).


Alarm Sounds When Refrigerator Door Remains Open Too Long

Not properly closing a refrigerator door will no doubt invite huge electricity bills. This gadget is an alert device that beeps if you leave the refrigerator door open for more than 20 seconds. When the door opens, the lamp illuminates and the IC (a 4060B counter/oscillator) starts counting down. After a preset delay of 20 seconds, the piezoelectric buzzer beeps intermittently for 20 seconds and then stops for the same amount of time. This cycle repeats until the refrigerator door closes.

Producing a small dc voltage from the ac mains to run an electronic control requires a step-down transformer or a capacitor dropper circuit. This design uses an innovative and easy solution. When someone opens the refrigerator door, the lamp receives power via the diodes in the bridge rectifier, D1-D4, and through the Zener diode, ZD1. The voltage drop across the Zener diode is smoothed by the filter capacitor, C1. This voltage is sufficient to run the rest of the circuitry.

To install the circuit, cut the existing wire as shown in the figure and connect the circuit (shadowed) at points A and B. The circuit can be conveniently placed in the compressor compartment where there’s ample space. With the door closed, the lamp is off and no power goes to the timer circuit.

The circuit runs directly from the mains. So care must be exercised and a little knowledge of refrigeration wiring will ease the job.


Automatic Battery Charger

The following automatic battery-charger design is created with a circuit that could qualify as the simplest window comparator ever built around a single transistor. It starts charging when the battery voltage drops beyond a preset value, and it stops when an upper preset voltage is attained.

With the help of a precise variable voltage supply, the upper and lower voltage levels were set. The normally connected (NC) lead of the relay isn't joined to the 15-V dc supply, which blocks this voltage from passing to the battery leads. This will accurately set the upper and lower levels. But the charging supply of 15 V dc was connected to the circuit.

First, the variable supply is fixed at 13.3 V dc—the voltage of a fully charged battery—and linked to the battery point of the circuit. The slider of VR1 is turned to the extreme end on the side that's attached to the positive terminal of the battery. VR2's slider should be turned toward the end that's connected to VR1. The transistor turns on, shunting VR1. Then the slider of VR1 is turned toward the other extreme, which is the end connected to VR2.

The test supply voltage is now set to 11.8 V dc, which is the voltage of a drained battery. VR2 is then adjusted so that it just turns off the transistor again. The test voltage is raised to 13.3 V dc again, and VR1 is adjusted so that the transistor turns on. With the upper and lower levels set, the NC point is connected to the circuit (15-V dc charging voltage). Now the battery charger is set and ready to go.


Thursday, February 19, 2009

Off Line-UPS Offers (100 -5000 Watts)

With increased dependency on electric power for various domestic, commercial purposes and the seemingly declining capacity of power utilities in many countries, the need for additional backup power sources is on the rise. Various modules are already available to address these different needs. However, most modules are too expensive, too bulky, or too rigid in their power capacity, capability, and flexibility.

The circuit described here is an off-line uninterruptible power supply. It has an expandable power stage design that can be easily modified for use with power ranges from as low as 100 W to as high as 5000 W with forced cooling.

The design is based on the LM3524D, a popular industrial-grade, pulse-width-modulation (PWM) controller. This device is fully self-contained and has all of the necessary logic built-in to ensure a compact and cost-effective product. The controller offers:

  • Complementary power drive output stage with 180° out-of-phase switching.
  • Stable oscillator with shutdown capability and noise suppression.
  • Current-sense and voltage-sense circuitry.
  • Dead time between the two complementary switching stages to prevent cross conduction and subsequent core saturation.
  • Low voltage cutoff.

The on-chip oscillator frequency is controlled by the RC pair R2 and C1, and the required frequency is calculated as fOSC = 1/(R2 × C1). Depending on the country, fOSC = 50 Hz or 60 Hz. The low-power drive stage, containing transistors Q1 and Q2, is driven by the outputs at pin 12 and 13 of the controller; out-of-phase switching is handled by its own internal logic.

The final switching stage is comprised of transistors Q5 and Q6 wired across the main step-up transformer. These transistors are switched on in tandem to drive current through the two halves of the primary winding of transformer T2, independently.

Power diodes D2 and D3 are included to protect the power-stage transistors against reverse currents. Switching pulses from the drive transistors are transferred to the power stage via transistors Q3 and Q4, which serve as current amplifiers.

Resistor R10 is used to feed back the current to the current-sense monitor via pin 5. The current-sense logic monitors this feedback and triggers shutoff in event of either overload or power-stage failure.

The value of R10 is calculated as IMAX = 200 mV/R10, where IMAX is proportional to the power rating of the load, and 200 mV is the minimum required potential drop across R10 to trigger a shutdown by the current-sense logic.

The transformer T1 is connected to the standard mains supply and the rectified dc is fed to the shutdown pin (pin 10). This pin serves to turn off the LM3524 during the presence of the standard mains supply. If the mains supply fails, the bias on the shutdown pin is removed, thus starting the oscillator and hence the drive stages.

The power stages and transformer T2 must be appropriately rated for the required output power. The power transistors (Q3, Q4, Q5, and Q6) can be paralleled with similar devices for increased power-handling capacity. In addition, forced cooling can be used to achieve an even higher output power rating. However, for low- or medium-output power ratings, these transistors must be mounted on a large heat sink. Transformer T2 is a standard 12 V: 0:12 V to 230-240 V step-up winding, while T1 is a small 230-V to 12-V step-down transformer rated for 50 mA on the secondary windings. Additional filter and shaping circuitry can be added on the secondary winding of T2 to obtain a near sine-wave output. The RC network comprising R3 and C2 is included for compensation.

Thursday, February 12, 2009

13.8V 40A Switching Power Supply By LM3524,LM324

This article was originally published (in a slightly modified form) in the QST magazine, December 1998 and January 1999, and in the Radio Amateur's Handbook, 1999. Visit the American Radio Relay League for information on these publications, and a world of ham radio related things!
Design decisions
There are several different topologies for switchers in common use, and the first decision a designer must take is which of them to consider. Among the factors affecting the decision are the power level, the number of outputs needed, the range of input voltage to be accepted, the desired tradeoff between complexity, quality and cost, and many more. For this power supply I decided to use the half bridge forward converter design. This topology connects the power transformer to a bridge formed by two power transistors and two capacitors. It is reasonably simple, puts relatively low stress on the power transistors, and makes efficient use of the transformer's magnetic capabilities.The second basic decision is which switching frequency to use. The present trend is to use ever higher frequencies. But by doing so it becomes more difficult to filter out the RF noise inevitably generated by the switching. So I decided to stay at a low switching frequency of only 25 kHz for the full cycle, which due to the frequency doubling effect of the rectifiers results in 50 kHz on the output filter.

For the main switching elements, bipolar transistors or MOSFETs can be used. Bipolars have lower conduction losses, while MOSFETs switch faster. As in this design I wanted to keep the RF noise at an absolute minimum, very fast switching was not desired, so I used bipolar transistors. But these tend to become too slow if the driving is heavier than necessary. So, if the transistors have to switch at varying current levels, the drive to them must also be varied. This is called proportional driving, and is used in this project.

The half bridge converter is best controlled by pulse width modulation. There are several ICs available for this exact purpose. I chose the 3524, which is very simple to use and easy to find. Any 3524 will do the job. It can be an LM3524, SG3524, etc.

This basically ends the big decisions. From now on, designing the circuit is a matter of calculating proper values for everything.

More Source :

Wednesday, February 4, 2009

High Current Regulated Supply By LM317

The high current regulator below uses an additional winding or a separate transformer to supply power for the LM317 regulator so that the pass transistors can operate closer to saturation and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055 pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm) equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used that is a few volts higher than the output voltage.

The LM317 will provide over 1 amp of current to drive the bases of the pass transistors and assuming a gain of 10 the combination should deliver 15 amps or more. The LM317 always operates with a voltage difference of 1.2 between the output terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base. The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904 base when the output is 5 volts. For 13 volt operation, the 1K resistor could be adjusted to around 3.6K. The regulator has no output short circuit protection so the output probably should be fused.

Saturday, January 31, 2009

preamplifier 88-108Mhz

This VHF amplifier working on Band 2 Radio Spectrum tuning approximately 88 - 108 Mhz
The Preamplifier circuit uses two 2N3819 FET's in cascade configuration. The lower FET operates in common source mode, while the upper FET, operates in common gate, realizing full high frequency gain. The bottom FET is tunable allowing a peak for a particular station.

Coil details follow:
L1 4 turns of 18swg air spaced with a 1cm diameter, the tap is one turn up from earth end...
L2 4 turns of 18swg air spaced with 1 cm diameter. The coupling coil is 1 turn interwound from the supply end. Enamel coated wire must be used. More info ...