20M, 4W QRP TRANSMITTER

Using this small yet powerful 4W transmitter Hams (licenced amateur radio operators) can transmit Morse code signals to long distances in 20-metre band. Morse code communication with European and neighbouring countries is possible. The transmitter (refer Fig. 1) comprises an oscillator, driver, and power amplifier. The oscillator is crystal-controlled. For this, an inexpensive and readily available crystal with fundamental frequency of 14.314 MHz is used with the horizontal dipole antenna oscillator. The oscillator delivers power of about 200 milliwatts. The next stage is a class-C driver that delivers power of nearly 1 watt. The final stage is a class-C power amplifier wired around transistor BD139, which delivers power of 4 to 5 watts.

After assembling the circuit, connect a 12V, 5W bulb across the antenna’s terminals. Apply regulated 12-15V DC to the circuit. Adjust gang condenser’s knob until the bulb glows, which indicates that the

Fig. 2: Connection of the transmitter to a horizontal dipole antenna

transmitter is okay. (Caution. Don’t switch on the transmitter without an output load.) After checking with bulb as load, the transmitter can be connected to a horizontal dipole antenna via 75-ohm coaxial cable as shown in Fig. 2. Each arm of the dipole antenna is about 5 metre long. The correct length L (in metres) of single pole of the antenna can be calculated using the following relationship: L = 71.6/ frequency (MHz) Winding details of coils are as follows: L1: 9 turns of 24 SWG insulated copper wire over 8mm dia. oscillator coil former with ferrite bead L2: 3 turns of 24 SWG insulated copper wire over coil L1 L3: 9 turns of 24 SWG insulated copper wire over 8mm dia. former with movable ferrite bead L4: 14 turns of 20 SWG insulated copper wire over 1.5cm dia. PVC former. (The coil is to be tapped from 9th to 10th turn from the top end.) RFC: 12 turns of 36 SWG insulated copper wire using TV balun core Warning. Use of this transmitter without Ham licence is illegal, hence only Ham licence holders should assemble this project.

FM Booster

Here is a low-cost circuit of an FM booster that can be used to listen to programmers from distant FM stations clearly. The circuit comprises a common-emitter tuned RF preamplifier wired around VHF/UHF transistor 2SC2570. (Only C2570 is annotated on the transistor body.)

Assemble the circuit on a good-quality PCB (preferably, glass-epoxy). Adjust input/ output trimmers (VC1/VC2) for maximum gain.

Input coil L1 consists of four turns of 20SWG enameled copper wire (slightly space wound) over 5mm diameter former.It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but hasonly three turns. Pin configuration of transistor2SC2570 is shown in the figure

Telephone Receiver

This simple telephone receiver without a dialling section can be connected in parallel to a telephone line. It can be easily assembled on a small vero board or a PCB. A geometry box made in the shape of a telephone receiver will be an excellent cabinet for it. No external power supply is needed, which makes the circuit handy. The ringer section comprises R1, C1, and a buzzer. If your telephone has a loud ringer, this circuit can be avoided. A bridge rectifier consisting of diodes D1 through D4 protects the circuit from any polarity change in the telephone line. PNP transistor MPS-A92 (T1) is the main interface transistor. The output of T1 is regulated by zener diode ZD and capacitor C2 to get 6.8V for powering the amplifier section. This power is also used to bias the transmitter section.

The transmitter section comprises transistor BC548 (T2) together with a few discrete components and a condenser microphone. The transmit signal is fed to the base of interface transistor T1. The voice input for the amplifier comes directly from the positive end of the bridge
rectifier. The amplifier section is built around high-performance, low-wattage power amplifier IC LM386. This circuit is designed as a high-gain amplifier. A small 8-ohm speaker is good enough for the output. After all soldering is done, adjust presets VR1 and VR2 to their middle position and connect the circuit to the telephone line in parallel. Adjust VR1 and VR2 for optimum reception as well as transmission

Quality FM Transmitter

This FM transmitter for your stereo or any other amplifier provides a good signal strength up to a distance of 500 metres with a power output of about 200 mW. It works off a 9V battery. The audio-frequency modulation stage is built around transistor BF494 (T1), which is wired as a VHF oscillator and modulates the audio signal present at the base. Using preset VR1, you can adjust the audio signal level. The VHF frequency is decided by coil L1 and variable capacitor VC1. Reduce the value of VR2 to have a greater power output. The next stage is built around transistor BC548 (T2), which serves as a Class-A power amplifier. This stage is inductively coupled to the audio-frequency modulation stage. The antenna matching network consists of variable capacitor VC2 and capacitor C9. Adjust VC2 for the maximum transmission of power or signal strength at the receiver.

For frequency stability, use a regulated DC power supply and house the transmitter inside a metallic cabinet. For higher antenna gain, use a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from

the end point for coil L1. L1: 5 turns of 24 SWG wire closely wound over a 5mm dia. air core L2: 2 turns of 24 SWG wire closely wound over the 5mm dia. air core L3: 7 turns of 24 SWG wire closely wound over a 4mm dia. air core L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core

MULTIBAND CW TRANSMITTER

A radio frequency oscillator is at the heart of all radio transmitters and receivers. It generates high frequency oscillations, which are known as carrier waves. Here’s a continuous wave (CW) transmitter for transmitting Morse code signals in the shortwave band (see Fig. 1). It is basically a variable frequency oscillator (VFO) whose frequency can be varied from 5.2 MHz to 15 MHz. The signal can be received in the shortwave band by any radio receiver. The circuit works off a 9V battery. Connect the Morse key (S1) across capacitor C5 as shown in the figure. Attach a telescopic antenna (capable of transmitting over a short distance) at the output t e r m i n a l .

The coil and gang capacitor C2 form the tank circuit. The coil (L) has a total of 60 t u r n s . Winding details are given in Fig. 2. Tappings on the coil allow selection of the r e q u i r e d band. The frequency can be varied using C2 (main tuning). On reducing turns of the coil (using selector switch S2), the oscillator’s frequency increases because frequency is inversely proportional to inductance. Capacitor C1 couples the signal from the tank circuit to the base of transistor

T1 (2N2222). Transistor T1 provides the required positive feedback for oscillation and transistor T2 (BC547) functions as the emitter follower. The output is taken from the emitter of T2. For stable oscillations, use a polystyrene capacitor as C1. All other capacitors may be ceramic disk type. Enclose the circuit in a metal box for better shielding.

MOSFET-BASED PREAMPLIFIER FOR FM RADIO DXing

FM transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. FM DXing refers to hearing distant stations (1500 km or more) on the FM band (88-108 MHz). The term ‘DX’ is borrowed from amateur radio operators. It means ‘distance unknown’; ‘D’ stands for ‘distance’ and ‘X’ stands for ‘unknown.’ For an FM receiver lacking gain, or having a poor signal-to-noise ratio, using an external preamplifier improves the signal level.

The dual-gate MOSFET preamplifier circuit shown in Fig. 1 gives an excellent gain of about 18 dB. It costs less and is simple to design. Field-effect transistors (FETs) are superior to bipolar transistors in many applications as these have a much higher gain—approaching that of a vacuum tube. These are classified into junction FETs and MOSFETs. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate.

In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain. In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate. The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The MOSFET is more flexible because it can be controlled by a positive or negative voltage at gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer. Thus the MOSFET has an extremely high input impedance. Dual-gate MOSFETs (DG MOSFETs) are very popular among radio amateurs. These are being used in IF amplifiers, mixers, and preamplifiers in HF-VHF transceivers.
The isolation between the gates (G1 and G2) is relatively high in mixer applications. This reduces oscillator pulling and radiation. The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the (large) incoming signal. A DG MOSFET can also be used for automatic gain control in RF amplifiers. DG MOSFET BF966S is an n-channel depletion-type MOSFET that is used for general-purpose FM and VHF applications.

In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor. Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies. Capacitor C4 (100 kpF) at the gain control electrode (gate 2) decouples any variation in G2 voltage at radio frequencies to maintain constant gain. Set preset VR (47k) to adjust the gain or connect a fixed resistor for fixed gain. The output of the circuit is obtained via capacitor C5 and fed to the FM receiver amplifier.

For indoor use, connect a ¼- wavelength whip antenna, ½-wavelength 1.5m wire antenna, or any other indoor antenna set-up with this circuit. You may use a 9V battery without the transformer and diode 1N4007, or any 6V-12V power supply to power the circuit (refer Fig. 1). The RF output can be taken directly through capacitor C5. For an improved input and output impedance, change C1 from 1 kpF to 22 pF and C5 from 1 kpF to 100 kpF. For outdoor use at top mast, like a TV booster, connect the C5 output to the power supply unit (PSU) line. Use RG58U/ RG11 or RG174 cable for feeding the power supply to the receiver amplifier. The PSU

for the circuit is the same as that of a TV booster. For TV boosters, two types of mountings are employed: The fixed tuned booster is mounted on the mast of the antenna. The tunable booster consisting of the PSU is placed near the TV set for gain control of various TV channels. (For details, refer ‘High-Gain 4-Stage TV Booster’ on page 72 of Electronics Projects Vol. 8.) Mount the DG MOSFET BF966S at the solder side of the PCB to keep parasitic capacitance as small as possible. Use an epoxy PCB. After soldering, clean the PCB with isopropyl alcohol. Use a suitable
enclosure for the circuit. All component leads must be small. Avoid shambled wiring to prevent poor gain or self oscillations. Connecting a single-element cubical quad antenna to the circuit results in ‘Open Sesam’ for DXing.

You can use a folded dipole or any other antenna. However, an excellent performance is obtained with a cubical quad antenna (refer Fig. 2) and Sangean ATS- 803 world-band receiver. In an amplifier, FET is immune to strong signal overloading. It produces less cross-modulation than a conventional transistor having negative temperature coefficient, doesn’t succumb to thermal runaway at high frequencies, and decreases noise. In VHF and UHF, the MOSFET produces less noise and is comparable with JFETs. DG FETs reduce the feedback capacitance as well as the noise power coupled to the gate from the channel, giving stable unneutralised power gain for wide-band applications. This circuit can be used for other frequency bands by changing the input
and the output LC networks. The table here gives details of the network components for DXing of stations at various frequency bands.

Infrared Cordless Headphone

Using this low-cost project one can reproduce audio from TV without disturbing others. It does not use any wire connection between TV and headphones. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones. Without using any lens, a range of up to 6 metres is possible. Range can be extended by using lenses and reflectors with IR sensors comprising transmitters and receivers. IR transmitter uses two-stage transistor amplifier to drive two series-connected IR LEDs. An audio output transformer is used (in reverse) to couple audio output from TV to the IR transmitter. Transistors T1 and T2 amplify the audio signals received from TV through the audio transformer. Low impedance output windings (lowergauge or thicker wires) are used for connection to TV side while high-impedance

windings are connected to IR transmitter.
This IR transmitter can be powered from a 9-volt mains adapter or battery. Red LED1 in transmitter circuit functions as a zener diode (0.65V) as well as supply-on indicator. IR receiver uses 3-stage transistor amplifier. The first two transistors (T4 and T5) form audio signal amplifier while the third transistor T6 is used to drive a headphone. Adjust potmeter VR2 for max. clarity. Direct photo-transistor towards IR LEDs of transmitter for max. range. A 9-volt battery can be used with receiver for portable operation.

Dual-Channel Digital Volume Control

This circuit could be used for replacing your manual volume controlling a stereo amplifier. In this

circuit, push-to-on switch S1 controls the forward (volume increase) operation of both channels while a similar switch S2 controls reverse (volume decrease) operation of both channels. Here IC1 timer 555 is configured as an astable flip-flop to provide low-frequency pulses to up/down clock input pins of pre-setable up/down counter 74LS193 (IC2) via push-to-on switches S1 and S2. To vary the pulse width of pulses from IC1, one may replace timing resistor R1 with a variable resistor. Operation of switch S1 (up) causes the binary output to increment while operation of S2 (down) causes the binary output to decrement.

The maximum count being 15 (all outputs logic 1) and minimum count being 0 (all outputs logic 0), it results in maximum and minimum volume respectively. The active high outputs A, B, C and D of the counter are used for controlling two quad bi-polar analogue switches in each of the two CD4066 ICs (IC3 and IC4). Each of the output bits, when high, short a part of the resistor network comprising series resistors R6 through R9 for one channel and R10 through R13 for the other channel, and thereby control the output of the audio signals being fed to the inputs of stereo amplifier. Push-to-on switch S3 is used for resetting the output of counter to 0000, and thereby turning the volume of both channels to the minimum level.

Infrared Toy Car Motor Controller

This add-on circuit enables remote switching on/off of battery-operated toy cars with the help of a TV/ video remote control handset operating at 30–40 kHz. When the circuit is energized from a 6V battery, the decade counter CD4017 (IC2), which is configured as a toggle flip-flop, is immediately reset by the power-on reset combination of capacitor C3 and resistor R6. LED1 connected to pin 3 (Q0) of IC2 via resistor R5 glows to indicate the standby condition. In standby condition, data output pin of the integrated infrared receiver/demodulator (SFH505A or TSOP1738) is at a high level (about 5 volts) and transistor T1 is ‘off’ (reverse biased).

The monostable wired around IC1 is inactive in this condition. When any key on the remote control handset is depressed, the output of the IR receiver momentarily transits through low state and transistor T1 conducts. As a result, the monostable is triggered and a short pulse is applied to the clock input (pin 14) of IC2, which takes Q1 output (pin 2) of IC2 high to switch on motor driver transistor T2 via base bias resistor R7 and the motor starts rotating continuously (car starts running). Resistor R8 limits the starting current. When any key on the handset is depressed again, the monostable is re triggered to reset decade counter IC2 and the motor is switched off. Standby LED1 glows again.

This circuit can be easily fabricated on a general-purpose printed board. After construction, enclose it inside the toy car and connect the supply wires to the battery of the toy car with right polarity. Rewire the DC motor connections and fix the IR receiver module in a suitable location, for example, behind the front glass, and connect its wires to the circuit board using a short 3-core ribbon cable/shielded wire. Note. Since the circuit uses modulated infrared beam for control function, ambient light reflections will not affect the circuit operation. However, fluorescent tube lights with electronic ballasts and CFL lamps may cause malfunctioning of the circuit.

LASER Link Communicator

There's something rather futuristic about talking 'over' a laser beam, which is what this inexpensive project allows. It will easily give a communication distance of several hundred metres, and with a parabolic light reflector, up to several kilometres. It transmits high quality audio and the link is virtually impossible for anyone else to tap into.

by PETER PHILLIPS
ELECTRONICS Australia, July 1997

In the February 1993 issue, we described a laser beam communicator project developed by Oatley Electronics. It was an extremely popular project, but this latest version not only makes the device better and simpler, but cheaper as well. Unlike the previous version, a visible laser diode (5mW 65Onm) is used as the transmitter. This makes alignment between the transmitter and receiver much simpler, as you can now see the beam. As well, the laser has a greater output power. The circuitry is also simpler, and uses basic components.
As before, there are two sections: the transmitter board and the receiver board, both powered by a separate 9V battery or a fixed voltage power supply, depending on your needs. The transmitter board has an electret microphone module at one end, and the laser diode at the other end. The electronics modulates the intensity of the laser beam according to the output of the microphone. The laser diode has an inbuilt collimating lens, and is simply a module that connects to the transmitter board. The previous design required brackets for the laser diode assembly.

The receiver uses a photodiode as the receiving element, and the onboard amplifier powers a small 4-36 ohm speaker. This board is therefore a high gain amplifier with a basic audio output stage.

But what about results - are they better? Sure. Because this design uses a higher power (and visible) laser beam, the range is improved, and alignment is easier and not all that critical, especially over a few hundred metres. The quality of sound transmit ted by the link is quite surprising.

As a simple test, we set up the prototype with the transmitter microphone near a radio. The received sound was clear and seemed to cover the full audio bandwidth. We haven't tried feeding an audio signal directly to the transmitter, but that will undoubtedly give even better results.

So clearly, this project is ideal for setting up a speech channel between two areas, say adjacent houses, or offices on opposite sides of the street. Or you could use it as a link between the work shop and the house. For duplex (two way) communication, you'll obviously need two laser 'channels'.

An important feature of transmission by laser beam is privacy. Because a laser beam is intentionally narrow, it's virtually impossible for someone to tap into the link without you knowing. If someone intercepts the beam, the link is broken, signalling the interception. Fibre-optic cables also have high security, as it's very difficult to splice into the cable without breaking the link. However it's theoretically possible; so for the highest security, you probably can't beat a line-of-sight laser beam.

You can also use an infrared laser, as in the previous project. While this gives even better security, as you can't see the laser beam without special IR sensitive equipment, it also makes alignment more difficult. (An IR laser diode is available for the project; see end of article for details.)

The Transmitter
fig.1 (above): The circuit for the transmitter. The output of the microphone is amplified by IC2a, which feeds the modulating transistor Q1, which varies the laser current according to the signal. The quiescent current of the laser diode is set by VR1.
The Receiver
fig2 (above): The circuit for the receiver, where light from the transmitter is detected and converted to a voltage by the photodiode. The signal is amplified by Q1 and IC1, which drives the speaker.

Where the transmission distance is no more than metre of so, a LED (or two for increased power) can be substituted for the laser diode. For instance, where the link is being used for educational purposes, such as demonstrating fibre-optic coupling, or the concept of communication over a light beam. Obviously the security of the transmission is much lower as LEDs transmit light in all directions. While this laser link can be adapted for use as a perimeter protector (as in the previous version), Oatley Electronics has developed a project especially for this purpose. Contact Oatley Electronics for further details if that is what you are really after.

Now to a description of how it all works. As you'll see, it's really very simple. We'll start with the transmitter...
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Transmitter

A laser diode needs a certain value of current, called the threshold current, before it emits laser light. A further increase in this current produces a greater light output. The relationship between output power and current in a laser diode is very linear, once the current is above the threshold, giving a low distortion when the beam is amplitude modulated. For example, the 65Onm 5mW laser diode used in this project has a typical threshold current of 3OmA and produces its full output when the current is raised by approximately 1OmA above the threshold to 4OmA. Further increasing the current will greatly reduce the life of the laser diode, and exceeding the absolute maximum of 8OmA will destroy it instantly. Laser diodes are very fragile and will not survive electrostatic discharges and momentary surges!

However, if used within specifications, the typical life of one of these lasers is around 20,000 hours. In the transmitter circuit (Fig.1) the laser diode is supplied via an adjustable constant-current source. Since the lasing threshold also varies with temperature, a 68ohm NTC thermistor is included to compensate for changes in ambient temperature. Note that the metal housing for the laser diode and the lens also acts as a heatsink. The laser diode should not be powered without the metal housing in place. The quiescent laser diode current is controlled by Q2, in turn driven by the buffer stage of 1C2b. The DC voltage as set by VR2 appears at the base of Q2, which determines the current through the transistor and therefore the laser diode. Increasing the voltage at VR1 reduces the laser current. The setting of VR1 determines the quiescent brightness of the laser beam, and therefore the overall sensitivity of the system.
The audio modulation voltage is applied to the cathode of the laser diode, which varies the laser current around its set point by around +/-3mA. The modu- lation voltage is from the emitter of Q 1, which is an emitter follower stage driven by the audio amplifier stage of 1C2a. Diodes D4 to D7 limit the modulating voltage to +/-2V, while C4 and C5 block the DC voltages at the emitter of Q 1 and the cathode of the laser diode. The audio signal is coupled to the laser diode via R10, which limits the maximum possible variation in the laser diode current to a few milliamps.

LED1 gives an indication of the modulating voltage. Diodes D2, D3 and resistor R8 limit the current through the LED and enhance the brightness changes so the modulation is obvious. The LED flickers in sympathy with the sound received by the microphone, giving an indication that a modulating volt- age is present.
The inverting amplifier of 1C2a includes a form of compression, in which the output level is relatively constant and independent of how soft or loud the audio level is at the microphone. This is achieved by FET Q3 and its associated circuitry.

The cascaded voltage doubler of C9, D8, D9 and C8 rectifies the audio signal at the emitter of Ql, and the resulting negative DC voltage is fed to the gate of Q3. An increase in the audio signal will increase the negative bias to Q3, increasing its drain-source resistance. Because the gain of 1C2a is determined by R7 and the series resistance of R5 and Q3, increasing the effective resistance of Q3 will lower the gain.

Since the compression circuit takes time to respond, the clamping network of D4-D7 is still needed to protect against sudden voltage increases. This system is rather similar to the compression used in portable tape recorders.
The electret microphone is powered through R1 and is coupled to the non inverting input of 1C2a via C6. This input is held at a fixed DC voltage to give a DC output to bias Ql.

The supply voltage to the transmitter circuit is regulated by ICI, a 5V three terminal regulator.

Receiver
The transmitted signal is picked up by the photo detector diode in the receiver (shown in Fig.2). The output voltage of this diode is amplified by the common emitter amplifier around Ql. This amplifier has a gain of 20 or so, and connects via VRI to ICI, an LM386 basic power amplifier IC with a gain internally set to 20.

This IC can drive a speaker with a resistance as low as four ohms, and 35OmW when the circuit is powered from a 9V supply. Increasing the sup- ply voltage will increase the output power marginally.

The voltage to the transistor amplifier stage is regulated by ZD I to 5.6V, and decoupled from the main supply by R2 and C2. Resistor R3 supplies forward current for the photodiode. (Incidentally, the photodiode used for this project has a special clear package, so it responds to visible light, and not just infrared.)

Construction

As the photos show, both the transmitter and the receiver are built on silk- screened PCBS. As usual fit the resistors, pots and capacitors first, taking care with the polarity of the electrolytics. IC sockets are not essential, although servicing is obviously made easier if they are used. In which case, fit these next, followed by the transistors, diodes and the LED.

Take care to use the right diodes for D8 and D9. These are larger than the 1N4148 types, and have two black bands (the cathode end) around a glass package. Note that the regulator IC has the tab facing outwards.

The photodiode is mounted directly on the receiver PCB. When first mounted, the active side of the diode (black square inside the package) will face towards the centre of the board. You then bend the diode over by almost 180' so the active surface now faces outwards.

The polarised microphone element solders directly to the transmitter PCB. The negative lead is marked with a minus sign and is the lead that connects to the metal case.

The laser diode is also polarised, and has three leads. Of these, only two are used, shown on the circuit as pins 2 (cathode) and 3 (anode). Take care when soldering the laser in place, as too much heat can destroy it. The diode can be mounted on the board, or connected with leads to it.

Finally, connect the speaker and 9V battery clips, then check over the boards for any soldering errors or incorrectly installed components.

Testing

First of all, it's most important that you don't look directly into the laser beam. If you do, it could cause perma- nent eye damage. Also, you are respon- sible for the safety of others near the laser, which means you must stop others from also looking into the beam, and take all necessary safety steps. This is covered by legislation.

Both the receiver and the transmitter can be powered by separate 9V batteries or suitable DC supplies. Before apply- ing power to the transmitter PCB, set VRI to its halfway position, to make sure the laser current is not excessive. To be totally sure, you could set VRI fully anticlockwise, as this setting will reduce the laser current to zero.

Then apply power to the board. If the laser doesn't produce light, slowly adjust VRI clockwise. The laser diode should emit a beam with an intensity adjustable with VRI. At this stage, keep the beam intensity low, but high enough to clearly see. If you are not getting an output, check the circuit around IC2b.

You should also find that LED 1 flickers if you run your finger over the microphone. If so, it indicates that the amplifier section is working and that there's a modulation voltage to the laser diode. You won't see the laser beam intensity change with the modu- lating signal.

To check that the system is working, place the two PCBs on the workbench, spaced a metre or go apart. You might need to put a sheet of paper about 2Omm in front of the photodiode to reduce the intensity of light from the laser beam. Set the volume control of the speaker to about halfway. If the volume control setting is too high you'll get acoustic feedback.

Move the laser diode assembly so the beam points at the receiver's photodi- ode. It's useful to adjust the beam so it's out of focus at the photodiode, to make alignment even easier. You should now be able to hear the speaker reproducing any audio signal picked up by the microphone. When the receiver and transmitter are in close range, the strength of the beam can cause the receiver to respond even if the laser beam is not falling on the photodiode.

Setting up a link

Once you've tested the link, you'll probably be keen to put it to use. For a short link of say 100 metres, all you need do is position the receiver so the laser beam falls on the photodiode. Once the link is established, adjust VRI higher the laser current, the shorter will be its life.

If you have an ammeter, connect it to measure the current taken by the trans- mitter board. Most of the current is taken by the laser, so adjust VRI to give a total current consumption of no more than 45mA.

Also, focus the laser so all of the beam is striking the photodiode. At close range, there's probably no need to focus the beam. In fact, because of the high output power (5mW) of the laser diode, excellent results will be obtained over reasonably short distances (20 metres or so) with rough focusing and quiescent current adjust- ments. But the longer the dis- tance between the transmitter and the receiver, the more critical the adjustments. For example, for distances over 20 metres, you might have to put a piece of tube over the front of the photodi- ode to limit the ambient light falling on it. This diode is responsive to visible light, so a high ambient light could cause it to saturate. For very long distances, say a kilome- tre, you'll probably need a parabolic reflector for the laser beam, to focus it direct- ly onto the photodiode.

For short ranges (a metre or so), or for educational or testing purposes, you can use a conventional red LED. Adjust the quiescent current with VR1. The light output of a LED is not focused, and simply spreads everywhere, so a reflector might help the sensitivity. Warnings The laser diode in this project is a class 3B laser and you should attach a warning label to the trans- mitter. Labels will be sup- plied by Oatley Electronics. Remember that, as for any hazardous device, the owner of a laser is responsible for its proper use.
Transmitter:
Resistors
All 1/4W, 5% unless otherwise stated:
Rl 4.7k
R2,3 1 00k
R4 68k
R5 10k
R6 4.7M
R7 220k
R8-1 0 220 ohm
Rl 1,12 47k
R13 56 ohm 1/2W
R14 68 ohm NTC thermistor
VR1 1 00k trimpot
Capacitors
Cl,2 1OuF 16V electrolytic
C3 4.7uF 16V electrolytic
C4,5 10OuF 16V electrolytic
C6,7,9 68nF ceramic
C8,1 0 0.47uF monolithic ceramic
Semiconductors
LED1 5mm green LED
Laser 5mW/65Onm laser diode (or LED)
Ql,2 BC557 PNP
03 2N5484 N-ch JFET
Dl-7 1N4148 signal diode
D8,9 1 N60 germanium diode
lci 7805 5V regulator
1C2 LM358 op-amp
Miscellaneous
PCB 65mm x 36mm; electret microphone element;
8-pin IC socket; 9V battery and battery clip.

Receiver:
Resistors
All 1/4W, 5% unless otherwise stated:
Rl 680 ohm
R2 22 ohm
R3 4.7k
R4 39k
R5 3.gk
R6 10k
R7 1 k
R8 220 ohm
Rg 4.7 ohm
VR1 50k trimpot
Capacitors
Cl,2,5,7 10OuF 16V electrolytic
C3,4 1 uF 16V electrolytic
C6 15nF polyester
Semiconductors
Qi BC549 NPN
ici LM386 power amp
ZD1 5.6V 40OmW zener
Miscellaneous
PCB 36mm x 64m; photodiode with clear
casing; 9V battery and battery clip, 4-16 ohm
speaker; 8-pin IC socket.

InfraRed Proximity Detector

This proximity detector using an infrared detector (Fig. 1) can be used in various equipment like automatic door openers and burglar alarms. The circuit primarily consists of an infrared transmitter and an infrared receiver. The transmitter section consists of a 555 timer IC functioning in astable mode. It is wired as shown in the figure. The output from astable is fed to an infrared LED via resistor R4, which limits its operating current. This circuit provides a frequency output of 38 kHz at 50 per cent duty cycle, which is required for the infrared detector/receiver module. Siemens SFH5110-38 is a much better choice than SFH506-38. Siemens SFH5110-38 is turned on by a continuous frequency of 38 kHz with 50 per cent duty cycle, whereas SFH506 requires a burst frequency of 38k to sense. Hence, SFH5110-38 is used.

The receiver section comprises an infrared receiver module, a 555 monostable multi vibrator, and an LED indicator. Upon reception of infrared signals, 555 timer (mono) turns on and remains on as long as infrared signals are received. When the signals are interrupted, the mono goes off after a few seconds (period=1.1 R7xC6) depending upon the value of R7-C6 combination. Thus if R7=470 kilo-ohms and C6=4.7μF, the mono period will be around 2.5 seconds.

Both the transmitter and the receiver parts can be mounted on a single breadboard or PCB. The infrared receiver must be placed behind the infrared LED to avoid false indication due to infrared leakage. An object moving nearby actually reflects the infrared rays emitted by the infrared LED. The infrared receiver has sensitivity angle (lobe) of 0-60 degrees, hence when the reflected IR ray is sensed, the mono in the receiver part is triggered. The output from the mono may be used in any desired fashion. For example, it can be used to turn on a light when a person comes nearby by energizing a relay. The light would automatically turn off after some time as the person moves away and the mono pulse period is over.
The sensitivity of the detector depends on current-limiting resistor R4 in series with the infrared LED. Range is approximately 40 cm. For 20-ohm value of R4 the object at 25 cm can be sensed, while for 30-ohm value of R4 the sensing range reduces by 22.5 cm.

TV Transmitter

One of the most useful gadgets a video enthusiast can have is a low-power TV Transmitter. Such a device can transmit a signal from a VCR to any TV in a home or backyard. Imagine the convenience of being able to sit by the pool watching your favorite movie on a portable with a tape or laser disc playing indoors. You could even retransmit cable TV for your own private viewing. Videotapes can be dubbed from one VCR to another without a cable connecting the two machines together. When connected to a video camera, a TV transmitter can be used in surveillance for monitoring a particular location. The main problem a video enthusiast has in obtaining a TV transmitter is that a commercial units are expensive. However, we have some good news! You can build the TV Transmitter described here for less than $30 in one evening! The easiest way to do that is to order the kit that‚s available from the source given in the Parts List (a custom case for the kit is also available). Nevertheless, we present enough information here to build the TV Transmitter from scratch. The TV Transmitter combines linelevel audio and video signals, and transmits the resulting signal up to 300 feet. The circuit can be powered from a 9- volt battery. It is suggested that a 12-volt DC supply during be used during the alignment procedure. This would insure maximum transmission range and best possible picture. Aligning the TV Transmitter requires no special equipment whatsoever, and it is a very simple

procedure. The Transmitter's output can be tuned to be received on any TV channel from 2 to 6. The range of channels is wide enough so that the unit will not interfere with other TV viewers who are nearby. To comply with FCC rules, it is mandatory the nearby TV viewers are not disturbed by the transmission. If your activities interfere with the reception from a licensed station, regardless of the reason, you must shut down your unit.

Circuit Description
Above is the schematic diagram of the TV Transmitter circuit. Video signals input at jack J1 are first terminated by resistor R6 and coupled through capacitor C1 to clamping-diode D1. The clamping forces the sync pulses to a fixed DC level to reduce blooming effects. Potentiometer R3 is used to set the gain of the video signal; its effect is similar to that of the contrast control on a TV set. Bias-control R7 can be used to adjust the black level of the picture so that some level of signal is transmitted, even for a totally dark picture. That way, a TV receiver can maintain proper sync. As we'll get to later, potentiometers R3 and R7 are cross adjusted for the best all-around performance. RF-transformer T1 and its internal capacitor form the tank circuit of a Hartley oscillator that's tuned to 4.5 megahertz. Audio signals input at J2 are coupled to the base of Q3 via C2 and R4: the audio signal modulates the base signal of Q3 to form an audio subcarrier that‚s 4.5-megahertz higher than the video-carrier frequency. The FM modulated subcarrier is applied to the modulator section through C5 and R9. Resistor R9 adjusts the level of the subcarrier with respect to the video signal. Transistors Q1 and Q2 amplitude modulate the video and audio signals onto an RF-carrier signal. The operating frequency is set by coil L4, which is 3.5 turns of 24- gauge enameled wire on a form containing a standard ferrite slug.That coil is part of a Colpitts tank circuit also containing C7 and C9. The tank circuit forms Q4's feedback network, so Q4 oscillates at the set frequency The RF output from the oscillator section is amplified by Q5 and Q6, whose supply voltage comes from the modulator section. Antenna matching and low-pass filtering is performed by C12, C13, and L1. Resistor R12 is optional; it is added to help match the output signal to any kind of antenna. (More on that in a moment.)
Construction

Before we go on, while it is certainly possible to build the unit from scratch. However, unless you are an experienced builder and an accomplished parts scrounger, it is strongly recommended that you purchase the complete kit, or, at the very least, the component kit from the source mentioned in the Parts List. While most of the parts are readily available, some can be a real headache to obtain.
The 4.5-MHz RF transformer (T1) used in the kit is an OEM Toko part that is not available via traditional sources. While just about any 4.5-MHz RF transformer that is similar to the one described in the article (internal capacitor, tapped secondary) can be used, such units are hard to obtain from hobbyist-friendly sources. If you are determined to go that route, your best bet is to contact Toko directly (1250 Feehanville Dr.. Mt, Prospect, IL 60056; Tel. 708-297-0070) to obtain the location of your nearest full-line distributor. Also, coil L4 is a custom unit. It can, however, be home made using the parameters given earlier. The Transmitter should be built on a PC board for best performance. You can make a board from the foil pattern provided in Fig. 2, or use the one that’s included with the kit. Parts are installed on the board as shown in the parts-placement diagram [see Fig. 3). Pay careful attention to the orientation of the transistors, electrolytic capacitors. and the diode. If resistor R12 (not included in the kit) is used, it must be tack- soldered on the solder side of the board between the antenna output and ground. That resistor should be installed if you intend to use anything other than the built- in whip to provide proper matching between the antenna and the circuit.
The outline of the switch (S1) that is shown in Fig. 3 is the same as the one that comes with the kit, an SPST push-button switch that is normally open. You can use any kind of toggle switch as a replacement. A simple whip antenna mounts to the board with a single machine screw: The whip antenna is suitable for most applications. The battery holder can be soldered to the board with scraps of jumper wire or mounted with doublesided tape or screws.When the board is finished, it must be mounted in a case. The case available from Ramsey Electronics allows the board to be mounted in the bottom half, and by lifting the top off, still be aligned. That also protects the underside of the board against shorts during alignment. You should inspect the solder side of the board carefully before mounting it in the case.
Alignment
To align the TV Transmitter, you'll need a TV receiver and a source of video such as a VCR or camcorder. You'll also need a non-metallic tool to adjust coil L4 and transformer T1. A fresh 9-volt battery can be used for alignment, but if you find it is difficult to align, try doing it with a 12-volt supply. Note that during alignment and testing, we found that the unit operated much better from 12 volts. If you find the same to be true, it is a simple matter to add an external power jack to the unit and wire it to the appropriate points on the PC board.

Tune a TV receiver to an unused channel between 2 and 6. The TV must have an indoor antenna
connected directly to it; an outdoor antenna or cable won't work. Make sure both potentiometers are in midposition and apply power to the Transmitter. Adjust L4 with a nonmetallic tool until the TV screen goes blank. Then fine-adjust L4 for the "most-blank" picture. Connect the video and audio outputs from a VCR to jacks J1 and J2 (respectively) of the Transmitter, Then set a video tape to play. You should see a picture on the TV screen:

if you do, readjust L4 for the best picture; if you don't, check the board for any bad connections. Next, adjust R3 for the best picture brightness and R7 for the best overall picture. You might have to make another minor adjustment to L4 after R3 and R7 are set. Finally, adjust T1 with a nonmetallic tool for the best sounding audio. That‚s all there is to it. The whip antenna should be fine for most in-home use. If you need more range, an external antenna can be connected to J3 (remember to install R12). But always keep in mind that it is your responsibility to make sure that your operation does not interfere with your neighbor's TV viewing.

PARTS LIST FOR THE TV TRANSMITTER

SEMICONDUCTORS
D1—1N914 silicon diode
Q1-Q—2N3904 NPN transistor
RESISTORS
(All fixed resistors are 1/4-watt, 5% units .)
R1, R2, R11—1000-ohm
R3, R7—1000-ohm trimmer potentiometer, PCmount
R4, R9, R10—10,000-ohm
R5—47,000-ohm
R6—75-ohm
R8—4700-ohm
R12—75-ohm (optional, see text)
CAPACITORS
C1, C8—100-mF, 16-WVDC, electrolytic
C2—2.2--mF, 50-WVDC, electrolytic
C3-C6, C11, C14, C15—001-mF, ceramic-disc
C7, C9—2.2-pF, ceramic-disc
C10—100-pF, ceramic-disc
C12, C13—68-pF, ceramic-disc
ADDITIONAL PARTS AND MATERIALS
ANT1—Antenna, telescopic-whip
B1—9-volt battery
J1-J3—RCA jack, PC-mount
L1—0.15-mH miniature inductor
L2, L3—2.2-mH miniature inductor
L4—0.14- to 0.24-mH adjustable, slug-tuned coil
(see text)
S1—SPST, push-button switch, normally open
T1—4.5-MHz 1F-can-style RF transformer (see
text)
Printed-circuit materials or pre-fab PC board, battery holder and connector, pair of RCA patch cords, solder, hardware, etc.

Modular Burglar Alarm

Description

The Basic Alarm Circuit has an automatic Exit/Entry Zone - an Instant Alarm Zone that will accept both normally-closed and normally-open triggering devices - and an "Always On" 24-hour Personal Attack/Tamper Zone. By using the Expansion Modules - you can add as many extra alarm zones as you require.

The Alarm is armed and disarmed by SW1. Before you move the switch to the "set" position - all the green LEDs should be lighting. You then have up to about a minute to leave the building. As you do so - the Buzzer will sound. It should stop sounding when you close the door behind you. This indicates that the Exit/Entry loop has been successfully restored within the time allowed.

When you re-enter the building - you have up to about a minute to move SW1 to the "off" position. If SW1 is not switched off in time - the relay will energize - and the main bell will ring. It will continue ringing for up to about 40 minutes. But it can be turned off at any time by SW1.

The "Instant" zone has no Entry Delay. The moment one of its normally-open switches is closed - the main bell will ring. Similarly - the moment one of its normally-closed switches is opened - the main bell will ring. If you don't want to use normally-open switches - leave out R8, C8 and Q2 - and fit a link between Led 3 and C7.

The 24 Hour Personal Attack and Tamper protection is provided by the SCR/Thyristor. If one of the switches in the normally-closed loop is opened - current through R11 will trigger the SCR - and the main bell will ring. In this case the bell has no time limit. To reset the PA/Tamper zone - first restore the normally-closed loop - then press SW2 momentarily. This will interrupt the current and reset the SCR.

The Support Material for this alarm includes a step-by-step guide to the construction of the circuit-board and the expansion modules - parts lists - detailed circuit descriptions - and more.

Parts list
1x CMOS 4001
1x BC547
1x Entry buzzer
1x Bell
Misc: diodes, resistors, capacitors, switches, etc.

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