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The circuit depicted is the receiver device of a transmitter/receiver combination that will prove extremely handy when tracing the path of electrical wiring in a building or to locate a break in a wire. The corresponding transmitter may be found elsewhere in this website. The transmitter produces a distinctive tone which alternates between 2100 Hz and 2200 Hz. The matching receiver for the wire tracer is possibly even simpler than the transmitter, as is shown by the schematic. It consists of no more than a short wire antenna (a piece of wire, 10 cm long is adequate), a high-pass filter (C1-R1), an amplifier stage (IC1), an output stage (T1) and a loudspeaker.

The prototype used a high impedance loudspeaker from a telephone handset, and this worked remarkably well. The purpose of P1 is to adjust the amplification. At the highest amplification, the wire energized by the transmitter can be traced from several tens of centimeters away. A direct electrical connection is therefore not required. However, it is important that you hold the ground connection (earth) in your hand.

This small circuit can be used to control model railway turnouts operated by AC voltages. A logic level in the range of 5–12 V can be used as the control signal. The coils of the turnout are switched using triacs. Changes in the logic level of the input signal are passed on by the buffer stage built around T1 and T2. The buffer stage is included to boost the current available at the gates of the triacs. If the input goes high, this positive change is passed through via C1. That causes a positive current to flow through D2 (D2 is reverse biased) to the gate of T3. That triac switches on, and power is applied to the turnout coil.


This situation persists until C1 is fully charged. No more current flows after that, so the triac does not receive any gate current and switches off. If the input is set low, a negative current flows briefly via C1. It can flow through D2, but not through D1. T4 is switched on now, and the other turnout coil is energised. This circuit takes advantage of the fact that triacs can be triggered by negative as well as positive gate currents. If the turnout coils are energised for too long, you should reduce the value of C1.

If they are not energised long enough, increase the value of C1. The TIC206D can handle several ampères, so it can easily drive just about any type of turnout coil. You can also use a different type of triac if you wish. However, bear in mind that the TIC206 requires only 5 mA of gate current, while most triacs want 50 mA. That will cause the switching times to become quite short, so it may be necessary to reduce the value of R1.

Here’s a project that could be useful this summer on the beach, to stop anyone touching your things left on your beach towel while you’ve gone swimming; you might equally well use it at the office or workshop when you go back to work. In a very small space, and powered by simple primary cells or rechargeable batteries, the proposed circuit generates a low-energy, high voltage of the order of around 200 to 400 V, harmless to humans, of course, but still able to give a quite nasty ‘poke’ to anyone who touches it.

Quite apart from this practical aspect, this project will also prove instructional for younger hobbyists, enabling them to discover a circuit that all the ‘oldies’ who’ve worked in radio, and having enjoyed valve technology in particular, are bound to be familiar with. As the circuit diagram shows, the project is extremely simple, as it contains only a single active element, and then it’s only a fairly ordinary transistor. As shown here, it operates as a low-frequency oscillator, making it possible to convert the battery’s DC voltage into an AC voltage that can be stepped up via the transformer.
Using a centre-tapped transformer as here makes it possible to build a ‘Hartley’ oscillator around transistor T1, which as we have indicated above was used a great deal in radio in that distant era when valves reigned supreme and these was no sign of silicon taking over and turning most electronics into ‘solid state’. The ‘Hartley’ is one of a number of L-C oscillator designs that made it to eternal fame and was named after its invertor, Ralph V.L Hartley (1888-1970). For such an oscillator to work and produce a proper sinewave output, the position of the intermediate tap on the winding used had to be carefully chosen to ensure the proper step-down (voltage reduction) ratio.

Here the step-down is obtained inductively. Here, optimum inductive tapping is not possible since we are using a standard, off-the-shelf transformer. However we’re in luck — as its position in the centre of the winding creates too much feedback, it ensures that the oscillator will always start reliably. However, the excess feedback means that it doesn’t generate sinewaves; indeed, far from it. But that’s not important for this sort of application, and the transformer copes very well with it.

The output voltage may be used directly, via the two current-limiting resistors R2 an R3, which must not under any circum-stances be omitted or modified, as they are what make the circuit safe. You will then get around 200 V peak-to-peak, which is already quite unpleasant to touch. But you can also use a voltage doubler, shown at the bottom right of the figure, which will then produce around 300 V, even more unpleasant to touch. Here too of course, the resistors, now know as R4 and R5, must always be present. The circuit only consumes around a few tens of mA, regardless of whether it is ‘warding off’ someone or not! If you have to use it for long periods, we would however recommend powering it from AAA size Ni-MH batteries in groups of ten in a suitable holder, in order not to ruin you buying dry batteries.

Circuit diagram:

Warning!
If you build the version without the voltage doubler and measure the output voltage with your multimeter, you’ll see a lower value than stated. This is due to the fact that the waveform is a long way from being a sinewave, and multimeters have trouble interpreting its RMS (root-mean-square) value. However, if you have access to an oscilloscope capable of handling a few hundred volts on its input, you’ll be able to see the true values as stated. If you’re still not convinced, all you need do is touch the output terminals...

To use this project to protect the handle of your beach bag or your attachecase, for example, all you need do is fix to this two small metallic areas, quite close together, each connected to one output terminal of the circuit. Arrange them in such a way that unwanted hands are bound to touch both of them together; the result is guaranteed! Just take care to avoid getting caught in your own trap when you take your bag to turn the circuit off!

Copyright : Elektor Electronics 2008

This circuit provides a simple means to determine the voltage of a low-impedance voltage source. It works as follows. P1, which is a 1-W potentiometer, forms a voltage divider in combination with R1. The voltage at their junction is buffered by T1, and then passed to reference diode D1 via R3. D1 limits the voltage following the resistor to 2.5 V. An indicator stage consisting of T2, R4 and LED D2 is connected in parallel with D1. As long as the voltage is not limited by D1, the LED will not be fully illuminated. This is the basic operating principle of this measurement circuit.