Here’s a challenge which is additionally useful this summer time on the beach, to prevent anyone touching your things left in your seashore towel whereas you’ve gone swimming; you might equally well use it on the office or workshop whilst you return to work. In an awfully small space, and powered by means of simple major cells or rechargeable batteries, the proposed circuit generates a low-energy, excessive voltage of the order of round 200 to 400 V, innocuous to people, of course, however still able to supply a rather nasty ‘poke’ to anyone who contactes it.
Quite apart from this sensible facet, this venture will even prove tutorial for younger hobbyists, enabling them to discover a circuit that all the ‘oldies’ who’ve labored in radio, and having enjoyed valve know-how specifically, are bound to be conversant in. As the circuit diagram presentations, the venture is extremely simple, because it comprises handiest a single active part, and then it’s handiest a fairly abnormal transistor. As proven here, it functions as a low-frequency oscillator, making it conceivable to transform 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 that that you may imagine to construct a ‘Hartley’ oscillator around transistor T1, which as we now have indicated above was used an excellent deal in radio in that distant technology when valves reigned supreme and these was no signal of silicon taking over and turning most electronics into ‘solid state’. The ‘Hartley’ is one in each of various L-C oscillator designs that made it to eternal reputation and used to be named after its invertor, Ralph V.L Hartley (1888-1970). For such an oscillator to work and produce a right variety sinewave output, the place of the intermediate faucet on the winding used had to be moderately chosen to make sure the correct step-down (voltage reduction) ratio.
Here the step-down is acquired inductively. Here, greatest inductive faucetping shouldn't be that you can think of seeing that we're the use of a normal, off-the-shelf transformer. However we’re in success — as its place in the centre of the winding creates an excessive quantity of comments, it ensure thats that the oscillator will all the time begin reliably. However, the surplus comments signifies that it doesn’t generate sinewaves; certainly, far from it. But that’s now not necessary for such a software, and the transformer copes very smartly with it.
The output voltage may be used immediately, by the use of the 2 current-limiting resistors R2 an R3, which must not under any circum-stances be neglected or modified, as they are what make the circuit safe. You will then get around 200 V peak-to-peak, which is already reasonably disagreeable to contact. But which you can also use a voltage doubler, shown on the backside proper of the figure, so that they can then produce round 300 V, much more unpleasant to touch. Here too of course, the resistors, now comprehend as R4 and R5, should all the time be present. The circuit simplest devours around just a few tens of mA, in spite of whether or not it's ‘warding off’ any individual or no longer! If you need to use it for lengthy periods, we'd alternatively suggest powering it from AAA dimension Ni-MH batteries in groups of ten in an acceptable holder, so as not to spoil you shopping for dry batteries.
Circuit diagram:
Warning!
If you construct the version without the voltage doubler and measure the output voltage with your multimeter, you’ll see a lower value than mentioned. This is because of the truth that the waveform is a lengthy means from being a sinewave, and multimeters have hassle decoding its RMS (root-mean-square) worth. However, if if you have get admission to to an oscilloscope in a position to handling a few hundred volts on its input, you’ll be able to see the authentic prices as said. If you’re nonetheless no lengthyer yes, all you need do is contact the output terminals...
To use this venture to supply protection to the handle of your beach bag or your attachecase, for example, all you want do is fix to this two small metal areas, slightly shut collectively, every related to one output terminal of the circuit. Arrange them in such a technique that undesirable palms are sure to touch both of them collectively; the outcome is guaranteed! Just take care to avoid getting caught in your own trap whilst you take your bag to show the circuit off!
Copyright : Elektor Electronics 2008
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This creates a useful ‘quick test’ option: point the tester at direct daylight, and the indicator LED should light briefly. The sensitivity of the tester is such that IR light from remote control is detected at a distance of up to 50 cm. The circuit is designed for very low power consumption, drawing less than 1 mA from the battery when IR light is detected, and practically no current when no light is detected. Hence no on/off switch is required. The construction drawing shows how the tester may be ‘cased’ using a small ABS case from Conrad.
