This circuit takes advantage of back-e.m.f. (electromotive force) to amplify the voltage across a piezo sounder. Ordinarily, IC1 would only achieve a gentle beep. However, the addition of a very high inductance choke of a few Henry — in this case, the coil of a miniature reed relay is used — achieves a penetrating screech, and represents an easy method of obtaining considerably more volume in such a circuit.
The usual protective diode (D1) may be included across the choke, at the expense of a little volume. In practice, it was found that no harm was done by omitting D1. The operating voltage of the relay is immaterial, as long as it is not less than the supply voltage. Preset P1 should be adjusted to find the piezo sounder’s resonant frequency. A higher supply voltage means greater volume — as long as T1’s ratings are not exceeded.