wet alert

The new Toko piezo buzzer shown in the photo can be used for all kinds of simple acoustic alarm circuits. Its greatest advantage is its extremely high efficiency — in other words, it needs only very little power to make a fiendish racket. One possible application is the "wet alert" circuit given here. When the two electrodes are immersed in water, oscillator N1 is enabled. It produces a 1 Hz squarewave, and this signal is passed via N2 to switch the second oscillator (N3) on and off. The result is as you would expect: "bleep-bleep-bleep-...". The "bleep" frequency can be set, by P1, at any- where between 3 kHz and 10 kHz. A single transistor, T1, is used as an amplifier stage to drive the piezo buzzer; since the basic signal is a squarewave, the coil helps to boost the peak level still further, ensuring an impressive sound output. There are all sorts of possibilities for the electrodes. One of the simplest is to etch two parallel strips on a piece of printed circuit board materi...