The beeper was designed by Chris Hereward of the Cambridge University Department of Zoology, who kindly gave permission for it to be reproduced here.

Circuit description

IC1 (a PXO 1000) is an integrated circuit crystal oscillator and divider unit. The interval between beeps (i.e., the sample interval) is determined by the settings of switches A–D, as shown in Fig. A4.1. The output of IC1 is inverted by a part of IC2 (a 4049UB) and used to turn on IC3 (a TLC 555CP) for about 0.1 s, timed by a 1 MΩ resistor and a 0.1 µF capacitor. IC3 is an astable multivibrator that produces the output tone. This is buffered and amplified by IC2 and fed into the positive battery line via a 390 Ω resistor. The battery is given a degree of isolation from the circuit supply by a silicon diode. This may seem a curious way of coupling the output, but it has the advantage that the circuit is turned off by unplugging the earphone. Plugging in the earphone switches on and resets the timer.