funny bird - chirps back at you

chirps back at you Birds of all sorts are lovingly owned by many people, but most of them have, unfortunately (?), not yet learnt to communicate with us (or we with them?). Our bird has taken a step in the right direction: when you whistle at it, it chirps back. The necessary circuit has been split into two sections. The first is con- structed around opamps Al . . . A4. The incoming whistle received by the microphone is amplified in Al the gain factor of which can be set be- tween 20 and 500 by P1. To ensure that the bird really reacts to a whistle, the input signal is filtered in A2. It is then rectified by DI and decoupled by C5. Opamp A3, fundamentally a trigger with hysteresis, functions here as a monostable multivibrator IMMV). Its output (with an incoming whistle) re- mains logic 0 until C5 has discharged via R8 to such an extent that the voltage across it lies below the hysteresis threshold of A3. As long as the output (pin 8) is logic 0, p-n-p 2A B transistor T2 conducts a...