stereo doorbell - audio interrupter

audio interrupter The stereo is on, playing loud enough to do justice to your favourite rock record. The doorbell rings but in your ecstacy you don"t realize it, even if you had been able to hear it. It doesn"t take long before the would-be guest gets fed up and decides to protect his (or her!) gentle ears by going someplace quieter, like a heavy metal concert. That leaves you with two options: cut the mains lead of the stereo or fit a more effec- tive doorbell. The circuit here is a combination of the two but we guarantee it is less destructive than the first. It cuts the volume of the stereo"s output drastically when the bell is operated. Then to make sure the message is received the bell gives a number of tones of different frequencies switch- ing from one channel to the other. The operation of the circuit is quite simple. When the bell button is pressed there is a voltage across the bell which is rectified by diode D1 to provide a logic "1". This causes a number of things to h...