With TJ Byers
I have a 120 VAC wireless doorbell that I want to modify so that I know if someone rang the bell while not at home. Is there an LED circuit I can install that would turn on when someone rings the bell (yes, it uses a speaker announcer), which I could clear with a simple press button to arm it for the next caller?
There are several sensor options that can be used for this design, including tapping into the speaker wires. But I decided that a sound-activated switch would better serve our readers because it could be used with chimes and all sorts of bell announcers (even a telephone ringer). The pick-up is an electret microphone that is mounted inside the announcer case close to the speaker. This isolates the mic from outside noise and prevents false triggering. The vibrations it picks up are amplified by the op-amp (any garden-variety op-amp will work) and coupled to a driver transistor through a 10µF cap. At rest, no sound comes from the op-amp and the transistor is turned off.
At the first hint of sound, the transistor turns on and triggers the SCR, which remains latched — lighting the LED — until you press the Reset button. The sensitivity of the switch is adjusted by the value of the Rf resistor; increasing its resistance increases the sensitivity of the alarm.