Sound Alarm Circuit
Électronique / Electronic
This alarm gives out an alarm light and sound when it detects your voice or any other sound. The earphone acts as a microphone. Sounds picked up by the microphone are amplified by IC 1.
Diodes DA and DB rectify the amplified signal-that is, they convert the sound signal from AC to DC. The signal travels through IC 2, the comparator, and activates the LED and the speaker.
When you finish building the circuit, rotate the control fully counter-clockwise, and set the switch to position ON. Then, rotate the control clockwise while speaking into the microphone, and set the control in a position where the LED lights only when you speak into the microphone.
Stop speaking and you will see the LED turn off.
Now, see what happens when you disconnect the wire between ground and pin 2 of 7400 and re-connect pin 2 whit A. When you blow into the microphone (earphone), the LED lights and the speaker sounds.