When the voltage supplying the circuit falls below the zener value, the LED lights up, and if a low current buzzer is used, it is activated. The circuit is from a 1999 American publication.
Today we can call this circuit a baby monitor. It is a microphone amplifier found in a publication by...
Found in Motorola documentation from the 90's, this circuit charges the capacitor with a constant...
This small transmitter can send its signals to a receiver placed at distances up to 100 m. The...