The configuration shown was found in an old Poptronics but can be easily assembled using a common calculator photocell. The relay is according to the supply voltage.
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...