Found in a Popular Electronics from the 80's, this circuit makes use of a time integrated to control a relay in a timed way. R2 determines the activation time.
This circuit was found in a Mullard transistor manual from 1960. Note the symmetrical power...
With the shadow over the sensor, it is possible to interrupt the sound of your amplifier using this...
This circuit puts the battery in the circuit when the voltage of a source is cut, preventing it...