This SCR relaxation oscillator was found in an 80's Electronic Design. It flashes a small incandescent lamp in frequency that depends on the capacitor.
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...