Found on a 1964 CQ this circuit is intended for tone frequency change modulation (FSK) telegraphic transmissions. It operates with signals of 2.975 and 2.125 Hz.
When the base of the general purpose transistor is grounded the circuit produces a signal that is given...
This circuit was taken from a popular 90's Electronics. The LDR is mounted next to the LED to feed...
Found in old documentation this circuit uses the NOR ports of 4001. The circuit has the frequency and...