This circuit was found in a Popular Electronics from the 90's. The components used are common and instead of the buzzer we can use an LED or a relay.
This circuit is from a technical documentation from Texas Instruments from 1978. In it we have the...
We found this circuit in a publication with experiments with operational amplifiers from 1973. The...
Using a varactor or varicap, this 1969 Electronics World circuit has a modulation range from 75...