Any sensor or piezo capsule can be used as a sensor in this circuit. The integrated circuit is also common in our days and a table of sensor voltage levels.
This circuit was found in an Electronic Design from 1967. The circuit uses two triacs being T2...
Found in old American documentation, this circuit can use crystals between 27.5 and 33 MHz...
This shield from the 60's uses a germanium transistor of the time and a common type of relay of...