Found in a 1980s American documentation, this circuit can use a SCR and a general-purpose transistor. R2 is 1 M and R1 is 10k. The capacitor is 47 uF.
Found in a 1965 Electronic Design, this circuit makes it possible to select signals with an...
This sonic relay circuit is driven by a high-pitched sound picked up by a microphone that can be a...
This circuit was found in a Delco 1965 publication. Transistors may be current equivalents according...