We found this circuit in an old American publication from the 1980s. Transistors can be general-purpose types like the BC548, and the tuning frequency is given by the L and C components.
When the sensor X1 is opened, the relay is energized. When waiting, the current drained is very...
Based on the LF256 operational amplifier, this circuit was found in the September 1989 issue of...
This circuit was found in a 1968’s Motorola documentation. The transistor is from that time. The...