Obtained in a 1969 Ferranti documentation, this circuit dissipates only 2.2 mW with 4.5 V. The circuit can be designed based on modern equivalent transistors.
Approaching the hand or an object to the sensor of this circuit, the relay closes its contacts. The...
For those who want to recover an old multimeter of this type, the circuit presented is worth...
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...