This circuit of a 1969 Electronic Design uses RTL logic. The setting can be adapted to TTL. The frequency is given by the crystal.
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...