This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset or speaker must be at least 80-ohm. Observe the low supply voltage.
When any of the sensors is interrupted, the relay closes its contacts and locks, not turning off...
We found this ladder-shaped signal generator circuit in Radio Electronics magazine from August 1966....
This antenna preamplifier can work with signals from about 20 MHz to 100 MHz, depending only on...