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.
We found this circuit in an Australian publication from 1991. It is a kit of the time with input...
This time base circuit was found in the January 1988 issue of Radio Electronics magazine. The...
This circuit was found in the December 1983 issue of Radio Electronics magazine. The sensor is an...