This circuit is powered by a voltage of 4.5 V, being common in radios of the 60's. The circuit is from Mullard's transistors reference manual.
This circuit of an Electronics Hobbyists Handbook uses a different configuration for a triac. The...
We found this circuit in the July 1988 Radio Electronics magazine. The frequencies are given by...
Found in an English documentation from the 90's, this circuit makes slow logical pulses audible,...