This small tone generator was found in the November 1987 issue of Radio Electronics magazine. The phototransistor is common and controls the sounds generated. The basic frequency is given by C1.
With the name of transitube we find this MW receiver in documentation from the 50's. It uses a...
This circuit is from a 1998 Electronics Now and can be easily assembled since it uses a common...
This circuit is suggested by National Semiconductor and makes use of the negative regulator LM2990...