If the battery is good, the LED flashes. This simple tester uses an LM3909 integrated circuit. We found this configuration in an old documentation.
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,...