Using the 2N2646 this circuit was found in a Radio Electronics magazine from July 1968. The transistors are from the time and the lamp can be replaced by a high-power LED in a current version.
This circuit is based on a 5 V LED flasher. We found it in October 1980 Radio Electronics...
This circuit was obtained in a 1978 publication. The operational amplifier must use a symmetrical source....
Using a 4046 PLL this circuit recognizes a 1.6 to 1.8 kHz tone by driving an output LED. The...