This circuit was found in the April 1971 Radio Electronics magazine. It uses common transistors and can be adapted to use an LED or audible indicator instead of a light bulb.
Other series amplifiers such as the MC3301, MC3401 and LM2900 can be used on this circuit. We...
Crystals from 100 kHz to 1 MHz can be used in this 1957 Raytheon circuit. 4 can have values...
This window comparator configuration is from a 1980 publication. It is a conventional circuit that...