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.
This is the basic configuration of an operational amplifier sample & hold circuit. The FET and...
The components of this filter are calculated using conventional formulas (see mathematics section of...
Operational amplifiers used in this 80's circuit is not very common today, but the setting can be...