This simple circuit was found in the August 1984 issue of Radio Electronics magazine. The circuit monitors the power supply to a computer and has a backup battery.
This very low frequency oscillator was found in a Popular Electronics from the 90s. The circuit...
In the figure we have an active high pass filter of 100 Hz, suggested by National Semiconductor in its 1994...
Constant currents from 10 mA to 1 A can supply the load, depending on the selection made by S1. Q1...