We found this circuit in an 80's manual. The same configuration can be adapted to make use of modern operational amplifiers. The power supply is symmetrical.
In the figure we show a timer, this simple one with only one of the four gates of integrated...
This circuit is a sensor for photodiodes that can be used in optical couplers, encoders and other...
This crystal tester was found on a 90s Ham Radio. Transistors are commonly used. The circuit is for...