This circuit was found in a 90s Electronic Engineering. The quadruple operational amplifier can be replaced by an equivalent. RL determines the tested resistance range.
This circuit controls a relay causing it to open and close its contacts at a speed that depends on the...
This circuit is from an Electronics World of 1968. The transistors are of the time and the power...
This circuit is from a Motorola manual from the 70s. The circuit can be implemented with more...