Found in documentation from the 1980s, this circuit uses an operational amplifier from that time. Transistors are also from the era, and general-purpose equivalents can now be used.
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...