This circuit was found in a 1964 documentation. It shows how to achieve automatic gain control for an oscillator. The transistors are of the time and the circuit controls signals from 2.6 to 3.1 kHz.
Using the Linear Technologies LT3080 IC we have a constant current source with a 10 V voltage...
This sensor circuit for currents of the order of nanoamperes was found in an American...
Found in an 80's Elektor, this circuit can use any equivalent optical coupler. The buzzer is of the...