We found this circuit in a 1977 factory documentation. The operational amplifiers can be 741 and the symmetrical power supply. Capacitors can be changed for operation at other frequencies.
We found this circuit in a 1973 documentation. The null adjustment trimpot is 10k. The circuit can be...
This circuit is from Radio Electronics from April 1977. The circuit is for analog lines using...
The indicator for this circuit is a mini buzzer with a 1.5 V oscillator. The circuit can also be...