This simple bridge circuit can be used for impedance or resistance measurement using a high impedance earphone as an indicator. The circuit is from an American 80's documentation.
This circuit was found in a 1976 NASA manual, making use of components of the time. The transistor...
The circuit that gave rise to this configuration is from 1978. In the original publication are...
We find this circuit in documentation from the 1980s. The triac and diac are of conventional types...