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.
We found this circuit in the National Semiconductor Audio Handbook of 1977. The circuit produces...
We found this circuit in an old American documentation. The operational amplifier is not common...
This circuit was found in a 1980s US publication. It indicates when the phone is off-hook by...