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.
Found in a Popular Electronics from 1992, this circuit uses a PLL567 in an audio meter bridge that...
This circuit was found in an old European documentation. When R2 is cut off the circuit triggers...
This circuit for the positive front of a rectangular signal was found in an old documentation, but applies...