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 found in a European publication from the 1980s, is based on PLL 566. The frequencies...
The circuit shown, with the printed circuit board, was found in an article in the June 1990 issue...
In the figure we have a simple timer with two voltage comparators of the four of an LM339. The...