Found in the September 1972 issue of Radio Electronics magazine, this signal injector is based on the HEP580. The circuit is powered by two batteries and its frequency is basically given by C1.
This circuit indicates whether the voltage to be measured is higher or lower than the zener diode...
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....