PURPOSE: To generate a warning and to prevent the electric power of a primary battery from being wasted by varying the output of a comparator when the primary voltage drops to a specific level or below.

CONSTITUTION: When the power source is turned on, a voltage is applied to a rectified power terminal 4 and the voltage VS at a point P2 becomes a constant reference voltage with a current by-passed to the series circuit of a resistor 5 and a Zener diode 6 and is inputted to an input terminal 12 of a comparator 7. Then when a plus voltage is applied to an input terminal 14 from an external electric circuit for battery voltage drop detection, a transistor 13 turns on and the voltage VB at a point P1 where the voltage of the primary battery 1 is divided by resistors 2 and 3 is inputted to an input terminal 11 of the comparator 7. When VB<VS, the comparator 7 operates to warn that the voltage of the battery 1 drops to the specific voltage or below.