PURPOSE: To enable the detection of voltage in a wide range from a low power source voltage to about several volts by comparing collector current of transistors which are different in the emitter area while commonly connected in the base.

CONSTITUTION: After a power source voltage to be detected is divided with a resistance R0, a voltage is applied in common to the bases of transistors (Tr) Q1 and Q2. The resistance R1 connected to the emitter of the TrQ1, the resistance R2 connected to the emitters of Trs Q1 and Q2 and the ratio of the emitter areas of the Trs are so designed that collector currents of the TrQ1 and TrQ2 coincide with each other at the state of the threshold voltage to be detected. Then, as the power source voltage rises, the collector current of the TrQ2 won't readily change because of the resistance R2 where as the collector current of the TrQ1 increases in a larger measure. This turns off the TrQ5 and turns on the TrQ7 which leads to a higher level of the output. On the contrary, when the power source voltage lowers, the TrQ5 is turned on and the TrQ7 turned off which leads to a lower level of the output.