PURPOSE: To prevent malfunction of a machine due to source voltage drop by providing a means for receiving a voltage detection signal and an operating signal and driving the machine, then setting a threshold voltage higher than the lowest operating voltage thereby disabling operation of the machine until stoppage of an operating means.

CONSTITUTION: Voltage of a power source 1 such as manganese cell or alkaline cell is fed to a voltage detecting means 3, and the output voltage thereof is varied according to the input voltage level. An operating means 4 has clock function and provides a machine 7 control signal to a machine drive means 6. Threshold voltage of the means 3 is set higher than the lowest operating voltage of the means 4 thus disabling operation of the machine 7 until stoppage of the means 4. By such arrangement, malfunction of the machine 7 due to source 1 voltage drop can be prevented.