PURPOSE: To provide a system in which a microcomputer can be protected from destruction, without generating any malfunction in a system for protecting the microcomputer. CONSTITUTION: This system is provided with a constant voltage circuit 3 connected to an alternating power source, which supplies direct currents converted from alternating currents at the time of power on, and direct currents from a built-in battery at the time of power off to microcomputer, rectifier circuit 4 connected with the alternating power source, charging/discharging circuit 5 which receives the output of the rectifier circuit, starts charging when the power source is switched from off to on, outputs a high level voltage after a prescribed time, starts discharging when the alternating power source is switched from on to off, immediately outputs a low level voltage, mode switching circuit 6 which receives the output voltage of the charging/discharging circuit, and outputs a voltage in a level corresponding to the level of the output voltage, and microcomputer 7 which receives the output voltage of the mode switching circuit, and switches a power consumption mode corresponding to the level of the output voltage.
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