An electronic circuit for use in a vehicle equipped with an internal combustion engine. The electronic circuit comprises a constant-voltage regulated power-supply circuit, a control circuit having a central processing unit for controlling electrical apparatus installed in the vehicle, and a detecting circuit for detecting variations in supply voltage supplied from the power-supply circuit. The detecting circuit is adapted to generate an output signal when the supply voltage of the power-supply circuit is out of a predetermined range. The electronic circuit further includes circuitry responsive to the output signal of the detecting circuit to render the central processing unit inoperative and an alarm responsive to the same output signal to give warnings.