A control circuit controls power supply to a device which includes a power storage device and an electricity generating means for intermittently supplying power to said storage device. The control circuit includes a microcontroller arranged to operate in either a sleep mode or an active mode when the device is switched on. The microcontroller intermittently enters the active mode from the sleep mode in order to test the storage device voltage and provide an indication if the storage device voltage is determined to be less than a predetermined level.
