Abstract: A control system for electrically powering from an external alternating-current power source a direct-current underwater hand-operated tool or light activated by a switch. The control system includes a relay for connecting and disconnecting the external AC power source to and from the system, a transformer connected to the relay for transforming the AC power to a desired lower voltage level, a converter connected to the transformer for converting the transformed AC to DC power, cables for supplying the DC power to the tool, and a control circuit for sensing the voltage level of the transformed AC power and in response to the sensed voltage level increasing beyond a predetermined safe level, causing the relay to disconnect the external AC power source from the system. The control includes a Zener diode for sensing the voltage level of the transformer AC power, and a transistor and an SCR for controlling the operation of the relay.