The Relay Demonstration Unit allows the investigation and study of how a relay functions, in a clear and practical manner. A useful property of this unit is that the circuit powering the coil is easily seen to be completely separate from the circuit switched on by the relay. This is one of the reasons relays are used where a safe low voltage circuit can be used to control a high voltage circuit. Coil Input 4-6V DC Bulb Input Max. 6V AC / DC.