Abstract: A circuit tester includes a light-weight portable transmitter and receiver, with the transmitter being provided with several interchangeable plugs for the testing of power circuits of different voltage. The plugs are alternately firmly electrically and mechanically coupled to apply power to the transmitter, which does not need a battery. The transmitter transmission circuitry operates on a relatively low voltage such as 9 volts, and the transmitter input circuitry operates without switching circuits to reduce the applied power to this operating level. The transmitter only weighs 21/2 or 3 ounces and the receiver only weighs 7 or 8 ounces. Unique modulation or interruption of the transmitter operates audio and/or visual signalling elements in the receiver.