Abstract: A circuit testing device, including a casing having opposite ends, a single probe rigidly projecting from one end of the casing so that the casing may be manipulated to forcibly insert the probe into an electrical power socket, the casing carrying a first socket terminal, and there being a second probe connected with the socket terminal, circuitry in the casing including at least one Zener diode and at least one half wave rectifier interconnecting the single probe and the socket terminal, a presence-of-voltage circuit in the casing and electrically connected with the single probe, the circuit including a presence of voltage indicator and rectifier and amplifier elements electrically connected between the single probe and the indicator, the rectifier element including a rectifier bridge having two terminals connected across the Zener diode, there also being a voltage level indicating meter connected in series with such circuitry.