Abstract: A circuit-energized voltage tester for determining whether or not the voltages on the tip and ring conductors of a telephone line are within their normal ranges is disclosed. The voltage tester is adapted for inclusion in a standard telephone repairman's handset and includes an energy storage circuit that becomes charged upon the application of handset test probes to tip and ring conductors if they are carrying a voltage greater than a predetermined level. Upon the closure of a switch, the energy thus stored energizes the remaining portion of the voltage tester. If suitable voltages are present on the tip and ring conductors, light emitting diodes forming a portion of a pair of optical isolators are controlled such that no measurement indication is provided. Specifically, the photo transistor portions of the optical isolators are alternately interrogated by a low frequency measurement control oscillator.