Abstract: A movable telephone circuit test instrument is provided, indicators mounted thereon which show the communicating state of a telephone circuit being tested. Supervision of the circuit under test is thus provided at the same location as the tester, thereby reducing the number of persons required to carry out the test. The supervising indicating means are preferably light emitting diodes and are provided in a unique driving circuit that includes a variable conductance parallel current path for maintaining the brightness of the light emitting diode substantially constant regardless of the magnitude of current being monitored by the supervising circuits.