The amplitudes of a signal on the microphone input of a telephone and a signal on the line input of the telephone are compared with each other and with two thresholds. The instrument includes a component for selectively activating and deactivating the illuminating component and a deactivating component for automatically deactivating the illuminating component, after a predetermined period of time has elapsed. The inventive principle is used to advantage in two-point modulators and allows inexpensive, integratable mobile radio transmitters with good noise characteristics. The mirror substrate is driven by electrostatic force between the movable electrode and the fixed electrode and vibrates centering of the beam.