A Device for the Measuring of Tracking Performances

Abstract

A transistorized circuit which responds in a binary fashion to the change in conductivity of a photoelectric cell is described here and shown in Fig. 1. The trigger of the photocell is a Schmidt trigger directly coupled to the amplifier of the photocell.1 It operates in a nonsaturated mode for the most rapid, possible response. The cycle of operation is essentially as follows: Assume that the photocell is being so activated by a source of light that its resistance is decreased. This increases the basecurrent of Ti, which, in turn, increases the collector-current of T1 and decreases the voltage at the base of T2. This decreases the collector-current of T2 increasing the base-voltage of T3 which takes over conduction from T2 via the feedback through the common emitter-resistor. T2 is cut off while T3 is conducting approximately 8 ma. causing the relay to be energized, and to remain energized until the light impinging on the photocell decreases sufficiently to cause the base-voltage of T2 to rise above cut-off. The collector-current begins to flow in T2, decreasing the base voltage of T3 which rapidly tends toward the cut-off, as conduction of T2 is reinforced by positive feedback until the T3 cut-off is reached. Trigger-time is approximately 50 gsec., reset-time being slightly longer due to the inductance of the relay. In practice, triggerand reset-time are limited by the recovery-time of the photocell. To calibrate the circuit the control of sensitivity (500 K pot.) is set to maximal sensitivity (minimal resistance) with the photocell receiving the maximal light. The threshold (50 K pot.) is then set slightly beyond the pull-in point of the relay. The control of sensitivity is then set to a point just before the drop-out of the relay. These settings are made easily with the aid of a voltmeter connected across the collector of T1 and the ground.