Display Intensity Control

This is a light dependent voltage source that regulates the supply to 7-segment dis- plays in accordance with the intensity of ambient light. The regulating action is positive, i.e., a higher ambient light intensity results in the circuit raising the supply voltage to the displays. Phototransistor Ti does not con- duct when it detects darkness, and the base of T2 is therefore grounded via R2 and Pl. This causes the voltage at the emit- ter of this pnp darlington tran- sistor to be about 1.2 V. The voltage across R5 is the refer- ence potential, 1.25 V, of the Type LM317 regulator, so that I is is about 5.7 mA, and the out- put voltage, Uo, of the circuit is Uo =1.2 + [5.7 x 10-3(R5+ R3)] =1.2 +1.82,=-, 3 volt DISPLAY INTENSITY when Ti detects darkness. When it detects a relatively high light intensity, the base and emitter voltage of T2 increase. When the base voltage of T2 exceeds 2.7 V, R4 limits the emitter voltage to 3.9 V due to the constant cur- rent of 5.7 mA. T2 no longer ...