The following post explains how the IC LM317 can be used for automatically controlling 7 segment LED display intensity with respect to the variations in the ambient light conditions, detected through a photo transistor.
- When it detects a relatively high light intensity, the based and emitter voltage of T2 increase. When the base voltage of Tz exceeds 2.7 Vj R4 limits the emitter voltage to 3.9V due to the constant current of 5.7 mA.
- This causes the voltage at the emitter of this pnp darlington transistor to be about LZVZ The voltage across Rs is the reference potential, 1.25 W of the Type LM317 regulator, so that lies about 5.7 mA, and the output voltage, UO, of the circuit is when T1 detects darkness.
- The maximum output current is of the order of 700 mA when IC1 is adequately cooled. The input voltage range of the circuit is 8 to 15 VC
- This is a light dependent voltage source that regulates the supply to 7-segment displays in accordance with the intensity of ambient light.
- The sensitivity of the regulator is adjustable with P1.
- T2 no longer conducts and the output voltage of the circuit is 5.7 L because the total resistance between the regulator output and ground is Rs+Ra+R4= 1,000 Q, and the current through it is still 5.7 mA.
- The regulating action is positive, a higher ambient light intensity results in the circuit raising the supply voltage to the displays.
- Phototransistor T1 does not conduct when it detects darkness, and the base of T2 is therefore grounded via R2 and Pt.
LM317 Dispaly Intensity Controller Circuit is shown below: