Light onphototransistorQ1 (Radio Shack 276-130) produces voltage change across R2 for amplification by opamp whose output drives array fiveLEDs forming bar graph voltage indicator. Adjust R3 initially for highest sensitivity by tuming off room lights and rotating until LED 1 just stops glowing. Now, as light is gradually increased on sensor, LEDs
come on one by one in upward sequence and stay on until all five are lit. Solar cells or selenium cells can be used in place of phototransistor. -F. M. Mims, "Opto-electronic Projects, Vol. 1, " Radio Shack, Fort Worth, TX, 1977, 2nd Ed. , p 85-93.