User Manual

Required parts: 9V battery*, 1K resistor (brown black red gold), 2x 10K resistor (brown

black orange gold), red LED, BC547 transistor, phototransistor (TR).

How it works: A LED lights up when

enough light hits the phototransistor

(TR). The phototransistor is light sensi-

tive, in dark conditions it has a high re-

sistance, and in light conditions it has a

low resistance.

Across the phototransistor is a positive

potential which is fed to the base of the

transistor enabling it to switch. Resistor

R2 creates a switching point which de-

termines when the transistor starts to

conduct. Resistor R1 limits the current

that ows through the phototransistor.