Advertisement
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
resistance, and in light conditions it
TR
has a low resistance.
Across the phototransistor is a positive
potential which is fed to the base of the
transistor enabling it to switch. Resis-
tor R2 creates a switching point which
determines when the transistor starts
to conduct. Resistor R1 limits the cur-
rent that flows through the phototran-
Pag. 19
www.vellemanprojects.com
Advertisement
