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Velleman EDU01 User Manual

Page 19

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Pag. 19

Required parts: 9V battery*, 1K resistor (brown black red gold), 2x 10K resistor
(brown black orange gold), red LED, BC547 transistor, LDR.

How it works: A LED lights up when enough
light hits the LDR. The LDR is a light sensitive
resistor (Light Dependent Resistor), in dark
conditions it has a high resistance, and in light
conditions its resistance becomes low.
Across the LDR is a positive potential which is
fed to the base of the transistor enabling it to
switch. Resistor R2 creates a switching point
which determines when the transistor starts to
conduct. Resistor R1 limits the current that
flows through the LDR.