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Required parts: 9V battery*, 100 ohm resistor (brown black brown gold), 2 x 1K resistor
(brown black red gold), 2 x 10K resistor (brown black orange gold), 100K resistor (brown
black yellow gold), 2x BC547 transistor, 10µF electrolyte capacitor, phototransistor (TR), Red
LED, 2x wire jumper
How it works: This circuit will make a
LED switch on when it becomes dark-
er. Transistors T1 and T2 form a
Schmitt-trigger circuit. The output of a
Schmitt trigger is switched over at a
certain voltage at the input (the trigger
level). The purpose here is the switch
the LED on or off completely. As long
as there is no light on the phototransis-
tor, there is no base current to T1 and
it will not conduct. As long as this is the
case, T2 will receive a base current via
R2 and R3 and conducts; the LED will be on. When light shines on the phototransistor,
the voltage on the base of T1 rises. T1 will start to conduct when this voltage is higher
than the voltage over R5 + U
drops and T2 does not longer conduct and the LED goes off. Because of the change in
flow through R5 when the LED switches off, the threshold level for which T2 will conduct
again in darkness also changes.
www.vellemanprojects.com
of T1. When this is the case, the base voltage of T2
be
R6
100K
Pag. 27
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