Velleman EDU01 User Manual download pdf (Page 27)

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Pag. 27

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, LDR, Red LED, 2x wire

jumper

How it works: This circuit will make a

LED switch on when it becomes darker.

Transistors T1 and T2 form a Schmitt-

trigger circuit. The output of a Schmitt

trigger is switched over at a certain volt-

age 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 LDR 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 LDR the

voltage on the base of T1 rises. T1 will start to conduct when this voltage is higher than

the voltage over R5 + U

of T1. When this is the case, the base voltage of T2 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.

100K

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Velleman EDU01 User Manual Page 27