User Manual

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 darker. Transis-

tors 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 phototransistor,

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 +

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 ow through R5 when the LED switches off, the threshold

level for which T2 will conduct again in darkness also changes.

100K