Project 9: Light switch
Make a LED switch on when it becomes dark
Required parts: 9Vbattery*, 100 ohm resistor (brown black browngold), 2x IKresistor (brown black red gold), 2x 10K resistor (brown black orange gold), 10OK resistor (brown black yellowgold), 2x BC547 transistor, 10microF electrolyte capacitor, LDR, Red LED, 2xwire 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-* âge at the input (the trigger level). The purpose hère 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 + Ube 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.