Project 8: Timer circuit
Make a LED switch off after a certain time
Required parts: 9V battery", 1K resistor (brown black red gold), 100K resistor (brown black orange gold), 1M resistor (brown black green gold), Red LED, push-button, 2x BC547 transistor, 10|jF electrolytic capacitor, 3x wirejumper
How it works: When pushing the push button shortly, the LED will turn on and after a while it will go out. By pushing the push button the capacitor will quickly charge; when releasing the button the capacitor will release its stored energy via both transistors - both will start to conduct and the LED will light up. The current needed to make T2 conduct is limited as T1 and T2 form a Darlington transistor circuit. The time needed to discharge the capacitor is also determined by resistor R1. The smaller R1 the faster the capacitor is discharged and the LED switches off. When R1 is removed, the capacitor discharges solely via the base current of T1. Switching off is a lot slower now and takes ± 1 minute.