User Manual

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 tran-

sistor, 10µF electrolytic capacitor, 3 x wire jumper

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 releas-

ing 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 cir-

cuit. The time needed to discharge the capaci-

tor is also determined by resistor R1. The

smaller R1 the faster the capacitor is dis-

charged 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.

The total gain of both transistors can be calculated: ß = ß(T1) x ß(T2)

In electronics, the Darlington circuit is a

structure consisting of two bipolar

transistors connected in such a way that

the current amplied by the rst transistor

is amplied further by the second one. This

conguration gives a much higher current

gain than each transistor taken separately.

10M