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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, 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
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 tran-
sistor 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.
www.vellemanprojects.com
R1
10M
In electronics, the Darlington circuit is a
structure consisting of two bipolar
transistors connected in such a way
that the current amplified by the first
transistor is amplified further by the
second one. This configuration gives a
much higher current gain than each
The total gain of both transistors can be calculated: ß = ß(T1) x ß(T2)
transistor taken separately.
Pag. 25
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