User manual

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Fig. 16: Delayed switching-off (Schaltung3.jpg)
The discharging time is considerably extended by the large base resistor. After about two seconds the electrolytic
capacitor is already mostly discharged. But after this time, the base current still suffices for a slight modulation of the
transistor. The collector current decreases only gradually.
Fig. 17: The afterglow electric torch (F_NPN3.jpg)
The LED shines with full brightness as long as you keep the key switch pressed. But a brief press of the key switch
suffices to turn the LED on. Afterward it remains fully switched on for about two seconds and after that shines more
and more weakly. After about a minute a weak glow can still be seen. Actually, even after a longer period the LED
does not go out completely. But the current drops to such small values that it no longer has any visible effect.
5. Step: Contact sensor
You can connect a lamp with a simple switch. However, a contact sensor can also be constructed with a suitable
transistor circuit. Two wires or metal contacts do not directly touch, but instead you only have to touch them with your
finger.
The current gain factors of two transistors can be multiplied if you amplify the amplified current of the first transistor
once more as the base current of the second transistor. The circuit shown in Fig. 18 is also called a Darlington circuit.
Fig. 18: The Darlington circuit (Schaltung4.jpg)
If you assume a gain factor of 300 for each of the transistors, the Darlington circuit has a gain of 90,000. Now a base
resistor of 10 MΩ already conducts sufficiently to switch the LED on. In a real experiment, you can use a physical
contact instead of the extremely high-ohm resistor. Due to the large gain, a light touch with a dry finger is already
sufficient. The additional protective resistor in the input lead to the battery protects the transistors in case the physical
contacts are inadvertently directly connected.
green red
red