User manual

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Fig. 13: Current gain (F_NPN1.jpg)
The LEDs are for indicating the currents. The red LED shines brightly, the green one barely. Only in a completely
darkened room can the base current be seen as the weak shining of the green LED. The difference is an indication of
the large current gain.
3. Step: Positive and negative reversed
A PNP transistor has exactly the same function as an NPN transistor, but with reversed polarity. The emitter is thus
now at the positive pole of the battery.
Fig. 14:
A PNP transistor in the emitter circuit (Schaltung2.jpg)
Set up the circuit with the PNP transistor BC557 and examine the current gain here, too, with different base resistors.
The BC557B likewise has about a 300-fold current gain.
Fig. 15: Examining the current gain of the BC557 (Aufbau2.jpg)
4. Step: Follow-up control
The aim of this circuit is to make an LED electric torch with automatic afterglow. The interior lighting of automobiles
often works according to this principle: after you leave the car, the light still shines for a certain time and then slowly
goes out.
If you hold an electrolytic capacitor with the correct polarity to the battery, it takes an electric charge. After separation
from the battery this charge remains for a long time. The electrolytic capacitor can then be connected to an LED.
There is a brief flash of light. The electrolytic capacitor discharges in a brief moment.
The current gain of a transistor can be used to extend the discharging time of a capacitor. The circuit shown in Fig. 16
uses an electrolytic capacitor with 100 µF as the charging capacitor. After briefly pressing the key switch it is charged
and now delivers the base current of the emitter circuit for a longer period.
green red