User manual

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7. Step: LED as light sensor
This light sensor controls the brightness of an LED. When light strikes the sensor, the LED goes on; in darkness it
stays off. Actually, practically no current flows through a diode if it is put to a voltage in reverse direction. However,
there is actually a very small reverse current, e.g., in the range of a few nanoamperes which normally can be
disregarded. The high gain of the Darlington circuit, however, allows experiments with extremely small currents. Thus,
e.g., the reverse current of an LED is itself dependent on the illumination. An LED is thus at the same time a
photodiode. The extremely small photocurrent of the red LED is amplified with two transistors to such an extent that
the green LED shines.
Fig. 22: Gain of the LED reverse current (Schaltung6.jpg)
In a practical experiment, the right LED is already clearly switched on in normal ambient light. A shading of the sensor
LED with the hand becomes visible in the brightness of the display LED.
Fig. 23: The LED light sensor (F_Darlington3.jpg)
8. Step: Constant brightness
Sometimes you need a constant current which is as independent of voltage fluctuations as possible. An LED would
thus shine with the same brightness even if the battery had lost some of its voltage. The circuit depicted in Fig. 24
shows a simple stabilisation circuit. A red LED at the input stabilises the base voltage to about 1.8 V. Since the base
emitter voltage is always around 0.6 V, there is a voltage of about 1.2 V at the emitter resistor. The resistor therefore
determines the emitter current and thus also the collector current of ca. 2.5 mA.
The LEDs in the collector circuit do not need a series resistor because the LED current is regulated by the transistor.
The constant current source also works with different loads. Regardless of whether you use both LEDs in the collector
circuit or short-circuit one of them – the collector circuit remains the same.
Fig. 24: A stabilised current source (Schaltung7.jpg)
red green
green
yellow
red