Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 54

Circuit 31: Automatic bicycle light with phototransistor
Setup information
Install the two transistors as shown in Fig. 2.31. Then the two resistors and the electrolytic capacitor are
installed. The five jumper wires come next. Check the placement on the board again and only then connect
the battery.
Experiment
Even if everything is wired correctly, nothing will happen at first; the yellow LED remains dark. As long as
light falls on the phototransistor, it will keep the LED turned off by means of T2. But once it is dark, the
transistor T2 will switch on the LEDs. C1 ensures that switch-off is delayed so that a momentary shadow or
light incidence won't lead to switching operations that are not intended.
If you are indeed using the circuit as bicycle light, you should use a super bright white LED and there ought
to be a hub dynamo installed. Traffic regulations must be observed.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.31.
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Components required:
1 x 2N3904 transistor,
1 x PT331C phototransistor,
1 x yellow LED, 1 x 470 Ω
resistor, 1 x 47 kΩ resis-
tor, 1 x 100 μF electrolytic
capacitor, 1 x breadboard,
1 x battery, 9V battery with
clip connector
Fig. 2.31: Automatic bicycle light
with phototransistor