Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 78

Circuit 55: Light sensitive switch with phototransistor
Setup information
Install the three transistors, the LED and the two resistors as shown in Fig. 2.55. The five jumper wires
come next. Check the placement on the board again and only then connect the battery.
Experiment
This circuit tells you whether the sensor surroundings are bright or dark. The sensor functions as phototran-
sistor. When lit, this becomes electrically conductive. Transistor T1 is thus triggered and the red LED turns
on. It can be utilised in many ways: Light barriers, remote release for animal photography, room surveil-
lance and the like. With wiring adapted, also an exposure meter for enthusiastic photographers could be
accomplished.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.55.
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Components required:
2 x 2N3904 transistors,
1 x PT331C phototransistor,
1 x red LED, 1 x 1 kΩ resis-
tor, 1 x 100 kΩ resistor,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.55: Light sensitive switch
with phototransistor