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VMA501
V. 02 – 05/04/2019 13 ©Velleman nv
7.6 The Photosensitive Transistor
A phototransistor is a transistor whose resistance varies according to different light strengths. It is based on the
photoelectric effect of a semiconductor. If the incident light is intense, the resistance reduces; if the incident
light is weak, the resistant increases. A phototransistor is commonly applied in the measurement of light, light
control and photovoltaic conversion.
Let’s start with a relative simple experiment. The phototransistor is an element that changes its resistance as
light strength changes. Refer to the PWM experiment, replacing the potentiometer with a phototransistor. When
there is a change in light strength, there will be a corresponding change on the LED.
Required Hardware
 1 x phototransistor
 1 x red M5 LED
 1 x 1 KΩ resistor
 1 x 220 Ω resistor
 1 x breadboard
 jumper wires as needed
Connection