Starter Guide

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Lesson 15 Photoresistor
Introduction
In this lesson, let’s learn about the use of the external photoresistor of the
micro:bit.
What is photoresistor?
A photoresistor is a light-controlled variable resistor. The resistance of a
photoresistor decreases with the increasing incident light intensity; in other
words, it exhibits photoconductivity. A photoresistor can be applied in light-
sensitive detector circuits.
A photoresistor is made of a high resistance semiconductor. In the dark, a
photoresistor can have a resistance as high as a few megohms (MΩ), while
in the light, a photoresistor can have a resistance as low as a few hundred
ohms. If incident light on a photoresistor exceeds a certain frequency,
photons absorbed by the semiconductor give bound electrons enough
energy to jump into the conduction band. The resulting free electrons (and
their hole partners) conduct electricity, thereby lowering resistance. The
resistance range and sensitivity of a photoresistor can substantially differ
among dissimilar devices. Moreover, unique photoresistors may react
substantially differently to photons within certain wavelength bands.
A Analog output
+ 3.3V
- GND