
www.picaxe.com/products/axe171
AXE171 PICAXE-14 Audio Kit © Revolution Education Ltd 2015 v1.1
May be copied for educational use.
14
4.3 Using the LDR
4.3.0 What is an LDR?
A Light Dependent Resistor (LDR) is special type of resistor that reacts to changes in light level. The
resistance of the LDR changes as different amounts of light fall on the top 'window' of the device.
This allows electronic circuits to measure changes in light level.
4.3.1 What are LDRs used for?
LDRs are used in automatic street lamps to switch them on at night and off during the day. They are
also used within many alarm and toys to measure light levels.
The LDR is a type of analogue sensor. An analogue sensor measures a continuous signal such as light,
temperature or position (rather than a digital on-off signal like a switch). The analogue sensor
provides a varying voltage signal. This voltage signal can be represented by a number in the range 0
and 255 (e.g. very dark = 0, bright light = 255).