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PICAXE-08 ALARM PROJECT
revolution
© copyright 2002 - AXE102 Alarm Student Project Notes Version 1.1
Revolution Education Ltd. Email: [email protected] Web: www.rev-ed.co.uk
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Supported by:
L
IGHT
D
EPENDENT
R
ESISTOR
(LDR)
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.
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).
R(
W
)
Light intensity
(Lux)
dark
light
LDR
0V
5V
Time
Volt
age
dark
light
