This is a resistor which has a resistance that changes with the amount of
LIGHT that falls on it. The light energy produces more
free electrons which increases the current for a certain voltage across the LDR which means
a drop in resistance.
In the DARK its resistance is LARGE
(millions of ohms), in the LIGHT its resistance is SMALL (tens of
ohms). Very little current will flow through it in the dark.
A graph for the LDR of
resistance against light intensity is shown in Figure 1.
LDRs are used as light
sensors to switch on devices such as street lamps when it gets dark and to operate curtains
at night time. They can also be part of a fire alarm that detects the flame of a
fire.
This is a type of resistor which has a resistance that changes
with TEMPERATURE. The increasing temperature
produces more free electrons and so the resistance falls.
At LOW
TEMPERATURE its resistance is LARGE (thousands of ohms), at HIGH
TEMPERATURE its resistance is SMALL (tens of ohms). Very little current
will flow through it when it is cold. This means that its resistance increases as the
temperature falls and this type of thermistor is called a negative temperature coefficient
thermistor or ntc thermistor.
A graph for the thermistor of resistance against its
temperature is shown in Figure 2.
You can investigate this property practically by
doing the experiment described in the 14-16 Experiment guides called Thermistor
resistance.
(See: Thermistor resistance variation)
Thermistors are used as temperature sensors in thermostats in
ovens and irons, in fire alarms and on the wing of a plane to detect when the temperature
falls low enough for ice to form. They are also in use in premature baby units to detect when
a baby may have stopped breathing.