An LDR is a component whose resistance is dependent on the amount of light that shines
• When it is dark it has a high resistance.
• When it is light it has a low resistance
They are used in street lamps to measure the light level in order to switch the lamp
on and off as appropriate.
Light Dependent Resistors (LDR)
A thermistor is a component whose resistance is dependent upon its temperature.
There are two different types:
• Negative Temperature Co-efficient (NTC)
• Positive Temperature Co-efficient (PTC)
The resistance of NTC thermistors decreases as the temperature increases.
The resistance of PTC thermistors increases as the temperature increases.
Thermistors (NTC and PTC)
Magnetic Reed Switch
A reed switch consists of two metal strips housed inside a glass envelope. These
two strips are arranged to form a switch contacts. Because one of the strips is steel,
when a magnet is brought in proximity the switch contacts close.
The picture below shows the components often found on security systems in shops and
houses. The magnet is fixed to the door and the reed switch to the doorframe. They
are arranged in a fashion so that they are in proximity when the door is closed and
apart when the door is opened. A circuit connected to the reed switch can then activate
Reed switch symbol
Magnet in plastic casing which is fixed to the door
Reed switch in plastic casing which is mounted on the doorframe.
Tilt switches are a type of switch that opens and closes its contacts depending on
its position. They are either metal canisters containing a tiny metal ball or a glass
bulb containing a small amount of mercury. The use of mercury is restricted due to
it being highly toxic.