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Design & Technology Resources
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An LDR is a component whose resistance is dependent on the amount of light that shines upon it.

 

• 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)

 

NTC

The resistance of NTC thermistors decreases as the temperature increases.

 

PTC

The resistance of PTC thermistors increases as the temperature increases.

Thermistors (NTC and PTC)
-t
+t

NTC

Symbol

PTC

Symbol

Temperature

Resistance

Resistance

Temperature

NTC Thermistor

PTC Thermistor

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 an alarm.

Reed switch symbol

Resistance

Light

Level

Magnet in plastic casing which is fixed to the door

Reed switch in plastic casing which is mounted on the doorframe.

Tilt Switches

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.