4.3 LDR devices - light dependent resistors - how they work and what they are used for

Circuit 44 shows how you can investigate the resistance of an LDR in varying light conditions.

The voltmeter is wired in parallel with the LDR, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

You must decide on the initial p.d. and see how the current varies.

You calculate the resistance of the thermistor from Ohm's Law equation: V = IR, so R = V/I

Somehow you need to vary the light intensity shining on the LDR resistor e.g. using a lamp working of variable resistor and taking a reading with a light meter as well as the p.d. and current readings.

Since an LDR is a light dependent resistor and you should find ....

The higher the light intensity, the lower an LDR's resistance, the greater the current flow for a fixed p.d..

i.e. high resistance in darkness and low resistance in bright light.

You can see this trend clearly in the resistance - temperature graph for an LDR.

Therefore an LDR can respond to changes in light intensity e.g. daylight/night time.

 

 

At constant temperature and constant light intensity, the current voltage graph for an LDR is linear, the same as for a fixed resistor (left graph 1), so it is an ohmic resistor at constant temperature.

Since the circuit system will sense the presence of light, that is the basis of a thermistor's applications.

 

 

Uses of an LDR

LDR resistors are used automatic control of lights at night - outdoor lighting, burglar alarm circuits, light intensity meters.

Circuit 33 shows in principle how to control the output of a lamp bulb.

(real LDR circuits are more complicated)

In this case the 'active' component, the bulb, is wired in parallel with the LDR response resistor.

In this case the p.d. across the LDR and the lamp bulb is the same, though the LDR is a variable resistor.

In dim light or darkness, the p.d. across the LDR and bulb is high because LDR's resistance is high.

The greater the p.d. across the lamp the greater the power output (P = IV), so the bulb lights up - glows more brightly as the surroundings get darker.

If the surroundings e.g. a room or a garden path gets brighter, the LDR's resistance decreases, the p.d. decreases, so the power output decreases and the lamp glows dimmer or 'goes out'.

 

As well as automatic night lights, an LDR can be used in a burglar alarm circuit.

A small and constant beam of light is shone on an LDR (it can be from an invisible infrared emitting LED). If the shadow (of the burglar) crosses the light beam, the intensity of light falling on the LDR is reduced. Therefore the resistance of the LDR is reduced and this triggers an alarm.

A simple light meter can be made by connecting an LDR in series with a battery and an ammeter.

The brighter the light, the lower the resistance of the LDR.

The lower the resistance, the greater the current flow, so the ammeter reading is a measure of the light intensity.