What does it do?
| The light sensor is used to detect changes in the amount of light in its surroundings. |
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How does it operate?
The usual light sensor circuit gives a low voltage when it is dark and a high voltage when the light is bright.
The light sensor can be followed by a subsystem that processes analogue signals. If the changes in the light level are small and the signal is to be fed to a digital subsystem then the light sensor needs to be followed by a comparator or Schmitt inverter.
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Normal circuit (light sensor)
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Inverted circuit (dark sensor)
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The light sensing circuit uses an LDR (light-dependent resistor) to measure light.
In the normal circuit, the output voltage increases as the light level increases.
In the inverted circuit (the dark sensor) the output voltage falls as the light level increases. |
| Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The light sensor forms a potential divider whose output voltage is determined by the upper and lower parts of the circuit. |
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The resistance of the LDR falls as the amount of light falling on the sensor increases. |
Possible applications
- System to check if it is day or night
- An object detector (object blocks light beam)
Making
How part of the PCB might look
The PCB shows the light sensor.
In the dark sensor the position of the two components is reversed.
Testing
Make sure that the signal going out (on the green PCB track) changes from high to low when the light level changes from bright to dark.
In the case of the dark sensor, the signal should be high when it is dark.
How part of the PCB might look
Fault finding
If there is a fault, check that:
- The variable resistor is at its mid point.
- The resistance of the LDR changes when the light level changes (if not, replace it).
- The resistance of the variable resistor changes when it is adjusted (if not, replace it).
If there is a fault, check the tracks and solder joints.
Alternatives
- There are several alternative types of LDR. The traditional type is the ORP12, but smaller ones are cheaper (but may need a higher variable resistor)
- Photodetectors (phototransistors, photodarlingtons and photodiodes) are smaller and cost less.
- Modulated infrared (IR) receivers are useful for detecting the IR signal from TV-style remote controls.
- Optoswitches are useful for detecting when an object is near the sensor, but they are more expensive.
- A Hall effect sensor can be used to detect if a magnet is close
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