An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.
LDR circuit symbols
Typical LDR resistance vs light intensity graph
The most common type of LDR has a resistance that falls with an increase in the light intensity falling upon the device (as shown in the image above). The resistance of an LDR may typically have the following resistances:
Daylight = 5000Ω = 5kiloΩ = 5kΩ
5k for short. This may vary from design to design.
Dark = 20000000Ω =20Mega Ω = 20MΩ
This goes to show that there is a large variation between these figures. If you plotted this variation on a graph you would get something similar to that shown by the graph shown above.
Applications of LDRs
There are many applications for Light Dependent Resistors. These include:
1. Lighting switch
The most obvious application for an LDR is to automatically turn on a light at a certain light level. An example of this could be a street light or a garden light.
2. Camera shutter control
LDRs can be used to control the shutter speed on a camera. The LDR would be used to measure the light intensity which then adjusts the camera shutter speed to the appropriate level.
A typical LDR controlled transistor circuit
The circuit shown above shows a simple way of constructing a circuit that turns on when it goes dark. In this circuit the LDR and the other Resistor form a simple ‘Potential Divider’ circuit, where the centre point of the Potential Divider is fed to the Base of the NPN Transistor.
When the light level decreases, the resistance of the LDR increases. As this resistance increases in relation to the other Resistor, which has a fixed resistance, it causes the voltage dropped across the LDR to also increase. When this voltage is large enough (0.7V for a typical NPN Transistor), it will cause the Transistor to turn on.
The value of the fixed resistor will depend on the LDR used, the transistor used and the supply voltage.
Please read to our blog on resistors in series and parallel to understand the circuit above more.