LDR is a light dependent resistor. LDR’s or
Light Dependent Resistors are very useful especially in light/dark sensor
circuits. Normally the resistance of an LDR is very high, sometimes as high as
1000 000 ohms, but when they are illuminated with light resistance drops
dramatically. LDR is used to measure the light intensity. It is an input
device, that senses the light and gives the value to microcontroller.
Two
cadmium sulphide (cds) photoconductive cells with spectral responses similar to
that of the human eyes. The cell resistance falls with increasing light
intensity. Applications include smoke detection, automatic lighting control,
and batch counting and burglar alarm systems.
The animation opposite shows
that when the torch is turned on, the resistance of the LDR falls, allowing
current to pass through.
When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light. However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor. The LED lights. The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.
There are just two ways of constructing the voltage divider, with the LDR
at the top, or with the LDR at the bottom:
There are just two ways of constructing the voltage divider, with the LDR
at the top, or with the LDR at the bottom:
You are going to investigate the behaviour of these two circuits. You
will also find out how to choose a sensible value for the fixed resistor in a
voltage divider circuit.
Remember the formula for calculating Vout
Sensitivity:
The sensitivity of a photo detector is the relationship between
the light falling on the device and the resulting output signal. In the case of
a photocell, one is dealing with the relationship between the incident light
and the corresponding resistance of the cell.
Applications:
Analog Applications:
- Camera Exposure Control
- Auto Slide Focus - dual cell
- Photocopy Machines - density of toner
- Colorimetric Test Equipment
- Densitometer
Digital Applications:
- Automatic Headlight Dimmer
- Night Light Control
- Oil Burner Flame Out
- Street Light Control
- Absence / Presence (beam breaker)
- Position Sensor
Related Projects
Temperature
based automatic fan speed ControllerRemote Control Based Home Appliances Control
Season Based Street Light Switching Based On Sensors
No comments:
Post a Comment