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| LIGHT DEPENDENT RESISTORS | |
| V. Ryan © 2002 - 2022 | |
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LDRs 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. |
| The animation below shows that when the torch is turned on, the resistance of the LDR falls, allowing current to pass through it. | |
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Circuit Wizard software has been used to display, the range of values of
a ORP12, LDR . When a light level of 1000 lux (bright light) is directed towards it, the resistance is 400R (ohms). When a light level of 10 lux (very low light level) is directed towards it, the resistance has risen dramatically to 10.43M (10430000 ohms). |
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Opposite is another example of a light sensor circuit : 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. 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. |
| CIRCUIT WIZARD SOFTWARE SIMULATION OF DARK SENSOR CIRCUIT | |
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| When the LDR is covered the LED illuminates | |
| PICTORIAL REPRESENTATION OF THE DARK SENSOR | |
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1. The circuit above is a light sensor. That means
light must shine into the LDR for the circuit to be activated. Draw a
circuit composed of the same components that activates when it is DARK
(when the LDR is covered). This is a typical examination question. 2. What is the role of the preset resistor ? |
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