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MODULE V - FUNDAMENTALS OF ELECTRONICS VOLTAGE DIVIDERS What is a voltage divider? You are going to find out but don't be in too much of a hurry. Work through the Chapter and allow the explanation to develop. The diagram below shows a light dependent resistor, or LDR, together with its circuit symbol:

The light-sensitive part of the LDR is a wavy track of cadmium sulphide. Light energy triggers the release of extra charge carriers in this material, so that its resistance falls as the level of illumination increases. A light sensor uses an LDR as part of a voltage divider. The essential circuit of a voltage divider, also called a potential divider, is: As you can see, two resistors are connected in series. with Vin , which is often the power supply voltage, connected above Rtop . The output voltage Vout is the voltage across Rbottom and is given by: It may help you to remember that Rbottom appears on the top line of the formula because Vout is measured across Rbottom . KEY FORMULA: This is an extremely useful formula which you should learn thoroughly What happens if one of the resistors in the voltage divider is replaced by an LDR? In the circuit below, Rtop is a 10 resistor, and an LDR is used as Rbottom : Suppose the LDR has a resistance of 500 , 0.5 , in bright light, and 200 in the shade (these values are reasonable). When the LDR is in the light, Vout will be: In the shade, Vout will be: In other words, this circuit gives a LOW voltage when the LDR is in the light, and a HIGH voltage when the LDR is in the shade. The voltage divider circuit gives an output voltage which changes with illumination. A sensor subsystem which functions like this could be thought of as a 'dark sensor' and could be used to control lighting circuits which are switched on automatically in the evening. Perhaps this does not seem terribly exciting, but almost every sensor circuit you can think of uses a voltage divider. There's just no other way to make sensor subsystems work. Here is the voltage divider built with the LDR in place of Rtop : What effect does this have on Vout ? The action of the circuit is reversed. that is, Vout becomes HIGH when the LDR is in the light, and LOW when the LDR is in the shade. Substitute the appropriate values in the voltage divider formula to convince yourself that this is true. More on Voltage Dividers >>>

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