# Light sensor using Op Amp

Posted on Feb 5, 2014

A light based sensor using an LDR (Light detecting resistor) and ofcourse an Op Amp. You can use this circuit to make line followers and other stuff. Here we go: Since we are using an op amp to compare we should first be clear about its functioning. Basically an Op Amp has two inputs, an inverting input and a Non-Inverting input. If the voltage at the Non-Inverting input(Denoted by +) is greater

than the input voltage at the inverting input(Denoted by -) the output of the Op Amp will be High, ie. , 5 Volts. But if the voltage at the non inverting terminal (+) is less than the voltage at the inverting input terminal (-) then the output will be Low ie. , 0 Volts. As the value of the resistors is what the voltage depends on, it should understood by now that any change in resistance leads to a change in voltage. Not its time to welcome the LDR in the scene. In the above resistor network replace R2 with the LDR. Now with the increase in the light intensity the resistance of LDR decreases which leads to the decrease in voltage and viceversa The whole theory turns upside down if we connect the LDR in the place of R1, ie. , The output voltage decreases with a decrease in the light intensity and vice versa. Using the same voltage division principle we use a variable resistor and connect it to the other input terminal of the Op Amp. This will be the reference to which the Op Amp will compare the voltage from the resistor network. We manually tune the potentiometer using a screw driver, so that the voltage given by it will be slightly above or below the voltage given by the resistor network. To get the above circuit we should tune the Variable resistor using a screw driver or a knife so that the Vout of the variable resistor is just below the Vout of the resistor network. Doing that is not as complicated as it sounds, You...

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