OP-amp mounted ? but why ?

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Zubeen

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I bought arduino board using atmega8 micro controller and i got IR sensing boards with it. the board are mounted with an op-amp LM358. I am not getting the purpose of that op-amp. please explain !
 

Zubeen said:
I bought arduino board using atmega8 micro controller and i got IR sensing boards with it. the board are mounted with an op-amp LM358. I am not getting the purpose of that op-amp. please explain !
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Op amps are of great importance in electronics, they can be used for may applications. I think the 358 op amp is used to amplify the input to the board.
 

peeyushsigma said:
In IR sensors LM358 Op amp is used as a comparator.
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yes i have read some where about comparators but i dont really know what the are......
please explain !

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OK OK maybe i got it ,
The sensor board on it have a potentiometer so the when we change the pot meter's resistance the reference level to the op-amp must be changing. when different intensities of ir are sensed by photo diode it must be producing different decrements in voltage across it . but whenever this voltage across the photodiode must be getting greater or lower to that being input to the op amp (as a reference level ) the op-amp must be telling by giving 1 or 0 .
Am i right ? if yes this must be the circuit diagram.
but there is one capacitor also ? why ?
 

Your description of a comparator is correct. The output switches depending on if the input voltage is above or below the reference voltage. The capacitor, if connected to the reference signal acts as a filter to stabilize the reference. If it's connected to the OP-AMP power supply input, it is acting as a decoupling cap and is helping to quiet the power supply noise.
 
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    Zubeen

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Thank you , but if same condition appears before me .... how should i then decide the rating of my capacitor to reduce the noise ?
i have usually seen using 104 but whats the logic behind ?
 

I don't remember where I read/heard/learned it but I have always used 0.1uF capacitors for decoupling the power supply and 1.0uF capacitors to stabilize the reference voltage. Just a rule-of-thumb I picked up someplace that has worked fine for meover the years.
 

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