# Capacitor Substitution

I'm working on this

Light Sensor - Photodiode Project

Based on the circuit, why is such a specific cap required?

Is there something I can substitute for that capacitor? I can't even find that cap on Digikey. Amazon has em if I buy 100 of them...

Secondly, just trying to understand why the circuit cares if it's polarized or not.

100nF is .1uF billions and billions out there. http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_151116_-1

Thanks! Shoulda thought of that.

bradix14: I can't even find that cap on Digikey.

The problem is not finding, it but choosing from so many. :grinning:

Which is exactly what you should do. You will want them sooner or later.

bradix14: Secondly, just trying to understand why the circuit cares if it's polarised or not.

In this case it does not as the voltage is always in the one direction, but it is very poorly explained so you would have to figure out which way it is (and in fact, the positive is the side that goes to the Arduino).

It does not matter as 0.1µF capacitors are mostly ceramic and non-polarised anyway - though you can get tantalum capacitors (polarised) of that value.

Turns out, the reason he found the capacitor was needed, is almost certainly because the circuit is laid out on a breadboard rather than a PCB, and he has not included the capacitor directly across the supply terminals of the op-amp.

Paul__B,

So would I be fine then to just use a .1uf mylar cap I already have?

Or, if I understand you correctly, I could put a cap across the supply voltage to the Op Amp instead of where he has it?

Of the capacitor is for decoupling then you need a ceramic capacitor because it handles high frequencies, a Myler might not be good enough.

http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html

You want one across the power supply AND one where he has it. It is across the feedback and so it adds more negitave feedback for high frequencies and so reduces the gain and so reduces noise on the output of the op amp.