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Light sensor w/ photoresistor & 741 Op-Amp

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wt200999

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I came up with a circuit for sensing light using a photoresistor and a 741 Op-amp. Here is my circuit diagram. (sorry for the crude drawing)

circuit.png

With a power supply at +5v The output I get from the op-am I was expecting to be either 5v or 0v, but instead I get ~4.5v and ~2v for either light or dark. I eventually wanted to interface 3 of these to inputs on 7400 series logic.
 

wt200999 said:
I came up with a circuit for sensing light using a photoresistor and a 741 Op-amp. Here is my circuit diagram. (sorry for the crude drawing)

View attachment 53254

With a power supply at +5v The output I get from the op-am I was expecting to be either 5v or 0v, but instead I get ~4.5v and ~2v for either light or dark. I eventually wanted to interface 3 of these to inputs on 7400 series logic.
Click to expand...

If you don't have a "rail-to-rail" op amp, the output won't swing all the way to 5V and 0V, you'll wind up a few diode-drops from it (due to the transistors internal to the op-amp that swing the output voltage around). You could run your signal into a comparator or schmitt-trigger to clean it up to a nice 5/0V range.
See also: Google
Schmitt trigger - Wikipedia, the free encyclopedia
 
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