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Author Topic: Read voltage drop from photoresister without a second resistor?  (Read 655 times)
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I am  starting to play with my Arduino Uno and have a question about hooking a photoresistor to my Uno. Most tutorials connect the photoresistor like this using a second resistor:

Code:
+5V --> PhotoResistor --> . --> 10K --> Ground
                          |
                        Pin X
 

I think this technique is called a "voltage divider" and I understand how it can be used to read the variable voltage drop that occurs from the PhotoResistor.

My question is why can't I just measure the drop without the 10K resistor? In other words, would the following work (where we read the voltage drop from Pin X)?

Code:
+5V --> PhotoResistor --> . --> Ground
                          |
                        Pin X

Thanks a lot!
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Because if the photoresistor goes to 0 ohms, you'll have a dead short across your 5v to 0v....
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Thanks! That makes sense.
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Also pin x is connected to ground, so your reading will allways be zero
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Remember the path to the answer is always the path of least resistance!
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What, don't have a resistor? Use a cap in voltage divider, here is tutorial:
http://learn.adafruit.com/photocells/using-a-photocell
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Also pin x is connected to ground, so your reading will allways be zero

Oh, yeah..... that too. I forgot about that bit.  smiley-red


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« Last Edit: January 17, 2013, 09:45:48 pm by JimboZA » Logged

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You can manage without the second resistor. Connect the photoresistor between the analog input pin and ground, and make a pinMode call to enable the internal pullup resistor by setting pin mode INPUT_PULLUP (or by writing a HIGH to the pin while leaving it set as an input).
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There are many ways to do it: you can use the internal pull-up; you can read it via a capacitor; or an inductor, or a diode, or a current source, etc.

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