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Topic: Read voltage drop from photoresister without a second resistor? (Read 1 time) previous topic - next topic


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: [Select]

+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: [Select]

+5V --> PhotoResistor --> . --> Ground
                        Pin X

Thanks a lot!


Because if the photoresistor goes to 0 ohms, you'll have a dead short across your 5v to 0v....


Also pin x is connected to ground, so your reading will allways be zero


Remember the path to the answer is always the path of least resistance!
Don't breath in the magic smoke!




Jan 18, 2013, 03:42 am Last Edit: Jan 18, 2013, 03:45 am by JimboZA Reason: 1

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:


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).
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


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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