Hi I am new to electronics, and I have made a light tripwire using an Arduino Uno. I made this on the idea that electricity follows the path of least resistance, which I now know to be false (https://physics.stackexchange.com/questions/5670/electricity-takes-the-path-of-least-resistance). False or not however, my tripwire works exactly as I would expect it to.

I programmed my board to sound an alarm if pin 2 reads low voltage I thought it worked because if the photoresistor has less resistance than the resistor, then it would read high voltage and vice versa. So can anyone explain what is happening?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Sorry, I'm new to electronics and don't know the resistance of R1 (I don't know how to read colour codes). \$\endgroup\$
    – TheSporech
    Dec 7, 2015 at 16:14
  • 4
    \$\begingroup\$ too bad that there are no resources on the interwebs on how to easily read them \$\endgroup\$
    – PlasmaHH
    Dec 7, 2015 at 16:14
  • \$\begingroup\$ Lookup voltage dividers and calculate the voltage at your pin for various light situations. \$\endgroup\$
    – Arsenal
    Dec 7, 2015 at 16:26

1 Answer 1


It's called a voltage divider. "The path" does not change. The current changes, because the total resistance LDR1+R1 changes while the voltage applied remains fixed, so the voltage across R1 (which is what you are measuring at U1 pin 2) changes.

  • \$\begingroup\$ Cool thanks, that sounds like what i'm looking for. \$\endgroup\$
    – TheSporech
    Dec 7, 2015 at 16:44

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