I am working on a phase detection project in which i am trying to find the order in which the incoming signal comes. I converted the 240v 50hz Ac signal to a 7v 50hz square pulse. when i connect one(7v 50hz) of the three lines to a digital pin on arduino it is properly detecting, but the problem is if i check the unconnected pins via oscilloscope i am getting the same signal which i connected as input, so when i connect another line to any other pin the two signals are getting added.

(Image 1)this is what i see in the DSO when i connect one line to the arduino pin and observe other pins .(Image 2)this is what i get after i give another line as input to arduino and observe the pins. i tried with 2 to 3 boards but i still get the same problem.


simulate this circuit – Schematic created using CircuitLab

this is the schematic i used for converting the sine wave into square wave, i used 4n33 optocoupler for this purpose.

  • 1
    7V into a 5V input pin...? It's not the input pin that you're seeing the noise on, it's the entire Arduino. If you operate out of specs then you can expect strange things to happen. – Majenko Jan 19 '19 at 22:14
  • actually i converted 7v to 5v and tried but still i got the same result, so i tot it was not the problem. – user2998792 Jan 19 '19 at 22:17
  • Is your signal purely positive, or is it ±7V? – Majenko Jan 19 '19 at 22:27
  • what exactly is the project that you are trying to build? – jsotola Jan 20 '19 at 3:29
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    An input pin has a very high impedance. It is so high, that it can read a signal from a nearby pin. Everything changes when you connect a oscilloscope to that pin. You know that the ground wire of a oscilloscope is connected to earth ground of the power plug? You are trying to read the mains 50Hz and that oscilloscope introduces probably the most 50Hz noise. Can you show us a schematic or drawing of your circuit? – Jot Jan 20 '19 at 3:54

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