I am new to Arduino and physical computing When I measure the wire coming from any UNSET Arduino uno pin, the result is approximately 2 volts even the "ANALOG IN" pins. Considering that the pins which been set to HIGH and LOW are giving 5 and 0 volts "which is cool". Is that the nature of Arduino or is that a failure caused by short or anything else?

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    Unconnected pins float near a certain voltage. If the voltage bothers you somehow just attach a pull-down resistor to them.
    – Avamander
    Feb 25, 2016 at 17:57
  • @Avamander I am trying to measure a small voltage from some kind of sensors which is between 0.2 - 0.6 volts to the analog in pin A0. I get my hands shocked when I do that because of the 2 volts getting out of the A0. I am worried about the sensor, I think the 2 volts is not good for it. Feb 25, 2016 at 18:03
  • If your getting your hands shocked when touching the Arduino pins, maybe you need to ground your power supply... You can get badly hurt or even dead in some conditions!
    – fabrosell
    Feb 25, 2016 at 18:18
  • @fabrosell my power supply is my laptop's USB Feb 25, 2016 at 18:21
  • I think this is a simple question for expert users. Feb 25, 2016 at 18:23

3 Answers 3


After a microcontroller reset, AVR IO pins default to input. In this state they have high impedance. For example, according to the Atmel ATmega328 spec sheet doc8271 Table 29-15, analog input impedance typically can be taken as 100 MΩ for design purposes. (Input pin impedance may in fact be orders of magnitude more, effectively an open circuit except for leakage currents of a few nanoamps.) Each pin has a few pF (5-10?) of capacitance [1,2,3,4] which in general is small compared to stray capacitances of attached wiring.

When stray electromagnetic fields charge the pin and stray capacitances, a small voltage develops on the pin. You could connect a 1 to 10 MΩ resistor between the pin and ground, to avoid the 2 V you are seeing.

  • Does that mean that it is natural fact to have the 2V and that the solution is to connect a resistor with the input wire to solve that problem? Feb 25, 2016 at 19:05
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    It's natural for arbitrary voltages to appear when you have an antenna (the wiring) attached to a capacitor (the wiring and pin). BTW, how did you measure the voltage? Many cheap DVMs have fairly low input impedance (about 1 MΩ) so you might see a voltage upon first connecting, but it should soon drop to near zero unless you are using long leads or have high EM fields in the area. Getting a shock suggests hundreds of volts, not 2 V. Feb 25, 2016 at 19:14
  • @AhmedAl-sabsab with the sensor connected to the Arduino there should no longer be 2 volts on the input pin that it is connected to; if there is then there must be an error in the way you have connected everything. You might need to provide more information on what you're actually doing - by editing the question. Feb 25, 2016 at 19:31
  • I have used an expensive multi-matter to measure the voltage. I also used Serial.println(volt); to print the result to the PC screen and I get the same result. the shock stills a mastery. it is a tiny shock not a killing one of course. Feb 25, 2016 at 19:32
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    Attach one end of a 10K resistor to ground, and the other end to a wire. Look very closely (for tiny sparks) as you move the probe end of the wire towards the input. If no sparks, probably the shocks you've seen are from static electricity in your body. You can also hold the probe end of the wire between thumb and ring finger, then touch the input with forefinger. (Generally, don't let shocks go across your body.) Having no shock in this case (where the probe is grounding your hand) would be consistent with static-electricity-in-your-body theory. Feb 25, 2016 at 20:32

The electric shocks being felt is due to leakage current from the laptop's switchmode power supply. All unearthed mains powered switchmode plug pack power supplies leak a few micro amps and voltages in excess of 80 volts AC as measured from the plug packs output socket to ground will be observed when measuring with multimeters having 10 meg ohm input impedance.

The solution is to have the GND pin of the arduino board connected to earth. Here in Australia all computer power supplies are grounded via the 3 pin power plug.


The pin is default to Tri-State or High-impedance(Hi-Z) which means they do not either sink or source current. Just as others say it's a matter about electromagnetic or some such. A voltmeter can measure its voltage because the current it takes is small enough(close to or smaller than the leakage current). The real value is undefined when you are not measuring it. Instead of reading analog pin values, You can try reading digital pins to take a better look at "floating".

A Hi-Z pin can absolutely power nothing from LED to other big stuffs. If all you want is just stable readings, well, this is where pull-up or pull-down resistors come in place. Typically these resistors can be from 10KΩ to 10MΩ, ensuring that when the pin is not connected they will provide the pins with stable values while when connected the influence of pulls can be safely ignored.

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