Floating input and voltage on d5 and d9

Continuing the discussion from Ask a littleBits engineer:

if you run a blank sketch on the arduino the arduino pins default to INPUT. if you hook the bargraph or the number module to the output of d5 or d9, you will see there is about 1.1v on the pins. It doesn’t really matter if the d5/d9 switches are in ANALOG or PWM. Looking at the schematic, presumably this voltage is related to the opamp/RC stuff on the module for d5 and d9. I’m curious why this is 1.1v?

(of course, if you set d5/d9 to OUTPUT, then you will have 0 volts on those pins)

Hi manitou,

In the default state, your GPIO pins are likely in tristate mode. In this configuration, when nothing is attached to it there is a very high impedance between the pin and ground and voltage. If there was nothing attached to the pin – which of course isn’t an option because of the filter/buffer soldered to the board – you would likely see the reading on that pin become unstable.

But why are you seeing 1.1V on that pin? I would think that the impedance from that pin to the external circuitry, while high, is low enough to drive the pin to a stable voltage reading, in this case 1.1V.

If you want to experiment further, set those pins as inputs but set the internal pull-up resistors (about 20k ohm). You will likely see the pin read high.

I did some more experiments (without the arduino) just using the proto module and the fork module. The fork module has the LV321 opamp on it as a voltage follower/buffer.
The setup is: power to proto to fork to number.

With all jumpers in place, number reads 99 (5.0). no surprise.

if you pull the signal jumper on the proto module (so you have a floating input to LV321), then the number module now reads 24 (1.2), and volt meter shows 1.176v from the fork module. SO floating input to LV321 results in 1.2v out.

(Rev 3 of arduino UNO and mega2560 exhibit same phenomenon with the on-board LED. The LED is fronted by an LM358 opamp. if LED pin (13) is in INPUT mode (floating), then the LED is on. The LM358 generates 3.8v with floating input (5v supply))