Resistance measuring problem


I recently had to make some changes in one of my installation in order to simplify its maintenance.
My installation is basically a 11 meter tall chandelier made of guitars & ukuleles with 2 metallic handles hanging at audience level. When 2 persons, or more, each grab one handle and made skin to skin contact, the arduino measure the resistance in the bodies of these persons and activate one or more dc motors installed close to the strings of the instruments.
It allows the audience to play the guitars and ukuleles by touching each other.
You can have a look on my website :’ingre.html

This installation has been working perfectly for the last 6 months. Unfortunately, for a lot of different reasons, I had to make some change in order to power the all installation with only 1 power box placed under the roof.
I realised that I actually could power my 3V DC Motors with 5V without damaging them (I checked online and made some tests), so I decide to increase my power box at 5V and use it also to power my arduino.
I had to make these changes very quickly so I decide to keep the usb cable that was originally plugged in my arduino, and I connect it to a female usb plug that I took from a charger. I then stripped the cable going out of the female USB plug and connect it directly to the power box.

Unfortunately, now the motors are playing on their own. It looks like the resistance can’t be measured any more. I think it comes from the fact that the changes I made, have kind of connected the digital ground with the analog ground from the arduino.

I made a schema from both the original situation and the one after my changes.

If you have any advice, please let me know.

Thank you for your help.

All the best,


I’d check those transistors. Do they perform identically after you changed from 3V to 5V

All 3 Ground pins on the Uno are connected together, so there is no digital Ground or analog Ground, just Ground.

You need to run a test sketch - such as "blink without delay" - and then some test sketches that simply cycle the motors on and off, to determine whether the Arduino is operating properly.

Don't feed the regulated 5V (which of course, you have very carefully checked to ensure it is 5V) through the USB cable, just connect it to the "Vcc" pin on the Arduino. You probably need a fairly large capacitor - about 100µF across the Vcc and ground connections where this feeds the Arduino. You should have separate supply wires from the 5V supply to the Arduino, and to the motor board. You should have separate ground wires from the supply ground, to the Arduino, and to the motor board. You should have no connections from the Arduino ground to the motor board, only those ground connections to each from the power supply.

The most likely problem is interference - transients - generated by the motors. When you were running them from a separate supply this was not such a problem.

I don't know that those diodes are doing between the emitter and collector of the drive transistors - very little I suspect. They should be across the motors instead.

Thank you Paul.

There seems to have some interferences generated by the motors as you said and I will follow your advices.

All the best,