I have connected an Arduino Pro-Mini 3.3V 8MHz to an ESP8266 via I2C.

The Arduino is in charge of collecting data from a thermistor (analog temperature) (T2) and a SI7021 (digital temperature) (T1).

The ESP8266 is in charge of sending the data received from the Arduino to an endpoint.

Here is my repo: https://github.com/ClemRz/Solar-Water-Heater-Monitor. In the res folder you will find a picture of the circuit.

The issue I am having is that as long as I leave the FTDI RS232 programmer connected to either the Arduino or the ESP I'm having a smooth response from the thermistor but as soon as I disconnect it, the response get some noise. What is happening?

Here is a graph where we can see 3 phases:

  1. Programmer disconnected: some noise can be seen
  2. Programmer connected: smooth noiseless curve
  3. Programmer disconnected again: noisy

enter image description here

If I remove Rx and Tx and I leave GND connected to the FTDI I also observe that the reading is smooth and noiseless.

EDIT: I switched the wall adapter with a better quality one and it fixed the noise issue.

  • \$\begingroup\$ Some breadboards are not connected all the way through. As in, for example in your picture, the left half of the ground of the breadboard is possibly connected to the right half through the ESP. Can you post an actual photo of the circuit and where you are plugging the FTDI? \$\endgroup\$
    – Wesley Lee
    Commented Oct 6, 2017 at 3:52
  • \$\begingroup\$ Thanks Wesley Lee, I'm not using a breadboard, I'm using this kind of board: i.ebayimg.com/images/g/mO0AAOSw32lY1FX7/s-l500.jpg Here is a photo of the actual circuit with FTDI's GND connected: !pic \$\endgroup\$
    – ClemRz
    Commented Oct 9, 2017 at 20:36
  • 1
    \$\begingroup\$ If you solved your problem, post an answer and accept it. Then the system will know you have a solution and not keep promoting the question to the front page for more answers. No need to change the title. \$\endgroup\$
    – The Photon
    Commented Oct 11, 2017 at 17:15

2 Answers 2


It is a common mode noise coupling to the signal from Rx,TX. Use CM choke on thermistor or shielded twisted pair with shield grounded at input only.

This is the result of an unbalanced input impedance and common mode stray coupling.

  • \$\begingroup\$ Hi Tony, I'm not sure I follow, do you mean that Rx and Tx from the FTDI RS232 is inducing? In that case why would it cancel the noise when the FTDI is connected? Thanks. \$\endgroup\$
    – ClemRz
    Commented Oct 9, 2017 at 20:39
  • \$\begingroup\$ Shielding,twisted pairs and Common mode chokes help reduce CM noise while earth grounding it drops the common mode voltage at that point. ( but opens possibility for ground faults) There are many ways to solve it depending on noise source and wire layout. \$\endgroup\$ Commented Oct 9, 2017 at 20:47

Switched the wall adapter with a better quality one and it will fix the noise issue.


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