AD8232 - Measuring heart rate only if USB connector is touched. Grounding issue?

Question

TL;DR

I can read out my heart rate but I have to physically touch GND of the Arduino or else the signal is just noise.

Description

So I've started to play around with an Arduino and wanted to try out a heartrate sensor module (AD8232).

The following is the code I am using for this:

const int CONNECTED_LED = 13;

const int LO_MINUS = 10;
const int LO_PLUS = 11;

bool disconnected = true;

enum State {
  CONNECTED,
  DISCONNECTED,
  LO_MINUS_DISCONNECTED,
  LO_PLUS_DISCONNECTED
};

enum State currentState = DISCONNECTED;


void setup() {
  Serial.begin(9600);
  pinMode(LO_MINUS, INPUT); // Setup for leads off detection LO +
  pinMode(LO_PLUS, INPUT); // Setup for leads off detection LO -
}


void loop() {
  State nextState = getState();

  if (nextState != currentState) {
    // Store the current state
    currentState = nextState;
    switch (nextState) {
      case DISCONNECTED:
        break;
      case LO_MINUS_DISCONNECTED:
        break;
      case LO_PLUS_DISCONNECTED:
        break;
    }
  }

  if (currentState == CONNECTED) {
    digitalWrite(CONNECTED_LED, HIGH);
    Serial.println(analogRead(A0));
  } else {
    digitalWrite(CONNECTED_LED, LOW);
    // Serial.println(-1);
  }

  delay(1);
}

State getState() {
  if (digitalRead(LO_MINUS) == 1 && digitalRead(LO_PLUS) == 1) {
    return DISCONNECTED;
  } else if (digitalRead(LO_PLUS) == 1) {
    return LO_PLUS_DISCONNECTED;
  } else if (digitalRead(LO_MINUS) == 1) {
    return LO_MINUS_DISCONNECTED;
  }
  return CONNECTED;
}

This is how I am connecting the boards (I double checked already twice)

AD8232  | Arduino UNO R3
------------------------
GND     |  GND
3.3v    |  3.3V
OUTPUT  |  A0
LD-     |  ~10
LD+     |  ~11
SDN     |  <not used>

I've followed this diagram for connecting the electrodes:

However, the result is a random signal unless I am touching the USB-connector or GND of the Arduino.

Below you can see the random signal and the moment I touched the connector. I noticed this by accident after hitting the reset button after wondering why on earth I only get this random signal:

So.. what's going on here? I have all three electrodes connected.

I've even tried to different modules (I bought the AD8232 two times) but I see the same issues with both modules.

If I touch something else e.g. a radiator (heater) then the signal becomes another kind of random. It only works touching the connector.

Since this appears like a grounding issue I checked for continuity from the GND pin of the AR8232 to the USB connector and continuity is given.

I'm not expert enough (yet) to see the problem. Anybody?

Answer 1

One of your three ECG leads is equivalent to (human)body ground, and it's not making contact. I spent too long making this image to explain it, which should help you a lot more than it will me. I mean, I don't have the board in my hands, so I cannot see which lead goes to which pin. You can, easily, with a multimeter testing for continuity between the: RIGHT ARM, RIGHT LEG, LEFT ARM (RA, RL, LA) solder spots, and the tip, ring, barrel of the jack. Something to keep in mind though is the headphone jack switch. When the plug is physically inserted, this switch opens. Again, I'm not there to guide your multimeter, so this image is the best I can do for you. One of your leads is going to be a grounding lead. It may be the "black" one, it may be the (traditionally) "RL" one, it may be another one, i can't tell.

When you touch USB ground you are effectively replacing the body contact of the missing ground lead. I made these images for you to better understand it. You should be able to fault find this in a few minutes via you MM and continuity testing.

Answer 2

Make sure you unplug the PC charger. I unplugged it and the module then worked fine for me as shown in this video Here