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There are many inexpensive ear clip pulse sensors such as this one available with a standard 1/8" audio jack. I would like to know how to read this sensor with a microcontroller and calculate heart rate.

I found a Grove kit for an ear clip sensor but I don't see the design files for it anywhere (The link on the wiki for Eagle files only contains an Arduino sketch).

Does anyone know where I can find a pinout for this sensor or an example amplifier circuit?

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  • \$\begingroup\$ I guess it has a ground, a supply voltage and an output pin just like IR sensors. The output varies according to pulse. You just count the oscillations in a time period. \$\endgroup\$
    – Cornelius
    May 21, 2014 at 15:38

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I'm pulling off of their wiki, but I don't think you need an amplifier. http://www.seeedstudio.com/wiki/Grove_-_Heart_rate_ear_clip_kit If you check out their measured signal, it's a very clean 5 volt pulse. Sensors are never that nice. The white box inline with the signal is likely cleaning up the signal and doing some thresholding. This may be either good or bad for you, depending on how much control you need.

Their example code will be a nice starting point for actually measuring heart rate, although it could definitely be improved significantly.

As for the pin-out, we're left guessing. If we look at the base shield, we can see that it's labelled, from the top-most pin, "D1, D2, V, G". Comparing that to the ear-clip's connector, that tells us that yellow should be our data pin, red is 5 volts, and black is ground.

Now, if you're looking at using just the clip, without the additional signal processing module, things get a little saucier. Now you probably have to do some reverse engineering. If you have the signal processing box available, plug a 3.5 mm plug in, and measure for a voltage. It may or may not be 5 volts. You may be able to assume that ring2 is ground (maybe). Tip might be signal, but who knows.

If you're really lucky, you may be able to figure this out without even the signal processing box. Chances are, the ear clip is just an opto-coupler. One side will be a transmitter, maybe an infrared LED, and the other will be a receiver, maybe a photodiode. Your pulse will change the amount of light allowed to pass through your ear, giving you a nice, noisy, analog signal. You could test this by grounding what you hope is ground, and putting a current limited voltage through something like a 100K resistor into another input. Keep the clip in darkness so the photodiode won't allow any current to flow. Slowly increase the voltage until three volts or so, at which point the IR LED should have turned on. Measure from the resistor-clip node to ground. If this is less than 3 volts, that means current is flowing, which means you've found the LED and ground. This leaves the remaining pin to be the photodiode (i.e; your signal).

Hopefully that will get you started.

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Geert Langereis has published some detailed info on PPG circuits here:

https://www.cs.tau.ac.il/~nin/Courses/Workshop12a/PPG%20Sensor%20System.pdf

http://www.geertlangereis.nl/sensorwiki/doku.php?id=methods:ppg:ppg_kyto_hrm2511

The tip is a photodiode input, the ring is the photo-transistor collector, and the sleeve is ground.

A typical electronic circuit for PPG has four functions:

  • Biasing of the LED
  • Biasing of the photo-transistor
  • Remove low-frequency motion artefacts and drift (high pass filter)
  • Isolate the heartbeat pulse (lowpass filter at about 2Hz)

In the first stage, the LED is biased. For an operational voltage of 5V, a resistor of (Vcc - Vf) / If = (5V - 1.2V) / 65mA = 58Ω. (based on spec sheet for photodiode forward voltage and current)

In the next stage, the photo-transistor is biased. The choice of this biasing resistor is made based on the example circuits in literature and a value of 22kΩ was used with satisfying results.

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    \$\begingroup\$ David, link-only answers are generally frowned upon because links can either go dead or the content can change. Your answer then becomes useless or misleading. Links also don't help much with search engine optimization. Perhaps you could edit your answer with a short outline of the linked content? \$\endgroup\$
    – Bort
    Apr 4, 2018 at 21:54

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