Measuring A Pulse With Infrared Light

The next time you’re unfortunate enough to make your way to a hospital, emergency room, or urgent care clinic, you’ll be asked to attach a small pulse monitor to your finger. The device the nurses clip on to one of your remaining digits is called a photoplethysmographic sensor, and basically it is able to read your pulse through reflected light. In the search to find out how these devices actually work, [Raj] sent in a great tutorial covering the theory behind photoplethysmographicy, and also built a simple device to detect a pulse without using a microcontroller.

These photoplethysmographic sensors operate by shining light into someone’s flesh – usually a finger or ear lobe – and recording the light reflected back to the source. The volume of blood in the finger will have an effect on the amount of light reflected back, and makes for a perfect way to automatically measure someone’s heart rate.

To build his device, [Raj] used a TCRT1000 reflective optical sensor. Inside this sensor is an infrared LED and a phototransistor. Of course with a finger over the sensor there is a ton of noise both from ambient light and the base rate of reflected light from a piece of flesh. [Raj] filtered this out, leaving only the small variations in the amount of reflected light, thus creating a very simple – and very inexpensive – electronic pulse meter.

18 thoughts on “Measuring A Pulse With Infrared Light

  1. Cool, I’ve been wanting to play around with making some manner of bio-sensors, and the other day I saw a video somewhere about this exact sort of hack. (Can’t remember where, think it was a Make youtube vid)

    Wonder how well this would work in a wristband or something, wireless, with one of those absurdly-low-power transmitters from TI to send the data to an MCU…

    Off to order samples I go! <3

    1. grab some clear tape, and some wire tape. Put the clear tape on you (after washing the area so you actually adhere to the skin), lay the goodies down, and then cover with wire tape.

      No need for a clamp, this would probably work anywhere on your body.

      Though I would suggest you put it somewhere that doesn’t flush when you exert yourself, because that might throw the filter off.

      1. You do understand the title of the project report was
        “Introducing Easy Pulse: A DIY photoplethysmographic sensor for measuring heart rate” right, or do you expect HAD to be critical of the projects they report on?

      2. Damn, I imagine the application to work for HAD staff position mentions a requirement that the applicant should have skin as thick as a Bull Elephant, or at least that of any President of the US. anyway. I once had a watch/stop watch that had one of the built in, but it was a pain to find the happy medium the as to the amount of pressure to apply to the sensor, because the sensor was part of the switch that activated that function. Seemed light the amount of pressure required to activate the switch interfered withe the sensor’s ability to take reading.

    1. I have tried it, and to be fair I was quite disappointed by the sensor, considering its price tag. While it indeed looks good, the output is one continuous stream of noise. The filtering has to take place in your µC, which takes significant processing time and yields buggy results at best.

      I really like the solution in this article, because it outputs a clean signal you can instantly feed you µC. It might not look as sexy as the Pulsesensor does, but I can perfectly manage that.

  2. Jeez, what a mouthful. That’s kinda like addressing a blood-pressure cuff as an aneroid sphygmomanometer. I usually document it as it pulse-oximetry but whatever blows your hair back.

    Typically, it’s used on fingertips or toes because the emitter is on one side of the clamp and the sensor is on the other. Alternatively, you may use it on earlobes and in pediatric patients, either foot.

    Along with pulse readings, it also (and primarily) detects oxygen saturation on one’s hemoglobin. Except in the cases of carbon-monoxide poisoning where it will give false-readings of what has become a measurement of carboxyhemoglobin. Add some different wavelength emitters and some software to that package and it will be able to tell the difference.

    What would be a boon to the community would be a pinout of the different probes (finger clamps) sold by the various manufacturers of these products because they typically connect to diagnostic equipment via a d sub connector with extra grooves and ridges in the plastic to make connections proprietary whereby one manufacturer’s probe will not mate with another manufacturer’s equipment. Maybe when we get universal healthcare (that is agreed on by everyone), we’ll get universal connections for our equipment. Didn’t the computer hardware industry find some work around for that? Universal serial something….?

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