Let's Make Robots!

Homemade Pulse Sensor

Senses your heart-rate

I've been working on re-making the the Open Hardware Pulse Sensor so it'd be easy to send off to OSHPark and to make at home. I'm not sure, but I think I started this project in March and I've just now finished it.

The bit of encouragement I needed was when hackaday.com put it up as their "Fail of the Week."  I thought I was going to be mature about it.  But those four red letters started eating at me, so I gave it another go.  Weirdly, I got it working.  

I believe there were three problems:

  1. I had mixed up the op-amps again.  In my defense, I've got 5 different ICs flying about in the same package as the op-amp.
  2. The Arduino I'd been plugging into was sitting on a surface that provided enough conductivity to create noise between the 3.3v pin on the underside and A0, which I was using for the op-amp in.
  3. Every time I touched the sensor the exposed vias were shorted through my own conductivity.  Stupid mineral water.

VIDEO:


 

I've already detailed how I went about making it; so, I'll try to stick to repeatability.

1. Order the parts.

  • Op-amp:   .29  (Digi-Key)
  • Light Photo Sensor: 1.23 (Digi-Key)
  • LED:   .79   (Digi-Key)
  • 0603 Schottky Diode: .50 (Digi-Key)
  • Passives:  ~2.50 - Resistors: 1 x 470k, 1 x 12k, 2 x 100k, 1 x 10k, 1 x 3.3Meg - Capacitors: 3 x 4.7uF, 2 x 2.2uF
  • OSHPark Boards$.67 (minimum 3 boards, costing $2.00. 3/2.00 = ~.67)

 Total (approximate): $ 5.98 

2. Make sure you have theses tools.

3. Solder the light-sensor.

The light sensor is the hardest bit, so take your time.  I put a little bit of solder on each pad with my soldering-iron, then, cover the soldered pads in flux.  Next, I attempt to align the light-sensor with the pads as close as possible.  After, I put the board with the backside on an over-turned clothes iron.  Let the iron heat up until the solder reflows and the sensor is attached.

 

4. Flip the sensor and lock it to your surface with tacky-putty to solder the LED, passives, and op-amp.  I won't detail this, since my video shows the entire process.

5. Wrap it with tape, cutting a small hole for the LED and light-sensor.  (I'll come up with a better solution, and a way it to clip it to your body, on the next iteration).

6. Wire it up to the Arduino 

Left ---- Middle ---- Right

A0 ------ 3.3v --------GND

7. Run the Arduino and Processing sketches these amazing guys provided.

8. Yell at me if you have problems.

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They have a fail of the week!?  Makes me want to be careful NOT to document my failures.

Very cool though you got it working.  This has some pretty cool potential uses...I am impressed

Regards,

Bill

Not harsh at all.  They are very polite and openly communicate.  I think it is one of the best ideas they've had in a long time.  I'm pursuing a degree in a research field and we discuss openly how failures never get published, which leads mistakes to be repeated by hundreds in the dark, without end.

I should be clear, they wouldn't have put it up without my permission, which I gave.  I just wasn't ready for the cognitive dissonance that followed.  But I've got my big-boy panties on.  And I know I would not have finished these projects unless something spurred me on.

Again, I hope this post doesn't deter anyone from documenting their mistakes.  I've learned so very much reading the Fail of the Week articles and the replies to them.  

Hope you're well, sir.

Good point about failure.   Success is dynamically unstable like trying to balance a tennis ball on top of a soccer ball.  At any second, it falls and goes boom.  Failure happens more often and certainly teaches more than success. 

Again, congrats on figuring it out on this sensor.  Very cool.

Regards,

 

Bill

 

 

I've always heard that there is no such thing as failure, only temporary setbacks. I thinks it's awesome you publish setbacks so we can all learn from them instead of having to learn ourselves. And in the end you suceed one way or another by learning from setbacks. :-) - Stephen

Thank you, sir.

I'm glad we share that belief.

Hope you're well,

-Thomas