Homemade Pulse Sensor

Reading time ~2 minutes

Originally posted on www.letsmakerobots.com

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 projects 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.

** **

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

  1. Make sure you have theses tools.
  1. 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.

  1. 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.

  1. 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).

  2. **Wire it up to the Arduino **

Left —- Middle —- Right

A0 —— 3.3v ——–GND

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

  2. Yell at me if you have problems.

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