Pulse Necklace 29July2009
Pulse Choker Hack Notes July 29th, 2009
Using the circuit that I found, I wired up a heart beat sensor. It WORKS! But, like the watch, it's difficult to get the optical sensor and your finger "aligned" correctly, so it often doesn't give a signal. I had the best luck when I pressed my thumb into the sensor, with my index finger holding the other side of the sensor with low to moderate pressure. When it does give a signal, the LED seems to blink at *half* the speed of your heart, i.e. each beat causes a state change (from on to off). This is really puzzling, since it should blink at the same speed. It also seems to miss a fair number of beats. Is it really working? Picture:
You can see the sensor, it's the little black box on the left of the group of three. The IC is a dual op-amp, the other black boxes are the transistors. The RBBB arduino clone is being used only for the 5V regulator.
Given this experience, and having had the same experience with the pulse watch, I think that the idea of just pressing the sensor into our necks and hoping it's going to work is totally out. However, it's still possible that a solution involving clipping the sensor to your ear-lobe might work, since that can (hopefully?) be made secure and non-variable. We need to experiment with this.
At the same time, I think we should move forward with a different mechanism for sensing heartbeat, such as the industry standard electrocardiogram units. The downside here is that we will probably have to have some kind of chest band or similar. I found the Open ECG Project's "Small 2-lead ECG, which actually has complete schematics and BOM, along with nice graphs showing how great the heartbeat signal is.
The problem with ECG from our perspective is that it requires electrodes on your body, and that's not cool. The Open ECG project has a page on reusable electrode development which actually links to Textronics, a company that is developing a *fabric electrode* (awesome! - they are doing great work!). They sell a dev kit for $100. Before we purchase that though we should prototype an ECG circuit and use the Red Dot electrodes, to prove that this stuff works for us. Also, I think we could build our own fabric electrodes once we've seen theirs - I mean, some conductive fabric and a strap of some sort to hold it against your skin, right?
I will protect the heart-beat sensor circuit breadboard and we can continue to play with it over the coming weeks. For instance, we have many more sensor heads, maybe some of them will be more forgiving than the Honeywell HLC1395 that the circuit calls for? We also need to feed the signal into the Arduino and see if we can output heart rate info, plus control any display we eventually build. Lots of good stuff to look forward to!