3 ms·

Not a direct response to you, but if you calculate Carnot thermal efficiency with a skin temp of 37 C (likely too high), and ambient temperature of 20 C (probab

by nickff 10y ago

Not a direct response to you, but if you calculate Carnot thermal efficiency with a skin temp of 37 C (likely too high), and ambient temperature of 20 C (probably about right), you get 5.5%.[1] Using this result (combined with yours), the watch has to run on less than 5 mW of electrical energy. [1] http://calistry.org/calculate/thermalEfficiencyCalculator http://calistry.org/calculate/thermalEfficiencyCalculator P.S. It would also be interesting to see how much waste heat is removed in breathing.

nom 10y ago
5mW should be enough for a simple e-ink watch. CPU and display have to stay idle most of the time though and you can't communicate a lot over radio. It should be possible, even with that little power if you can accumulate the energy in a capacitor. However it's not an easy task and poses a real engineering challenge, that's for sure.
- nickff 10y ago
  Many of the Cortex M0s are very good at conserving power, and have a number of low-power and sleep modes, which make running at these levels quite easy. On the other hand, while Bluetooth 4.0 is low-power friendly, cost and size limits inherent to the watch will probably force the team to do a custom antenna and FCC certification, which is expensive, slow, and painful. I would say the electronics are not too complicated or difficult, but the energy harvesting and design will be tough, and the whole project will be quite arduous. It would be very interesting to see their development plan, and whether they are going for an iterative/modular design, or all-in.
  - nom 10y ago
    I agree, the CPU is probably the smallest problem. I'm curious with how little power you can get away with using a BLE radio. Especially if you constrain yourself to some kind of polling mode where the watch reaches out to the smartphone every minute or 5 minuetes or so.

3 ms·

Not a direct response to you, but if you calculate Carnot thermal efficiency with a skin temp of 37 C (likely too high), and ambient temperature of 20 C (probab

by nickff 10y ago

nom 10y ago
5mW should be enough for a simple e-ink watch. CPU and display have to stay idle most of the time though and you can't communicate a lot over radio. It should be possible, even with that little power if you can accumulate the energy in a capacitor. However it's not an easy task and poses a real engineering challenge, that's for sure.
- nickff 10y ago
  Many of the Cortex M0s are very good at conserving power, and have a number of low-power and sleep modes, which make running at these levels quite easy. On the other hand, while Bluetooth 4.0 is low-power friendly, cost and size limits inherent to the watch will probably force the team to do a custom antenna and FCC certification, which is expensive, slow, and painful. I would say the electronics are not too complicated or difficult, but the energy harvesting and design will be tough, and the whole project will be quite arduous. It would be very interesting to see their development plan, and whether they are going for an iterative/modular design, or all-in.
  - nom 10y ago
    I agree, the CPU is probably the smallest problem. I'm curious with how little power you can get away with using a BLE radio. Especially if you constrain yourself to some kind of polling mode where the watch reaches out to the smartphone every minute or 5 minuetes or so.