Energy Harvesting Wristwatch Uses a Versatile Photodiode

There’s some interesting technology bundled into this energy harvesting wristwatch. While energy harvesting timepieces (called automatic watches) have been around for nearly 240 years, [bobricius] has used parts and methods that are more easily transferable to other projects.

Unlike early mechanical systems, this design uses the versatile BPW34 PIN photodiode (PDF warning). PIN photodiodes differ from ordinary PN diodes in that there’s a layer of undoped ‘intrinsic’ silicon separating the P and N doped layers. This reduces the utility of the diode as a rectifier, while allowing for higher quantum efficiency and switching speed.

They are typically used in the telecommunications industry, but have a number of interesting ‘off label’ applications. For example, the BPW34 can be used as a solid-state particle detector (although for detecting alpha particles you’re better off with something in a TO-5 package such as the Hamamatsu S1223-01). The fast response speed means you can send data with lasers or ambient light at high frequencies – a fun use for an LED lighting system or scrap DVD-RW laser.

Some common solar panels are essentially large PIN photodiodes. These are the brownish panels that you’ll find in a solar-powered calculator, or one of those eternally waving golden plastic neko shrines. They specifically offer excellent low-light performance, which is the basis of the energy harvesting used in this project.

What is very interesting is that energy harvesting IoT sensors use a similar method to function without batteries or mains power: amorphous silicon solar cells combined with a high-efficiency supercapacitor charging circuit. This is immensely useful for wireless transmitters on rooftops and industrial or agricultural sites where mains power would be impractical. Also since there’s no need to replace batteries, weatherproofing your sensor is only an epoxy tube away.

Does it function well as a watch though? With a standby time of at least 7 days, a viewing time of maximum 20 minutes, and a USB port for fast charging, it’s not entirely impractical unless you obsessively check the time. As a comparison, automatic mechanical watches offer unlimited viewing time, but typically runs out of stored energy if unworn for a day or two, and we imagine a USB port is a more dignified way of winding a watch than flailing your arms wildly.

See it in action in the video below:

25 thoughts on “Energy Harvesting Wristwatch Uses a Versatile Photodiode

      1. Digikey has 30mm diameter round 2.5 digit E-ink 7seg displays. One of those would be great in a watch if you’re happy updating every 10 minutes and not knowing the last digit of the time. Weirdly they make 6 digit and 3 digit rectangular models, but no 4 digit ones that would suit a clock.

  1. BTW “eternally waving golden plastic neko shrines” are called maneki-neko if anyone was interested in finding one. Common on various internet markets like eBay.

  2. “… allowing for higher quantum efficiency …”
    You lost me here.
    – Whats a normal quantum efficiency?
    – Where do i find the memo that we can now sell devices based on guesswork in the quantum world?

    Sorry if im coming across as negative, i very well may just be misinformed, but how and if quantum works is still pretty much anybody’s guess right? so how do you optimize a device for it? Im super puzzled now.

    Does this guy have the secrets to quantum logic (how to read without ruining the output, how to verify anything?) worked out??

    1. Dang it HAD always strips out stuff, it should’ve said:
      “Sorry if im coming across as negative, i very well may just be misinformed, but how and if quantum ANYTHING works is still pretty much anybody’s guess right?”

    2. Not sure if I’ve just fallen for a case of Poe’s law, if so, whoosh…
      Otherwise: “quantum efficiency” is just the probability with which a quantum of light (be it visible, x-ray, gamma, whatever photon) is converted into an electron-hole pair that is subsequently detected or otherwise used electronically.

      Therefore “higher quantum efficiency” just means an increase in photon capture probability when using a PIN diode: photons interacting in the I layer are separated similar to the depletion zone between P and N, hence the active layer of a PIN diode is thicker than that of a PN diode, and less photons pass through unused.

      Has not much to do with quantum weirdness (which, btw, is pretty well understood in the sense that it makes solid, testable predictions. Not so well understood at gut-level, admittedly ;) – unless you mean *why* the diode does it’s thing and the photons interact the way they do. Then it’s full-on quantum weirdness all the way…

  3. I have a hard time believing a 5F capacitor can be charged with 6 photodiodes in 5 minutes. I use 12 diodes and a 1F cap in my energy harvesting jewelry and it takes hours to charge in full sun.

    1. Might have to use a different LED variant (7014, if you can still buy them), but the drive power requirements might make life difficult. Not having seen specs for other “4014” variants, the voltage/current requirements might be outside what is practical.

      The project page mentions the supercap (5F) charge time as 5min brightest, and 20min lowest [brightness]. OK, so “highest” would be out in the sun, but “lowest”… are we talking about a well lit office, or a bedside lamp?
      If (ignoring all efficiencies/inefficiencies) if I take the charge time as 4 times longer, that is quarter of direct sunlight, and is actually quite a bit brighter than a well lit office.

      Unfortunately the video looks more like an advertisement (with accompanying bad music which lasted about ten seconds before mute).

      It is still a very nice project though, and I really like the selection of very low power components! I doubt a 32 bit CPU could compete with what’s done here. Those 8 bitters still have life in ’em yet!

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