LED under the skin

So, I really do like the Northstar but I'm not sure I'm in a place where I can commit the money and the physical side of things. But, I really want an LED under the skin. Has anyone built one of these? I was thinking it could work something like a pared down version of the bone microphone in this post. It seems like it could fit in a pretty small area and keep enough charge to not have to be recharged more than once a day, and possibly less.


  • @AlexSmith is working on an NFC powered led. It's just a little coil attached to a surface mount led. There is no battery, and it is only powered when you place your phone near it. I don't feel like linking to the thread, but it should be fairly recent, just look around a bit.

    If you really just want an led, it wouldn't be that hard. All you would need it the led, battery, charging circuit, and the coils to send power to the charger. Would it stay on, or would you want to be able to turn it off? A switch or some sort of sensor is going to start adding to the overall size. The NorthStar I believe has some sort of reed switch or hall effect sensor, which lets people turn it on with their finger magnents. That means no mechanical parts to fail over time.

    How long would you want to keep the led on for? Is there a target size that you would be shooting for?
  • A magnetic switch would be nice so I can turn it off for work and not just have to have the power run down. I was tempted by the NFC powered LED but then I'd need a constant external power source. I wouldn't mind a little extra bulk for a switch I could use with my magnet. Reed switches seem like they can bee pretty small so it wouldn't be that much extra space.

    I'm just going to have to make this myself if I want it I guess. 

    My main issue currently is figuring out the charger. Are there any really small ones available? Also setting up a workspace where I can put this together I suppose. A studio apartment isn't the best place. Where do you all get your parts for these things?
  • edited March 2016
    Have you had any experience with electronics? Soldering? Circuits?

    The tools that are required for this sort of thing aren't exactly bulky or rare. A cheap soldering iron and a desk are really all you need. I have plenty of space right here in my bedroom, and I make stuff like this all the time. However, a decent amount of experience is required.

    With something like this, the best approach would just be to make a custom PCB. That would give you full control over the design, the features, and form factor. I would recommend first making one on a breadboard. Testing and iteration is key, very rarely will you be happy with your first attempt. Again, this isn't a project that would be beginner friendly necessarily. If you don't know what that is, then you might want to take a step back.

    I would be willing to help you out if you want to talk.
  • The electronics I'm not that worried about, I need some work but I've done some soldering and some basic stuff and this shouldn't be too tough. I was more concerned about the coating and keeping it sterile and what I need for that. The design specifics are going to be a learning experience as well, but I think with a bread board I should be fine. Can a home made PCB be made small enough? It doesn't have to be that big, right? It's just the connections?
  • A homemade PCB can be made small enough, but with this sort of thing you should try to leave as much as possible up to the pro's. I would recommend you take a look at OSHPark for making custom PCBs. They are dirt cheap and will get you your board in under a month.  

    Don't worry about the coating and keeping it sterile too much right now. Your first priority should be design. As far as size, no it shouldn't be that big. The hardest part is going to be the wireless charger. You should probably decide if you want to go with the wireless charging standard Qi (pronounced chee), or just want to roll your own. The guys making the NorthStar said that their next iteration will feature a Qi charger that can fit on the back of a coin! I personally believe that is the best option, but we will have to wait and see. Everything else, like the microcontroller, LiPo charging circuit, and led is pretty trivial. 
  • I've been thinking about this some more and I'm wondering if I can get away with something simpler. Here are my two thoughts.

    First, just have an LED with a battery and a reed switch. It wouldn't be rechangeable but it should last for a decent amount of time as long as I was careful not to just leave it on. This is definitely doable and the only down side is that I'd have to have it removed when the battery ran out.

    Second option: An LED with a capacitor and an inductive coil to charge the capacitor. I don't know how much of a charge one could get on a capacitor but I figure it could be an hour if it's big enough. I don't know how feasible this one is but if it could light up for an hour or two per charge I'd prefer this option since it wouldn't have to be removed.

    The idea of jumping right in and designing something that would be almost as complex as the Northstar but with just one LED is rather daunting and I'm not sure I want to do that without having some other success first. Any feedback on these two ideas is very welcome. I'm going down to rat shack today to grab a battery, reed switch, and LED to test out the basic design of the first one.

    Also, what color LED works best? Green?
  • @Cathasach, a reed switch only conducts when a magnet is held near. It sounds like you want a latching circuit which requires a second input to disengage.
    You'll want a super-capacitor because a regular capacitor will power an LED for only a couple seconds. I haven't seen those at the Shack but it's been awhile since I checked.
    Red light passes through skin better than other colors.
  • Something like this should work: https://standexelectronics.com/products/ksk-1e66-series-reed-switch-3/

    Although 14mm is a little bigger than I thought I'd be using it's not a deal breaker since it's still smaller than the battery I'd be using. That might be overpowered for my purposes though, but it's good to know it's doable.

    I'm not sure how to spec a supercapacitor at this point. I'm thinking of THESE LEDS since I really don't want to go with red. I want something more interesting than red, although I figured red would be the best through skin I was really hoping it would be something else.
  • Good find with the latching reed switch, I didn't know those existed but that's cool.

    Where are you planning to put this?

    A 5mm LED is huge for an implant and better surface mount options exist. The problem with a standard 5mm through-hole LED is that the bulk is a lens. You don't need the lens.

    What kind of battery are you using?

    Do you have a charger picked out?

    The reason size is so important is that you're adding material to a system, your body, that isn't meant to have anything extra. Have you gotten a piece of sand in your shoe? That grain of sand is tiny but you can feel it easily. Try putting an LED under your skin and the problem is way worse.

    If you insist on a green LED try buying a red LED of similar brightness and shine them both through your fingertip and see if enough light comes through. It might.
  • If I go with the battery I'm not going to have a charger, I'm just going to remove it when the battery dies. The size is the main reason for that. I don't know what battery I'm going to use but currently I'm looking at a CR2032, just because of the form factor though. I'm going to try out some different types of batteries and do my research on which ones will last the longest.

    I don't have a location decided on yet. I'm thinking somewhere on the arm or the chest. I hadn't thought about the LED and I'll have to look for something that's smaller. 

    Thank you all so much for your advice and input here. I can't tell you how much I appreciate it.
  • I hope we can keep each other safe.

    As soon as you get your parts together I'd love to see a video demonstration.
  • I got the basic supplies, LEDs, reed switch, and a battery and will be putting something together in the next couple days as a test to see how long the battery will work and what reed switch will work best with the project. I need to order some stuff after this to make something that can be implanted but I'm excited for the first steps.
  • use a switching hall effect sensor instead it will work better than a read switch (needed a pretty strong magnet to turn on my BLE audio implant) 
  • hall effect sensors, by the very working mechanism, require a permanent current to flow in order to detect magnetic fields. This might empty your battery faster than desired. 
  • yes @ThomasEgi they do. Example, the northstar uses a hall effects sensor and it lasts a very very long time (rev 2) with a very normal sized battery. the amount of battery loss (all implants should be chargeable) is negligible if you are doing daily or weekly charges  
  • @Benbeezy the initial plan was to use a non rechargeable battery for testing. And while you can sample a hall effect sensor every now and then leaving it mostly turned off, it does add extra circuitry.
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