powering LEDs in a necklace pendant - Page 1
Forum section: electronics / projects

author: mclien
author_rank: Contributor
timestamp: July 19, 2025, 05:22:52 pm
content: I want to light a pendant with 2 UV LEDs (https://www.bivar.com/parts_content/Datasheets/SM0603UV-395.pdf)The space in the pendant itself is very limited, so the ides is to have the power source behind the neck (where you usually close it). And then use the 2 sides of the necklace as the wire.I'd like to use a usual sterling silver necklace, but of course this is not a reliable conductor, as each link of it might be a circuit breaker. I did some simple test with some models. One doesn't even be conductive untill you put some tension on it, the other kind of works, but has 0 to 1k Ohm resistance with a complete interruption when moved a lot.I have a plan B, which is a stainless steel wire which can be designed without any links, but that would be a less preferred design.Power source is to be a LiPoly battery for easy recharge over USB.So, can I use some simple circuit additionally to the LEDs, which bridges the short interrupts. Somehow the right circuit with a capacitor should do the trick, I guess.As for the supply I was also thinking to use a constant current source (is that the right english term?) From what I understand this one would be working with 2 of the above mentioned LEDshttps://www.americanbrightled.com/pdffiles/led-components/ezyled/AB-EZPC.pdfLet me know if I'm at least aiming the right direction or (again) failed horribly to do the right internet search.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2025, 06:26:07 pm
content: Applying a DC voltage between two pieces of metal in prolonged skin contact is extremely dangerous. Even though the voltage is low (3.4V LED Vf) even the slightest trace of sweat will start an electrochemical reaction that can burn through the skin in as little as two hours. Permanent staining from silver ions passing through the skin then getting reduced to metallic silver is also a possibility. Fortunately its likely to cause severe painful skin irritation well before it does permanent damage.You need to rethink this design so the LEDs and conductors feeding them from the battery pack are fully insulated. Maybe consider hollow silver braid or even a braided silk cord with silicone insulated wire running through its center as the conductor.Also, strong UVA light is itself a skin irritant - seehttps://pmc.ncbi.nlm.nih.gov/articles/PMC9368482/so unless you pendant design only uses the LEDs to excite fluorescence within it and doesn't allow significant UV to directly hit the skin, it may be unsafe to wear.

author: mclien
author_rank: Contributor
timestamp: July 20, 2025, 10:18:32 am
content: Most of the problems are solved the moment it is worn above clothes, which indeed is the intention.I did search for small silicone insulated wire, but could only find quite thick ones (from 3mm in diameter upward)At the moment this is mainly a proof of concept for the illumination part and making the backside of the pendant light tight is an idea to keep in mind.And it is mainly intended to work as a reload/ reactivation of the fluorescence within the pendant. After the concept itself is proofed (mainly if it is working in general space wise, it might be an idea to figure iut, if the intensity of the fluorescence can be measured and light up the UV only if it gets dim.And thanks for the other design suggestions, which I'll consider, if the main concept works out.So any suggestions about the cirtuit itself?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: July 20, 2025, 02:30:17 pm
content: Good Morning Mclien, I'm intrigued by your concept and I hope you post your progress here. I am curious, though, as to why have you chosen UV LEDs. Thanks for sharing your ideas and best of luck

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2025, 03:09:00 pm
content: If its only going to be worn over clothes, a wireless charging coil setup might be possible, with the charging coil implemented as a wearable, secured to the inside of the clothes under the pendant with wires to it and power source concealed by the clothes. That avoids all the problems with DC and skin contact, and should be safe for anyone without a pacemaker or other electronic medical implant, and no piercings in close proximity.Incidentally, a CR2032 lithium coin cell with an efficient boost circuit should be able to power your LEDs at their recommended operating current for over three hours, so don't rule out built-in power unless you cant conceal a 20mm disc behind the pendant!If you don't mind replacing the wire frequently due to fatigue, you could use fine magnet wire for DC power. The polyurethane insulated variety should be fairly benign in skin contact and has the benefit of solder-through insulation.

author: edavid
author_rank: Super Contributor
timestamp: July 20, 2025, 03:22:23 pm
content: Quote from: mclien on July 20, 2025, 10:18:32 amHere's an example which is 0.8mm diameter:https://www.adafruit.com/product/3164QuoteIn that case, what does it matter if the LEDs flicker, or if the brightness varies? No need for a constant current source; a simple resistor is fine.I also don't see why you need a battery pack. Wouldn't 3 button cells or 2 coin cells provide enough energy for an evening's wearing? You could put those in the pendant.

author: OZ1LQB
author_rank: Regular Contributor
timestamp: July 20, 2025, 03:47:12 pm
content: mclieni se you are in germanyhere is my goto place for such thingshttps://www.az-delivery.de/en/products/kupferdraht-schaltlitzen-kit

author: mclien
author_rank: Contributor
timestamp: July 20, 2025, 04:48:47 pm
content: Some context. A friend of mine is making these:(mushrooms as example, she has flowers, galaxies, mountains)When she is on the road selling those on markets 18mm diameter is about the limit. At home with equipment for cooling down the glass more slowly (temperature regulated furnace) she can go up to 30mm, but that's it.All those include some florescent material, which flows for a while after activating (best effect with UV light). So my idea is to build a slightly bigger silver cap on top, which holds the smallest UV-LEDs I can get my hands on. First try will be to print a small resin part, which holds he LED in place and doubles as insulator and cover that with a Sterling silver cap.She made a special marble for me, which has a M1.6 thread as a mounting point. I didn't tell her what I'm about to do, but have to get a working prototype within 14 days, because then we meet at the next event and the deal is, either I have ist build or I have to tell her what I#m about to do.So lots of thanks for the sources for silicone wire.But I think there is not really room for an includes powersource. Even coils for wireless power might be to big to hide.