Wireless LEDs Aren’t Really Magic

[Atomic14] bought some wireless LEDs that receive power from a base station. They were very neatly packaged, but — we like it — he took one apart and made his own versions. They may not look as polished, but they work and they are undeniably cool.

The LEDs work by receiving power from an induction coil. Once you have power, lighting up an LED is no big deal. Reverse engineering found the transmitter sends 217 kHz into a 2.2 mH inductor. A capacitor resonates the coil and drives the attached LED.

Some experiments found that the circuit could supply about 2 mA -3 mA of current. [Atomic14] used two LEDs to get work out of each half of the AC waveform. He also dissects the transmitter, so you could roll your own there, too.

What would you do with a wireless LED? Maybe lighting in a model display or a checkerboard? We wondered if you could use two or more power frequencies to signal (for example, 200 kHz lights a red LED but 250 kHz lights a green one)? The original transmitter was fixed frequency, but if you used a microcontroller, it would be easy to make it frequency agile. In the end, there is an economic analysis of building these versus buying them ready-made, but we all know that isn’t always a decision that is strictly based on dollars. The commercial versions did look a little nicer, but with surface mount components, even the DIY version could probably look a bit cleaner.

We’ve actually seen these before. We wondered if you could draw power from something that was transmitting anyway.

18 thoughts on “Wireless LEDs Aren’t Really Magic

  1. Back in the day, there were AM band transmitters that were a couple thousand watts. KNX in Torrance CA comes to mind along with numerous tales of receiving the news on your showerhead. Should be enough rf to power a led

    1. Yeah, KNX was a pain – it came in on almost everything I tinkered together. I used to live about 4 miles from the antennas in Columbia Park (Torrance). Between the 235KV power lines next to the house and KNX, I had to give up on shortwave listening. Even with good LC-filters to get rid of KNX, I still had the power lines to deal with.

      BTW, according to the web, KNX is now putting out 50KW (which did tend to go down at night). At that power level, it’s hard not to receive it if you line close to the antennas!

      I’m not sure if it is enough power at 4 miles to light an led, but you have the right idea. I wish I had tried it when I still lived there. “KNX 1070, all news, all the time…” lighting up little LED glow-bugs in the back yard would have been a kick.

      Back in 1968 my parents had a house that was within eye shot of WBAL’s antennas in Baltimore, MD. My dad built my mom a crystal diode receiver into the wall (no antenna, just a loop-stick coil) that actually drove an 8 ohm speaker (with no impedance transformer either). His first version required a modification because the music was too loud. He actually had to put in a pot to turn the speaker volume down!

      We could also hear WBAL playing on the heating ducts and water heater in the basement too! It was really freaky.

      But all good things come to an end. During an electrical storm, there was a direct a strike on a power pole less than 50 yards from the house. After that strike, the plumbing and heater ducts no longer played WBAL, and the crystal radio’s volume went down drastically. But it still played! The lightning strike must have cleared whatever non-linearity there was that acted as a detector and blew open whatever was acting as an antenna.

      Now that I’m retired and out of the CA and will soon move into in a semi-rural area in another state, I look forward to firing up my National NC-46 and my old ship-borne surplus Hallicrafters S22 shortwave receivers. I just hope there is something still on the air to listen to. I know one of my old favorite stations, Radio Australia, has ceased shortwave broadcasts, and I fear many other stations I used to listen to have vanished as well. It’s just not the same listening to these stations on internet radio! It was always a treat waiting till dark to listen to the world. Before I moved closer to the KNX antennas, I lived on a hill and even in an apartment I had a long wire antenna strung on the apartment roof. There, the sun was the biggest noise source I had to deal with!

    1. Nice idea. Colour the pieces according to how much danger they’re in!

      Logic is the way to go here, that way you can have two pieces side-by-side with different (and changeable) colours. Bit difficult to do that with bandpass filters.

  2. I remember seeing, a long time ago, and probably on here, someone had made up a weather station on the roof of their apartment building, that they powered off of a nearby broadcast radio station.

  3. Back when I had a pool I wanted to build a bunch of these, put them in clear waterproof balls and ballast them until they were neutrally buoyant- coolest pool lighting ever.

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