Inductive LED Version 1
Inductively Charged LED from fatti_k on Vimeo.
Motivation
Frustrated with the lack of slim and discrete power sources for my LED tape, I decided to build inductively charged < self-powering LEDs as a future prototyping elements for my upcoming projects:
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Deconstructing an Electric Toothbrush
As a shortcut to find out how inductive charging works, I bought a $14.99 electric toothbrush with contactless charging feature (Crest SpinBrush) and took it apart:
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Building the LED Circuit
My next step is to reuse the brush side coil and build a circuit to replace the motor, and instead turn on an LED. Adam Kumpf kindly directed me to look at Bridge Rectifier circuit pictured below. This takes the AC power from the inductor coil and converts it to a DC power.
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A cleaner version: minimum required circuit:
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The LED turns itself on when it is barely above the base (oscilloscope is set at 10x, so all readings are ten times bigger than the actual value).
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Next, I built the same circuit in a different form factor, into a small cylindrical form.
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Again, Adam gave me these handy SMT bridge rectifiers, which simplified the circuit.
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Now built into the cylindrical form factor:
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Casting
After playing with Cati Vaucelle’s ClearFlex Urethane shapes, it seemed promising — it could magnify the LED if a round shape is imposed right on top of the LED.
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Unfortunately, it takes 15-16 hours to cure, so for the lack of time, I went with the 5-minute epoxy. I got a tray of quail eggs from a Korean grocery, used “Siligum” from Pearl Art Store to make positive mold of an egg, then again a negative mold. It is not clear or pretty, but it is nicely diffusing.
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Custom Base (EM Surface)
After a few attempts to take apart the electric toothbrush base (in fact, two of them) I found that they were all resin-poured. I could not find out what exactly was inside. (Thank you Daniel Leithinger!)
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Adam Kumpf gave me a rundown of the circuit I will need to build for the charging base.
- First I’d start with a function generator + a resistor and a capacitor + and a magnet coil (yellow). Eventually I’d build the oscillator and power supply (blue).
- Generally a source frequency below 100KHz is recommended. You can use a capacitor to tune the frequency. Resonance frequency formula (red) shows that in order to lower the source frequency 10 times, you have to increase the capacitor by 100 times, for example.
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So I rolled more magnet coil
- 22 awg, thicker is better for power generation,
- Less number of turns is better to build a step-up transformer
And hooked it up to a function generator, and a capacitor (resistor was removed later to boost the power output).
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With this source amplitude and frequency, you could get 6-8 times the boost in voltage when measured across the base coil.
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With 20V peak to peak and 500-600 KHz freq source, you could get over 10 times magnification of voltage.
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Using this custom coil, it takes the LED egg about 3 minutes to come fully on.
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Result
On the toothbrush base
- Input voltage: 5V DC
- Time to turn on after full drain:
- Time to turn off after full charge:
