Wirelessly-powered LED Christmas Tree
December 25, 2012
This Christmas, I made a few wirelessly-powered LED Christmas trees to give as gifts! These are powered via a phenomenon called resonant inductive coupling; they are essentially a miniature version of the Tesla coil I built 5 years ago.
Two RLC circuits consisting of a hand-wrapped magnet wire coil and a small ceramic capacitor were constructed. These circuits were tuned to resonate at the same frequency. One coil (the "primary") resides in the base, while the other coil (the "secondary) is connected to the LEDs arranged as a Christmas tree. The primary coil is connected to circuitry that excites the coil to "ring" at the resonant frequency. This ringing is picked up by the secondary coil, which receives the energy wirelessly! This effect is analogous to how a vocalist is able to shatter a wine glass using only their voice.
Below is a simplified schematic illustrating this. The primary coil is shown at the left, while the secondary coil is shown at the right.
The two RLC circuits were tuned by playing with two parameters: (1) The number of turns of wire in the coil, and (2) the value of the capacitor. The resonant frequency of RLC circuit was experimentally determined by connecting the circuit-under-test to a function generator via a 1k series resistor. The node between the circuit-under-test was then measured with an oscilloscope. The frequency was swept using a function generator to determine where the amplitude was maximized -- this is the resonant frequency!
A tiny little 8-pin PSoC 1 was used at the oscillator for this project. These are accurate, cheap (less than $1) and need far less external components than a comparable oscillator circuit using a 555 timer, for example.
This outputs a square wave at the experimentally determined resonant frequency. (In this case, 29.72kHz.) This square wave switches a transistor, which energizes the coil. (The RLC circuit of the coil essentially acts as a bandpass filter, so the actual transmission is a sine wave.)
Since the received power is alternating current, the LEDs in the Christmas tree are arranged in opposing polarities, so that both sides of the sine wave are rectified.
Below is a picture of the Christmas tree still lit while being held at some distance from the base.