Final Implementation

The bulk of my time on this project was spent trying to build a transmitter. My assumption was that if I could simply generate a large enough signal, harnessing it would be the easy part. Unfortunately, I was not able to build an adequate transmitter and with the semester’s end rapidly approaching I chose to concentrate on building a working proof of concept model. In the end, my power source was a Sonicare toothbrush base. I built a demonstration model that consists of:

Receiving Coil
Rectification State
Power Storage Circuit
Mechanical Stage (demonstration)

The receiving coil contains two levels of windings of 24 AWG magnet wire. The lower level (closer to transmitter) has 50 windings and the upper has 10 windings. This coil has an alternating current induced in it that must be rectified. I used a classic four diode H-Bridge configuration to achieve rectification. The power level at this point is quite low, hence the necessity of the power storage circuit.

The circuit I used is a modified version of a circuit designed by Mark Tilden for use in low power robotics. It is called a Type 1 Solar Engine because it was intended for use in solar powered applications. The basic operation of this circuit is to charge a 4700uF capacitor from a low power source. When the voltage across the terminals of the capacitors exceeds a predetermined threshold set by zener diodes the circuit switches from charging to discharging.

In this application, the capacitor charges on the secondary coil and discharges into a small micro-cassette recorder motor. The cycle takes approximately one second creating a clock-like movement as the motor is pulsed. Finally, through a series of gears and a rubber pulley belt, a small image is rotated.

Variations on Final Build

My first challenge was to build an oscillator. The oscillator is an essential feature of the power source. I intended to duplicate the Sonicare system and then make modifications to scale up the power output. Of course, there are numerous options when choosing an oscillator. My first choice was a Colpitts oscillator.

I chose the Colpitts because it easy to assemble from discrete components, inexpensive and simple to troubleshoot should problems arise once the circuit became more complex. A final benefit of the Colpitts is that it can maintain oscillations into the megahertz if necessary.

I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental.
At times it was difficult to initiate oscillation. Professor Joseph Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a soldered circuit board. When the Colpitts worked, the oscillations were stable and I measured 400mVPP on the output.

The next step was be to amplify this signal and drive a transmission coil. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely. I attempted to use multiple amplification stages of different designs. Unfortunately, none of them were effective. It seemed that any loading whatsoever on the output of the Colpitts made it collapse. I decided to abandon the Colpitts.

For the sake of brevity I will not go into as much detail about the subsequent design alterations. My next permutation was to use a 555 timer IC. The out put was very stable but not sinusoidal. I considered adding a low pass filter to the output to correct this. For the sake of simplicity, I replaced the 555 with a Wein-Bridge oscillator

Now that I had a stable oscillator, I focused my attention on an amplification stage. There were many variations. Ideally there would be two stages: voltage gain and subsequent current gain. I attempted this with FETs, BJTs, a combination of both and finally with Op-Amps. I was never able to generate enough gain to drive a significant amount of current into my primary coil.