50 Turns

I started with 50 turns of Radio Shack magnet wire on a 12 wooden embroidery hoop purchased at a craft store. These bare ends I connected to an oscilloscope and waited for a storm. As luck would have it, it was not long. Amazingly I would getting peaks of close to 5 volts out of the loop antenna for a storm that was many miles away. I also discovered that although I though the antenna would be fairly directional that it was not. The geek in my already considering if I could determine the direction of the storm.

With this bit of information under my belt, I started to design an amplifier section not so much for amplification but for impedance issues. The voltage from the antenna was high, but had very little ability to generate current.

During testing I could not wait for storms to occur. It occurred to me that a piezo spark would be like a tiny lightning strike. I hacked a piezo/butane candle lighter to remove the butane and just generate sparks. Help close to the antenna I was able to get a nice response.

But a simple unity gain circuit using a 741 failed miserably. Lots of playing around lead me to believe that I needed a current path for the antenna. Simply connecting one lead to ground and the other to the high impedance input of the opamp left no way for the current to circulate in the antenna. Adding a burden resistor across the antenna leads fixed this problem.

The next problem to be overcome was the fact that the antenna output was AC, swinging both positive and negative relative to the ground lead. Since the output was to go to the ADC on a micro, I had to fix this issue. One method would be to bias the antenna ground lead to the 1/2 point of the ADC, but this would reduce dynamic range. I opted to rectify the signal with a “precision diode” opamp circuit. Now the output of the opamp stage was a “camel hump” waveform with the negatives folded up into the positive range.

This worked fine for a signal generator input, but when I switched to the antenna and piezo sparker, I got not output from the opamp. I realized that the reason the signal was not being amplified properly was that the center frequency of my antenna was at about 200kHz. At that frequency a simple 741 opamp’s gain falls off to nothing. No amount of amplification was going to help. So I ordered a high speed opamp and replaced it in the circuit.

Now I got the amplification I needed (gain of 1!) without any loss in the signal. But Murphy was not to be vanquished so easily. Now I discovered that the sample rate of the MSP430 ADC was not fast enough to reliably capture the lightning strikes from my piezo sparker.

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