Assignment #6: Input Devices - Metal Detector

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Metal Detector (using an inductor)


Things I learned:




The Process

Some background

        Turns out there are many ways to make a metal detector, with varying properties of detection. There are about three ways to make an inductor based metal detector:

        For this assignment the goal was to make a "hello-world" level metal detector, and trying to make it as simple as possible.


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Before diving into the microcontroller board, I wanted to make a simulated "proof of concept" using a breadboard/wave-generator/digital-scope. The idea was to explore the different effects that metal can have on the output of an LC circuit (or an RLC circuit), so I can see how the detection can be done.  It was important to have the simulation's parameters similar to the inputs/outputs I can do with the AT-Tiny45 microcontroller.

I simulated on a breadboard sine this made it really easy to connect different inductor/capacitor/resistor combinations.

Frequency - oscillations of square waves desired frequency - since this is what I can generate with the AT45. It also has to be in a range that I can both generate in the microcontroller, and also read the relevant values in the right resolution/sampling rate.

Theory: frequency = 1/[2*pi*sqrt(LC)] - used this go get a feel for the right L-C combination, plus I wanted to get as low a frequency as possible, so that I can generate it and also read the output on the ATMEL micro controller.

Practice - after playing with many sizes and shapes of inductor coils, most of which seemed to have a very low inductance, I settled on a ~260mH (milli-henrey) coil.



~32Khz frequency had a good response, which is GREAT since it can be generated easily with the AT45. When getting metal close to it, the change seems to be mostly in the amplitude of the oscillating output, with no change to phase. This means that if we follow the peak of the steady output, we can detect changes in it caused by metallic objects.

Demo Movie




The Board

I used the hello3 board design, with an AT45 chip.

Making the new board:


The assembly code