An Unexpected Electronics Mistake

Finding the Source of the Error

By Michelle Meyer

Electronics projects don't always (or rarely, in my case) work the first time, which makes the debugging process very important. I learned a few things about just how important it is when I built a metal detector for my electrical engineering class. I started by researching a variety of approaches to building a metal detector, and I decided on a simple circuit using a 555 timer IC. I followed this circuit I found online, adding a switch and swapping the 2.2µF capacitor next to the inductor for a 470µF one.
Metal Detector DiagramBuilding a Metal Detector using a 555 Timer IC

In order to make the 10mH inductor, I made a cardboard core and wound copper insulated wire around it. Before winding the wire, I did some calculations and determined I would need 50 turns of wire. After winding the inductor, I wired the rest of the circuit onto a breadboard and flipped the switch to turn on the metal detector. The speaker emitted sound, which was a good sign, but unfortunately the pitch did not change when metal was introduced near the inductor.

Cardboard CoreWinding Wire in Cardboard Core

I checked to make sure I wired the circuit correctly, and I found no errors. Next, I pulled out a digital multimeter to make sure current was flowing through the inductor. There weren't any issues there, so I decided to look at the circuit on an oscilloscope to see if there was a small change in frequency that was too subtle to hear. The oscilloscope showed no change in frequency, which meant that my circuit was completely ineffective. Amazingly, I could hear the trash can mocking me, and that's when I considered ending the project because I didn't have a clue where my error was.

After regouping I stepped through the project in my mind looking for failure points. That led me to recheck my inductance calculations. Sure enough, I found the problem. I was off by almost a whole power of ten: instead of being 10mH, the inductor was only 1.3mH. This project was the first time I had to calculate and make a part by hand, so while I did check for short circuits, I didn't think to check the calculations until it was almost too late. Perhaps I was spending too much time working on electronics projects and not enough time in math class!

I was relieved that not all was lost, and I redid my calculations for a new inductor that would actually be 10mH. I triple checked my calculations before winding the 150 turns of wire. After replacing the inductor, my metal detector was working! I could not only see changes in frequency on the oscilloscope but also hear changes in the frequency.

While I quickly thought of ways to improve the project (move from a speaker based solution to an LED that would illuminate when metal was detected or even a beat frequency oscillator circuit), I ultimately decided I had invested enough time and moved on to new projects. Through this experience, I became aware that many things can go wrong beyond the circuit wiring. Trusty multimeters, though useful in many situations, can't help solve every issue. It is important to adapt the debugging process to each project. For example, if I do get a chance to build an Arduino version, I have to watch out for mistakes in the code as well as circuit mistakes. Hopefully, keeping this lesson in mind when I work on more complicated projects will help me avoid having to quit because I can't find the source of error.

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Michelle Meyer is a student at Menlo School in Atherton, California, and a summer intern at Jameco. She plans to study engineering in college. Her interests include math, physics, sports, and hiking