Vol. DIY Electronics Projects

Chapter 4 AC Circuit Projects

AC Lab - Sensing AC Magnetic Fields

In this hands-on AC electronics experiment, you will sense AC magnetic fields using a simple wire coil and learn about AC signal coupling, orientation effects, and the impact of shielding using different materials.

Project Overview

As you will learn from this project, in today’s modern society, low-level magnetic fields of all frequencies are easy to find. These magnetic fields can often cause problems with other electronic systems through what is called electromagnetic interference (EMI).

As illustrated in Figure 1, a coil of wire may serve as a sensor of AC magnetic fields when paired with the audio detector circuit, explained in an earlier project, to detect AC voltages in the audio frequencies.

Schematic diagram of the sensor system to detect AC magnetic fields.

Figure 1. Schematic diagram of the sensor system to detect AC magnetic fields.

The voltages produced by the coiled inductor will be quite small, so it is advisable to adjust the detector’s sensitivity control to maximum.

Parts and Materials

Audio detector with headphones from the previous project
Electromagnet coil from relay or solenoid

As a note, what is needed for an electromagnet coil (inductor) is a coil with many turns of wire so as to produce the most voltage possible from induction with stray magnetic fields. The coil taken from an old relay or solenoid works well for this purpose.

Learning Objectives

To determine the effects of electromagnetic induction.
To determine the electromagnetic shielding techniques.

Instructions

Step 1: Connect the sensing coil to your audio detector, as shown in the schematic diagram of Figure 1 and the simplified illustration of Figure 2.

Simplified illustration of the sensor system to detect AC magnetic fields.

Figure 2. Simplified illustration of the sensor system to detect AC magnetic fields.

Step 2: There are many sources of AC magnetic fields to be found in the average home. Test the AC magnetic field system by bringing it close to systems around your house or lab. Try, for instance, holding the coil close to a television screen or circuit-breaker box. Note that physical contact with a magnetic field source is unnecessary; magnetic fields extend through space quite easily.

Step 3: The coil’s orientation is every bit as important as its proximity to the source, as you will soon discover on your own. Experiment by changing the orientation of the sensing coil with respect to the source of the AC magnetic field.

How does the orientation impact the intensity of the signal? What angles (orientations) of coil position minimize magnetic coupling (result in a minimum of detected signal)? What does this tell us regarding inductor positioning if inter-circuit interference from other inductors is a bad thing?

Step 4: If you want to listen to more interesting tones, try holding the coil close to the motherboard of an operating computer (be careful not to short any connections together on the computer’s circuit board with any exposed metal parts on the sensing coil), or to its hard drive while a read/write operation is taking place.

Step 5: Try shielding the coil from a strong source using various materials. Try aluminum foil, paper, sheet steel, plastic, or whatever other materials you can think of. What materials work best? Why?

Step 6 (Optional): One very strong source of AC magnetic fields is the homemade transformer project described earlier. Try experimenting with various degrees of coupling between the coils (the steel bars tightly fastened together versus loosely fastened versus dismantled). Another source is the variable inductor and lamp circuit described in another section of this chapter.

Whether or not stray magnetic fields like these pose any health hazard to the human body is a hotly debated subject.