Build a Magnetometer

The parts needed to build this magnetometer include two linear Hall-effect chips (Allegro 3515, 3516, 3517, 3518, 1321, 1322, or 1323 will all work), a 7805 voltage regulator, an LED, a 1000 ohm resisitor, a junction box, a single-pole light switch, a telephone jack, a faceplate to hold the switch and jack to the junction box, four-conductor telephone wire, a 9V battery holder and connector, heat shrink tubing, and two cables with alligator clips on each end. Finally, you'll need a voltmeter to read the output from the sensor.

The Hall-effect chips have three legs; when looking at the printed side of the package, the left leg is the power source (5 V), the middle leg is the ground connection, and the right leg is the signal. With no magnetic field present, the signal will be at a 2.5 V potential relative to ground (or the 5 V power leg). Actually, the signal will be 1/2 of whatever voltage you use to power the chip, but it's 5 V for this. The presence of a north pole approaching the printed side will lower the output voltage relative to ground (down to approximately 0 V) while an approaching south pole will increase the voltage relative to ground (to a high of 5 V). In an effort to make a more sensitive meter, we will combine two of these chips back-to-back. The middle legs will be connected to each other and ground, and the left leg of each will be connected to 5 V. We can measure the voltage between the two right legs, and this will give us twice the sensitivity. (Varying sensitivities of chip are available, but a 1 Gauss field will generally produce a change of at least 2.5 mV. When the two chips are back-to-back, one chip's signal will go up by 2.5 mV while the other one drops by 2.5 mV, for a net voltage difference of 5 mV).

The two chips are super-glued together and then heat-shrink tubing is applied to the middle legs to keep them insulated from the other legs. While all the chips should ideally put out the same voltage (1/2 the supply voltage) in the absence of a magnetic field, in reality there are small differences between them. The chips can be individually examined and chosen so that they are paired with another chip which has a similar output voltage. Several chips are being tested in the photos below.

Once the two chips have been chosen and glued together, you can start drilling holes in the faceplate of the junction box. As you can see in the picture below, you'll need to drill 6 holes. (The sixth hole can't be seen in this view, but it should be just on the opposite side of the light switch. It needs to be large enough to hold an LED, and it's a good idea to keep it close enough to the switch to still fit inside the junction box.) The leftmost and rightmost holes provide a way to attach the assembly to the junction box. The lowest hole is for the alligator cables to pass through to the outside, and the two holes in the middle of the faceplate are to anchor the battery holder and 7805 voltage regulator chip (#4 machine screws work well for this).

Cut the alligator cables in half and strip the wire from the cut ends, feeding three of the half-cables through the hole from the outside. The picture shows only two half-cables, which is the minimum needed to work. The third one will be connected to ground and used mainly to diagnose problems. At this point, you can add heat shrink tubing to the wires, mainly for mechanical support. They can then be connected to the screw terminals of the phone jack. The alligator clips will be connected to the voltmeter leads when you're finished feeding them through the hole from the outside.

You can now cut the modular phone cord in half and strip the ends of the four wires inside it (not easy!). Once you have them, you should plug the cord into the jack and do a continuity test to see what color wire is connected to what color terminal. Because you're cutting a phone wire in half, one half will be what you expect (red wire = red terminal) and the other one will be a little different. An unexpected by-product of rising copper prices is the tendency for some manufactures to use wire with a high percentage of iron in it. Since iron will tend to distort the magnetic field lines you want to detect, this is not good news. When you go to the store to buy phone wire, take a strong magnet with you and check the package before you buy it.

It's a good idea to put some heat-shrink tubing on each individual wire, as well as a larger piece on the entire wire. The pieces on the individual wires below ended up being too small and were later replaced.

You're just about ready to connect the dual-chip assembly to the phone wire. Before you do, make sure the legs of the two chips are well insulated from each other! This is the most likely place for a serious problem to develop. Although it isn't shown on this model, it's a good idea to apply heat shrink to the signal legs (all the way to the top) as well. This is easier than trying to apply it to the power legs, as they're on opposites sides of the chips and don't really need any more mechanical stress. I'd recommend this order: solder ground wire to ground legs, cover with heat shrink tubing, solder one signal wire to one signal leg, heat shrink it, repeat for the next signal leg/wire, and solder the power legs to the power wire last. It takes some careful bending to get the two power legs together without breaking them, but it can be done. Put a large piece of heat-shrink over the whole assembly to protect the power legs and make a nicer probe.

To review, the four wires are connected to the 6 legs of the chip assembly as follows: one wire to the two ground legs, one wire to the two power legs, and one wire (each) to each signal leg. The phone terminals provide an easy way for us to make the power, ground, and signal connections from the sensor wire to the outside world.

Back to the faceplate. You can run the red wire from the battery clip to one of the light switch terminals, and then a wire from there to the left leg of the 7805 chip (left as seen looking at the numbers). Solder this in place and use more heat-shrink. The black wire from the battery clip can go directly to the middle leg of the 7805, but you should add another wire so that you'll have an available ground connection. Solder & heat-shrink again.

Finally, connect a single wire to the right leg of the 7805. This is your 5 volt supply line. Connect this wire and the ground wire to two of the telephone terminals. (This is why it's so important to know which terminal goes to which color of wire. If this is screwed up, it's not impossible to fix; you just have to remove the wires that were already attached to the phone terminals and move them around until you get the connections correct.) Also, you should add one more wire to each of the two terminals above (power and ground). These two wires will power your LED. This means that two of the phone terminals should have their original connections plus connections to the alligator clips, and the remaining two terminals should have their original wires plus two extra wires each (+5V in and +5V out for one, ground in and ground out for the other).Finally, the third alligator clip wire is connected to the ground terminal.

Now connect the +5 V "extra" wire from one of the phone terminals to the long leg of the LED. Connect the other leg of the LED to a 1000 ohm resistor and connect the other end of the resistor to your "extra" ground wire. You may want to use more heat-shrink here. (The resistor and LED are shown at the bottom of the picture below).

That's it! You should now be able to add the battery, plug in the phone cord/sensor, and screw the face plate to the junction box. As you can see below, the voltage output will change sign as the magnetic poles are switched.

If there are problems, you can connect the voltmeter between one of the signal alligator clips and the ground clip. You should get about 2.5 volts between the two if everything is wired correctly. If you want measure very small fields, like the Earth's or the field from small appliances, you can see how to construct a cheap amplifier here. Back to my home page