I saw a magnetic pole detector on a magnet website for $17, and that seemed pretty extreme and over-engineered, so I built my own, parts supplied via my own tech and consulting company, Vadien. I also compiled a kit including all parts that can be bought Here!
This device is extremely simple, and consists only of two independent circuits with a hall effect sensor (y3144), an LED, a couple resistors, and a PNP transistor. The polarity is simply defined by which direction the hall effect sensor is facing.
I know I could've done this just as effectively with only one hall effect sensor, but I liked the idea of having a different colored light for each pole, and the cost is pretty negligible. So, let's begin!
Step 1: Parts List and Principles of Operation
The parts required are as follows, and visible in the first picture:
(2x) Y3144 Hall Effect Sensor, interchangable with A3144, A3141, and a couple others.
(4x) 1k Resistors, I used 1/4W 1% metal film resistors, but it doesn't matter much.
(2x) LEDs, pick one for each color, I used red for north, and blue for south. Note, if you use clear LEDs like I did, don't mix them up (like I did). I solved this problem by putting a small red sharpie mark on the bottom of the red LED.
(2x) PNP transistors, I used a 2N3906 because they're cheap and easy.
(1x) Switch, not really necessary, any will do, I had these lever microswitches on hand so I used them.
(1x) Protoboard, I used a 2x8cm with 2.54mm pitched holes. Again, any will do.
Besides some wire bits, that's all you need. Now let's talk about how it works:
These hall effect sensors require a steady current to work, so they must be independently grounded to allow a small intentional leakage current, therefor the necessary circuits will be power to sensors, power to LEDs, ground from sensors, ground from LEDs, all separately. The LED circuit is interrupted by a PNP transistor, which is controlled by the output of the hall effect sensor, so that they turn on when the hall effect turns off. In this design, we're simply building this entire circuit twice with the hall effect sensors facing different directions, and with different colored LEDs.
Picture 1 is all the parts ready to go, picture 2 is the schematic, and picture 3 is the board layout I used. I'll note that how the transistors are mounted might not make it clear, but the bases (the middle wires) of the transistors is bent to H3 and H4, respectively.
If you understand the concept, then we should be ready to begin! I'm going to use the letter columns and the number rows on my prototype board to help tell where stuff goes, but don't get confused, because the letters are different from the bottom to the top. All coordinates will be given from top.
Step 2: Construction: Component Mounting
Now, all of the parts in this circuit are mounted on the top, which makes construction fairly simple, but there's a few important things to keep in mind. I'll go through the first few parts here.
1(Pictures 1-2): Hall effect resistors. These two 1K resistors are necessary for the hall effect sensor to work, so are placed adjacent. I start with these just because they're at the end of the board. Notice the resistors sit diagonally, this is so that their leads fit straight down the holes. Slide through, then just dab solder and they should be good to go. Leave the dangly long leads for now, we need some of them later.
2(Pictures 3-4): Hall effect sensors. These don't quite fit in the holes evenly because their pins are too close together, so they have to be bent out slightly. Notice it's very important that the power pins are on the outside, and the signal pins are on the inside. If you're reading the text on the hall effect sensor with the leads on the bottom, then the left one is power, the right is signal.
3(Pictures 5-6): LEDs. Be sure their polarity is correct, LEDs have a longer lead to indicate positive, make sure that's closest to the sensors. Refer to the diagram on step 1 if you forget. My LEDs also fit very loosely, so I used masking tape to hold them down (up?) while I soldered from the bottom. Again, leave the leads for now, we need them later.
4(Picture 7): LED protection resistors, also 1K. Again, they're mounted almost vertically so their leads can be in adjacent holes. Slide in and solder just as before.
5(Pictures 8-9): Transistors. These are a little tricky to mount, so again, don't hesitate to refer to the diagram on page 1 if you're confused. Collectors go towards the LEDs, emitters go opposite, and their bases go to the middle two rows. They should both be facing the same way, as in picture 8.
6(Optional, no pictures): Switch. I added mine later, so I'll go over it later, but you could mount it now if you want. See diagram. Any switch in any configuration can be used, it just has to interrupt power or ground.
Step 3: Construction 2: Connecting
These steps get a little trickier, but read twice, solder once, and don't forget the diagram.
1: Bend the hall effect power/signal wires towards the adjacent resistors, and vice versa. Once they overlap by a millimeter or so, feel free to clip the extra, then just dab solder so that they're connected. We're not dealing with any real power here, so solder joints don't have to be perfect.
2: LEDs to power. Take the extremely long positive leads of the LED, and bend them out and solder to the outer two connections you just made in the previous step. Both of these points will be connected to the power supply when you're done.
3: Connect LED negative terminal to resistors, and resistors to collectors of the 2N3906's. See picture and diagram.
4:Connect signal leads of the hall effect sensors to the bases of the 2N3906's. This will require a floating wire, I opted to use super-flexible 24AWG silicone wire, because it's what I had. It did make the process easy, but any wire will do, but make sure your floating connections are insulated from the other leads.
5:Bend the emitter leads of the 2N3906's together and solder, this will be our main ground point. Once you've done that, put floating leads from it to the ground pins of the hall effect sensors (which should be untouched).
6:Wiring power. How I chose to do this step was to solder in a bare un-insulated wire across the entire J column, on S1 and S6 I put floating leads to the hall effect sensor power leads, and on J4 I connected to one terminal of a switch.
7:Done! Just add a positive lead either from any power pin (A1, A6, B1, B6, any J, OR your switch if you have one), and run a ground lead from any ground pin (B2, B5, I2, I5). With this configuration, the circuit will run just fine anywhere from 4.5-15 volts, so a 9v battery clip is probably the easiest power source.
All parts, or fully assembled version, available at our webstore, Here!