Magnet Polarity Detector

Recently fellow Hackster Mirko Pavleski published a project called DIY Magnet Polarity Detector - Hall sensor from PC fan. It is a simple tool that determines the pole of a magnet. This project extends the detector by adding a 3D printed case that is designed to fit easily in your hand.

3D case

The case is printed in PLA using a 0.2mm layer height. You will need to orientate the parts on the build plate and add supports for the probe and case top parts.

Assembly

Connect four wires to the FS276 or FS277 hall effect sensor and slide the wires and sensor in from the top of the probe.

Ensure that the writing on the sensor is at the top and glue the probe to the case top part using super glue.

Cut the leads on the 3.5V 120mAh battery and solder the leads to the TP4056 charging module.

Put the TP4056 module into the case and hold it place with hot glue.

Press fit the switch and two LEDs. (Look at the picture below to make sure the switch is orientated correctly - note the placement of the small cut-out on the switch).

Solder a wire from OUT+ on the TP4056 module to the two center pins of the switch.

Connect the two anodes of the LEDs.

Solder a 100 ohm resistor from the anodes to the switch.

Cut and solder the wires from the sensor. The VCC pin goes to the 100 ohm resistor on the switch, the GND pin goes to the OUT- pad on the TP4056, the N output goes to the green LED cathode and the S output goes to the red LED cathode.

Finally add the back and charge the battery.

Issues

I followed Mirko's advice and disassembled a 80mm case fan. The sensor in my fan was a AH2984 - Two Phase Hall-Effect smart fan motor controller. The pin out was the same so I used it in this build. However instead of the lighted LEDs showing a continuous light, mine flashed briefly every second or so.

Looking at the datasheet, unlike the FS266 or FS267 sensors, the AH2984 has extra logic to detect motor stall. The datasheet states:

"To help protect the motor coils, the AH2984 provides Rotor Lock Protection which shuts down output drives if rotor lock is detected. The device automatically re-starts when the rotor lock is removed."

This I think is why my unit flashes. Because the field is not changing, the AH2984 logic thinks it has a motor lock and shuts down. A second later it powers up and tries again.

Conclusion

You can get 20 new ATS276 hall sensors from AliExpress for around AU $5 which saves gutting a fan which may or may not have the correct hall sensor for this project.

All-in-all a nice build. Thanks Mirko for another great project.