A Hall Effect Magnetic Field Polarity Detector

Hall effect sensors can be found in many common household gadgets. Most brushless DC motors for instance, like those used in computer cooling fans, contain Hall effect sensors in the driving circuits. I recently replaced one of the cooling fans in my computer and decided to do some experiments with the Hall effect sensor in that old fan.

After removing the fan rotor you can see the stator and the control circuits. This particular fan is driven by a single chip ATS 277(the data sheet can be found here), which is a complementary output Hall effect latch. It turns on and off the alternate side of the poles depending on the rotor’s position.

The stator of a cooling fan

The stator of a cooling fan

So, using this Hall effect latch IC, we can build a simply electronic circuit that can be used to detect the polarity of a magnet (the magnetic field has to be strong enough to trigger the sensor due to its limited sensitivity).

Here is the schematics I used to build such a detector:

Magnetic Polarity Detector Schematic

Magnetic Polarity Detector Schematic

and here is a picture of the detector after it was built.

The assembled detector

The assembled detector

When the circuit is powered on, one of the two output will be latched at low and thus the LED connected to that pin will light up:

Activated with one side of the magnet

Activated with one side of the magnet

If the Hall sensor detects a strong enough magnetic field, the original pin with the low output will turn high and the other pin will turn low. Thus the blue LED will be off and the red LED will be turned on (see the picture below, note that the red LED is illuminated albeit it is a little dark compared to the blue one).

Activated with the other side of the magnet

Activated with the other side of the magnet

By alternating the polarity of the magnetic field (e.g. use the opposite side of the magnet), the LEDs will alternate between blue and red, depending on the polarity used.

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7 Comments

  1. Trea says:

    Is the a part list to build this experiment? new at electronics…Trea

  2. godwin says:

    nice!!! is it possible to build a brushless dc motor with tis circuit schematic?

  3. Alex says:

    Thank you!
    It’s works but what the number LED 1 or 2 belong South magnetic field?

  4. Joy says:

    Hi,

    Can i turn this to rotation detection? just simple sensor that to detect the motor is rotor or not

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