WHEN: Most Friday evenings. TIME:

14 March 2014

Sort those traction magnets!

It is pretty much common knowledge that not all magnets are of equivalent strength and some even vary wildly in strength between the north and south poles! There is an obvious upside to sorting out the good from the dud magnets and also marking the side with stronger magnet attraction so that faces down to the track.

The magnet strength is termed "gauss" and there is a very cheap and simple way to make your own tester, using a hall effect device and voltage regulator, totaling twenty bucks from Mantech and other electronics shops. Most have their own multi-meter or if not, these are cheaply bought from places even like Osmans or your nearest electronics shop. The black mini multi-meter in the pic cost me twenty-five bucks so not a huge investment.

The hall device measures 0 to 2.5 volts for the north pole and 2.5 to 5 volts for the south pole. Either convert the south pole reading back to 0 to 2.5 volts on your calculator so the measurement is same and just use the voltage or convert both to gauss for a common reading, as well.

There are some links to the "techie" document at the end or simply go with the extract text I found the most useful and put below. A spreadsheet conversion excel file is downloadable to save you own number crunching or simply print off the mini summary, which seems to cover the range of 25 mm traction magnets we use.

Below is the two components built on to some vero (perf) board, showing a voltage reading of 3.39 volts with a tyre as spacer. reference to the table or calculator gives the gauss number. I cut out the completed circuit portion of the board and glued the whole caboodle under an old slot car case base to give somewhere between 4 mm and 6 mm gap between the magnet and hall reader - any jig will do. The guy in the article was only using a ferite magnet so was able to rest directly on the hall reader, our traction magnets are far too strong for that.

The invoice is for the un-calibrated hall effect device and five volt regulator that was purchased, reference the article for more detail. Also two small crocodile clips to clip on the multi meter probes. Below is the extract picture from the article that I followed in conjunction with the text extract below that. Note the writing faces up on both gizmos and you then can follow the logic:

"Now, how do you make it?
Connect the + (red) of the battery clip to the input of the 7805 (pin 1).
Connect the - (black) of the battery clip to the common of the 7805 (pin 2).
Connect the +5V input of the Hall device (pin 1) to the output of the 7805 (pin 3).
Connect the common of the Hall device (pin 2) to the common of the 7805 (pin 2).
Set the voltmeter to read 20Vdc max.
Attach the + of the voltmeter to the output of the Hall device (pin 3).
Attach the - of the voltmeter to the common of the 7805 (pin 2) or the common of the Hall device (pin 2).

You are now ready to snap a battery onto the battery clip.
The red lead from the 9V battery goes to pin 1 of the 7805. The black lead from the battery goes to pin 2 of the 7805. The output of the 7805 (pin 3) is connected by a green wire to pin 1 of the Hall device. Pin 2 of the 7805 is connected by a black wire to pin 2 of the Hall device. Please note that the marking on the Hall device (giving its part number) is facing the camera. The voltmeter common (black) is connected to pin 2 of the Hall device. The voltmeter input (red) is connected to pin 3 of the Hall device. (I got the voltmeter from a Home Depot store near here for about $20.) That's all there is! Great, or what?!
The voltage at pin 3 of the voltage regulator. Ideally it is 5.00 volts, but we measured 5.02, which is close enough.
The output of the Hall device when no magnet is nearby. Ideally it is 2.50 volts, but we measured 2.59. This would be our V0 as noted above. The Hall device I have here is an Allegro UGN3503U, with a sensitivity of about 1.3 mV/G."

One assembled and good to go, either download the full spreadsheet here:

Or click and print off this graphic that has the range most magnets tested operated in:

Having completed the project, I found this circuit I must have downloaded previously (apologies for not crediting its owner), which is a bit more larney with switch and led light and maybe easier to follow:

A small investment in time and money and gold dust for magnet racing!

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