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!

10 March 2014

BYO multi board thirty band controller for 1/32 racing

This one is for those tight on local Rands and willing to do a relatively simple bit of handiwork to give themselves the versatility similar to the 1/32 Difalco 30 band controller, especially if contemplating something like non magnet plastic racing in addition to magnet racing, where alternative resistance boards are a must.

One way is to source a Parma turbo controller and bolt on:
1.   1/24 difalco conversion module economy rheostat slot dd253-hd30
2.   for that "feel" Difalco ball bearing upgrade kit w/trigger pin
3.   If you really want to go the whole hog (not necessary) - Difalco diy genesis frame kit w/trig pin

Using the conversion module instructions and this useful little pictorial which shows how to modify the Parma Turbo frame and fit the board - the board is different but the assembly and hook up precisely the same:

Or one can go the cheapie approach I used for Craig's controller, using the Euro style pnp transistor and simply bolting (with bolt insulation) the pnp transistor to the Parma Turbo frame and following the simple hook up. I recommend using the 1/24 upgrade board with 148 ohm for magnet racing and purchasing a spare 290 ohm board only if you intend non magnet racing.

For this approach source a Parma Turbo controller and bolt on:
3.   R12     From Mantech or many electronic shops, the Tip36c Transistor
4.   £7        Brake pot Wirewound 4W Power Potentiometer 25-Ohms (50 ohms ok as well)
5.   £7        Sens pot Wirewound 4W Power Potentiometer 50 Ohms (25 ohms ok as well)

Just follow the hook up guide in this picture using colored wire, cut completely though the board rail shown between H and C so there is no continuity and you good to go. No cutting of the existing wiring other than splicing in to the existing white and black cable and chopping and extending the red cables to hook to the 25 ohm brake pot, which is bolted on along with the 50 ohm sensitivity pot, which is soldered on where shown. That's it you done.

This the assembly sequence for the Craig pnp controller:

Bolt on upgrade board and transistor and cut frame heat sink

Note screw insulator for transistor and home made "L" brackets to hold board to frame

See it fits and chop case to suit

Chopped Case

Wiring and wiper arm added

Hook up to back of upgrade board.
That is pretty much it. If you prefer to go with the original US npn transistor approach then you don't have to cut through the track between H and C but you do have to insulate the wiper arm from the wiper base using tape on similar, as per the detail hook picture below:

On my version I used a Difalco replacement power transistor 1/32 for $7 or you can local and cheap with the Tip35c npn transistor for about R14.

Once complete, if you wish one can add a 30 amp or more automotive relay. One has to disconnect the white cable from the full power stop and connect in a new "skinny wire" from the relay to the full power stop. The rest is hooked up as per Jim Difalco: The brake wire is on #86, the skinny control wire that turns on relay is terminal #85. Then the two main black and white controller wires go to terminals #30 and #87 (does not matter which goes where).

Changing the Wiper arm button to floating is also recommended, guide as per Jim Difalco:

A floating button is pretty simple. About 1/4" below the wiper button hole bend the arm down, towards the circuit board, at about a 30 degree angle. Next at the centre of the wiper button hole bend the arm up at a 30 degree angle. Make sure you make both bends parallel to the top of the trigger. This setup with all the wiper pressure on the centre of the button lets the button pivot top to bottom. This is needed so when you pull the trigger the top or bottom button edges do not lose contact because of the play in trigger bearings.

Next file the face of the wiper button flat. File the face of the stud and tin it with solder. Now install the trigger in controller. If you have an electronic type controller solder the skinny wire to the wiper button stud. If it is a resistor type controller solder a short 1" piece of leadwire from the stud to the wiper contact that is over the wiper arm to ensure connection. Leave a little slack here, do not run the wire straight from button to trigger contact. The button needs slack to pivot. 

Tips: if you are trying this on a resistor controller the wiper button edges should be rounded off. If you are trying this on a Parma resistor that resistor should be sanded flat or you will be causing the button to catch the edge of any high band.

Adjust the wiper arm tension by gently pulling it away from the wiper board. Check that the wiper button does not lose contact as you pull the trigger across the contacts. Also check that the button contacts the first band at the front edge of the button. If it does not, you can twist the front edge of the wiper arm down until this happens.