Racing sim update

Ok, as promised here is the details of the electronics.  Most of you would be interested in the load cell interface so i’ll start with that.  The interface is truly simple, the main chip, an INA122PA is only about $7 from RS and a little more from Farnell.  There are other equivalents but this is the one i used and is also one of the cheapest.  The pinout is very basic for our purposes.  You have a power supply (coming from the Mjoy) to power the chip and the load cell.  The load cell applies it’s voltage to 2 pins and the chip simply amplifies the voltage from the load cell.  The gain (how much it amplifies the voltage) is set by the resistor accross pins 1 and 8.  The capacitor is a simple 0.1uf filtering type. The cell puts out 2mv/v so at 5v excitation we get 10mv at full scale.  To get 0-5v we need to multiply that by 500.  For this chip that means a resistor of about 400 ohms.  Less resistance will give more gain.  Since my load cell is a 100kg item i have increased the gain on mine to about 600.

That’s about it!  Hook it up inline instead of a pot and you’re done.  Just make sure you get the wires around the right way…

lc_interface Load Cell interface

As for the main Mjoy board, I modified a layout i found around the net.  I have not removed any ‘branding’ so if one wanted to seek out the original designer i’m sure they could.  The only real change i made was to add pads to be able to vertical mount standard diodes for the buttons rather than use SMD types.  Most other changes were to add some distance between tracks etc.  Print, transfer, etch, drill and solder etc.  I made up my own paralell port programmer interface to program the chip in place with the provided header.

One thing i would like to add here is that originally i tried to use an AtMega88 as i was told they were basically the same.  Unfortunately they are not.  There are too many dfferences to allow the standard hex file (for the AtMega8) to work.  Someone has ported the code to the ’88 but i have not been able to get this to work.  I presume it has something to do with fuse settings on the chip but even though i have set them right, it still does not work for me. I ended up having to stick with an AtMega8 at about 5 times the price of the ’88.

mjoy_pcb p3210025_sml1

PCB Layout files (Sprint 5.0 format, free reader available to be able to print them

NB – The Mjoy PCB has incorrect axis lables, they are backwards so it should read X,Y,R,T,Z,Rx.  Not really important other than the fact that the XYRT axes are 10bit and the other 2 are only 8bit. Earlier versions of Mjoy only have 2 10bit axes (X and Y)


Please let me know if you download these files, it’s nice to know someone else is finding these useful.

I’ve also started work on my H Shifter for the sim.  I’m trying to keep the design simple but at the same time i want something robust and somewhat realistic.  Combining the ideas of some others, i have come up with my own design which should allow for this.

Shown below is the basis of the shifter (not complete obviously).  Between the 2 horizontal bars (the ones with holes in them) will go a block of delrin which supports 2 standard skate bearings.  I have not yet finished the profile on the horizontal bars, there will be another dip in the middle. The delrin will freely slide over the 1/2″ shaft and will be followed by a spring and then a retaining clip.  This will force the block/bearings down on the ‘ramps’.  This should give a ‘clunk’ into gear and also allow for a little freeplay between gears in the ‘y axis’ when viewed from above.  The centering mechanism will be another single spring around the shaft that sits on the rod end and is secured again by a retainer.  The compression of the spring will try to keep it centered.  From there it is only a matter of making a gate plate and hooking up the switches for the gears.  I intend to mount the switches in the base of the unit to keep them safe.

p3250031_sml p3250030_sml

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12 Responses to “Racing sim update”

  1. thren says:

    Im very interested in the load cell pcb as I have just started to make a cst style drop down pedal set.

    I dont know alot about electronics but can make my own PCb’s using the negative photo resist method and a UV light box.
    I take it, from reading your post, that this load cell pcb could connect to any mjoy pcb…. there are a few on the internet, in place of a pot ?

    Currently I am in the metal fabrication stage of my pedals and am looking at my options as to what PCB to use.

  2. Andrew says:

    That’s right, the load cell interface ’emulates’ a pot running at 5v, so any joystick interface that runs a standard pot as a voltage divider at 5v will work fine i.e all Mjoy versions. Just hook up the 3 corresponding wires and you’re good to go. The INA122PA will actually work at lower voltages but most load cells are not rated to work below 5v.

  3. Ken says:

    Been looking all over for the load cell amp circuit. Got the chip before RSC died but didn’t save the circuit diagram (Doh!). Thanks for making this available.

  4. Charles says:

    Hi. Can you please email me the software for the mjoy usb joystick which works with your pcb layout? I tried building this circuit previously from another website but had encountered strange issues eg. when all of the switch inputs where used, one switch would turn on another switch etc. Also, does this circuit disconnect automatically when you switch off your pc?

    The most nerve wracking problem was configuring all of the controls & then losing the configuration once your restarted or shutdown your pc.

    Does this version have similiar bugs/issues?

    Awaiting your response…

    Charles (South Africa)

  5. Andrew says:

    Sorry for the late reply. I used version 1.1 from here – Do not get the ‘mod’ ones, they use a different board and i had no luck with version 1.2 (autocalibration version). Stick with v1.1 and use dxtweak2 to do your calibration (not windows normal interface).

    The reason you will be getting ‘crosstalk’ on the buttons is because you need diodes on them – My board allows for 1n4148 signal diodes.

    I do not have any problems with having to reconfigure the joystick after each reboot with mine.

  6. Charles says:

    Thank you kindly Andrew! I will give this a try. I intend using this in my home arcade video game design.

  7. Derek says:


    I hate to be dumb, but I don’t know what program will open your .LAY files. I tried the swiftview program, but it tells me I need the pro version or something. I would like to see the PCB for the load cell in a PDF, but no worries. I can use the gif you have to make my own board. Your load cell amp is, seemingly, just like Bodnar’s, and I think this one here is much les money and will do the job. Thanks for putting this up.


  8. Andrew says:

    Sorry – My bad, they are ‘Sprint’ format files – The view/print version is here:

  9. Derek says:

    Thanks for the update! I was able to get all of it :)

  10. pablo says:

    hi, good job for your h-shift
    time ago y do the Mjoy and my h-shift, however i sued mm a version for flight simulator because it had more speed of connection than cars-versions.

    After when win 7 64 bit come, my mjoy dont work any more, maybe another version of mjoy can´t wok (i dont know, if you know something about tell me), I haven´t time to do another version of mjoy and check the problem of win7 64 bit

  11. Andrew says:

    I considered the flight versions but there was no real significant gain that was noticeable on a racing sim. My mjoy is running fine on win7 64 bit.

  12. […] or Derek Speares for example.  Or you could just build one your self on proto board, like I did.  Here’s an example circuit, with a INA122 instrumentation amplifier.  It connects to the three leads that […]

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