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Picture of Racing Sim brake pedal load cell interface

A little while ago I undertook the process of scratch building my own PC racing sim.  It's all done now but one thing that i'd like to share with you all is how i accomplished a load sensitive brake pedal.

See, in a real car, the pedal does not really move all that much, even less in a race car.  The braking force is then proportional to the force applied to the pedal and not the MOVEMENT.  The problem lies in that most commercial pedal/wheel setups simply use a spring under the pedal and it detects movement of the pedal, but it just doesn't feel real.

Now, i don't claim to be the pioneer of this load cell idea but i would like to share the interface that i have made to enable one to be used.  There is at least one commercial LC interface available by itself or built into a USB joystick board but it seems they have effectively stopped selling them.  Hence, they are effectively unobtainable.

 
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Step 1: How it works

A load cell (also known as a strain gague) is supplied power and outputs a very small voltage based on how much it is 'flexed' or 'strained'. This is usually about 2 milivolts per volt of 'excitation' at it's rated capacity. 

What this interface does is multiply the output voltage of the cell to produce a 0-5v scale which then becomes useful to ether replace a 'pot' on a commercial pedal set or interface with the likes of an mjoy USB Joystick interface (That's a DIY Atmel AVR based USB joystick interface with 24 buttons and 6 axes).

The amplification is done with the help of a INA122PA from Texas instruments.  It is pretty much purpose built for this task.  All you need is a 0.22uf filtering cap and a resistor sets the gain.

Step 2: Construction

Picture of Construction

All you need to do is source yourself the INA122 chip from your fav electronics outlet - RS, Farnell, Mouser etc.  It should set you back less than $10 even in a single item purchase.

Print, transfer, etch and drill the board.  I used screw terminals on mine but they can be just soldered directly to the board.

The board layout is attached in Swift PCB format.  A free viewer/printer is available for download from them.

Solder the chip and 0.22uf cap in place and select your resistor for the gain.  Theoretically, a gain of about 500 is perfect so accoring to the datasheet it needs a 400R resistor.  This means we get very close to 5v output when the LC puts out 2 milivolts.

Providing your existing pot is running at 5v, you can simply substitute the three wires on the pot with the 3 wires on the LC interface (negative, +5v and signal) and you then have yourself the beginings of a load sensetive brake.

ina122PA.pdf(612x792) 205 KB

Step 3: Wrapping up

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Once you have your interface connected to the load cell and your joystick interface, you should be able to give it a try and see the scales move in your joystick calibration interface.  Then it's just a matter of sussing out the mechanics of mounting the LC into your pedals.

This is a pic of my brake pedal/LC setup.

Enjoy!

you can know the code of the component capacitor. Looking with 0.22UF there are many and they are not exactly versed in electronics. tnks

andrew_h (author)  antoninopanella5 months ago
Just get whatever you want - There is no specific reason for any particular type. A normal Ceramic capacitor is fine, tantalum, monoloithic. The one pictured is an MKT capacitor. Only the value is important.
MattJ56 months ago

Okay I know this is an old post but maybe you can help. I made this and wired it to my tx racing wheel's stock 2 pedal setup. Does not work but when i press the accelerator pedal the brake presses. I can follow most instructions but do not have great knowledge of electronics. The positive wire from the base comes down to the positive or the accelerator pot and then is wired in series to the brake pot. The signal is from the bast to the brake pot and then the brake pot has to wires from the base and the ground in series from the accelerator pot. I just wired it to my board in the same fashion. Any ideas as to what is making this happen?

andrew_h (author)  MattJ55 months ago

Hi Matt, I'm not sure I follow your description of the connections but providing your pedals work with a basic 0 -5v signal on the output of the brake, this should be a drop in replacement (plus some tuning). The 3 pins on the output should match the 3 wires on the pot. Is your wiring like this?

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PKM5 years ago
You're designing your own racing sim- anything related to either of these two recent Hackaday posts?

The mjoy sounds useful but I can't find much information about it, can you tell me anything more?  I obtained a steering wheel/pedal set a while ago, but though it's nicely built (Momo pedals and a solid wooden case) it uses potentiometers and the gameport interface so has terrible precision and drift.  I contemplated rewiring it, perhaps with some optical encoders from old mice, and hooking it up to my Arduino, but didn't know how to make that act as a game controller to the PC.
andrew_h (author)  PKM5 years ago
 No, my sim is not related to hackaday, it was inspired by boredom.  

I had a few bits lying around so i thought i'd give it a shot.  I started with the wheel and using optical sensors but I just could not get it reliable. It would lose it's center and just generally sucked. You're much better off using pots.

The Mjoy interface was originally developed by a guy named Mindaugus.  Unfortunately he has gone missing from the net but his legacy remains in the likes of http://web.archive.org/web/20070228151145/www.mindaugas.com/projects/MJoy/ 

I used a board based on the one found here translate.google.com.au/translate (Click on the PCB link). I made some minor changes to it to increase some track gaps and added the ability to use normal diodes for the buttons rather than SMD.  I have got heaps of pics of my sim build as well as the mjoy board stuff on my site - http:///www.hux.net.au
andrew_h (author)  andrew_h5 years ago
Sorry, the link to the original board design is translate.googleusercontent.com/translate_c