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.
PKM4 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)  PKM4 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 

I used a board based on the one found here (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:///
andrew_h (author)  andrew_h4 years ago
Sorry, the link to the original board design is