Introduction: 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.

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

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.

Step 3: Wrapping Up

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.