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.
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