Racing Sim Brake Pedal Load Cell Interface

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.

Enjoy!

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    20 Comments

    0
    bebop47
    bebop47

    Question 11 months ago

    I am trying to make my board work with my logitech dfp but no luck. I measured the pots and it operates at around 0-3.8v.
    Im using a different capacitor which is 100nf. Do i need a different cap? Im using also a 1k ohm trimmer for adjusting gain. Any thoughts?

    0
    AussieMakerGeek
    AussieMakerGeek

    Answer 11 months ago

    The Capacitor should not be a problem, neither the trimmer as long as you have it connected correctly. When you say you measured the pots, I assume you mean the original pot on the pedal?

    0
    bebop47
    bebop47

    Reply 11 months ago

    yes the pots on the pedals measure 3.8v. also the signal on the brake goes 0v-3.8v (0-100%) and the throttle goes the other way from 3.8v-0v (0-100%).
    funny thing is when i connect the board the voltages on the wires on the pedals goes down to 1.9v

    ina122pa board.jpg
    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 11 months ago

    It seems ok, try leaving off the cap and see what it does. Have you tried with the load cell connected?

    0
    bebop47
    bebop47

    Reply 11 months ago

    i think the main problem is that the output signal from the board is the same as the supply voltage. if the board is connected to a usb 5v line, the board outputs 5v too. even if the load cell is connected or not,

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 11 months ago

    Yeah, that should not be the case. You could try adjusting the gain with a multi-meter attached to the output and see if you can get it to drop to near zero but failing that, there is something wrong with the chip. I am actually using a different design these days using an INA826 and with an improved circuit that includes gain and offset (setting 0). The downside is that the INA826 is a SOIC8 chip and not DIP8. However, the circuit would technically work with both, they just have different pinouts.

    0
    bebop47
    bebop47

    Reply 11 months ago

    yeah i think the chips are faulty. if this doesnt work for me im going the hx711 arduino route. i wanted the ina122 board because i read somewhere that analog is more reliable/less latency? not sure if thats true tho

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 11 months ago

    That is absolutely true. The INA chips are pure analog whereas the HX711 is laggy as it does AD conversion and then you have a DA conversion back. Plus the HX711 is limited to 80hz which seems fast but adding in the other delays makes it less than ideal

    0
    danielromero963
    danielromero963

    1 year ago

    Thanks for your help sharing this project. How are you supposed to provide - 5 v to the load cell ??? Is there any easy way to do this with arduino?

    0
    jeffkruger
    jeffkruger

    Reply 1 year ago

    I used a separate USB cable from a second USB port on the pc.

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 1 year ago

    The circuit board provides the excitation voltage to the load cell directly so I am not sure what you mean. You need an instrumentation type amplifier chip (Like the INA122PA) to amplify the load cell signal enough for an arduino to be able to read it, so no you can't do it directly with an arduino but this can be connected to an arduino analog input if you want.

    Edit: Or did you mean -5v? The answer to that is you don't, it's just +5v and Gnd which most pedals already have. It will also work with 3v pedals such as Thrustmaster but you might need some more gain

    0
    marcinpolniak
    marcinpolniak

    Question 1 year ago

    Why i cannot download the LC_Board.LAY? F62529VG91YH1GD.tmp - this file is downloading

    0
    MattJ5
    MattJ5

    6 years ago on Introduction

    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?

    0
    ClaudemirC
    ClaudemirC

    Reply 1 year ago

    TX works at 3.5v and not at 5v maybe that's the problem!

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 6 years ago on Introduction

    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?

    3-02-2015 8-55-32 PM.png

    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

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 6 years ago on Introduction

    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.

    0
    PKM
    PKM

    11 years ago on Introduction

    You're designing your own racing sim- anything related to either of thesetwo 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.

    0
    AussieMakerGeek
    AussieMakerGeek

    Reply 11 years ago on Introduction

     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