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!

Comments

[phanphamvietlong]

I using INA114PA, with ACC Pedal is good working, I can get the signal with loadcell 10Kg, Brake pedal with a load cell 100Kg but I can not see the enhancement of the signal, what should I do for this case? I tried to push with more force even stand on another side of loadcell.

[AussieMakerGeek]

What resistance are you using for the gain?
See image...

[phanphamvietlong]

Can i use AD627 to replace the INA122? i can not buy it at my country?

[Binleo]

Hi, I using 1 AD627 and INA114AP but the out is so small, that I can not measure but out put just rise about 1%. (i can see output change but it is not enough to use). have any recommend for me. I use 330R for gain :(

[AussieMakerGeek]

yeah, most of the chips are the same function. I have used many different ones.

[phanphamvietlong]

thank you so much for reply. Let's me do it :D

[bebop47]

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?

[AussieMakerGeek]

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?

[bebop47]

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

[AussieMakerGeek]

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

[bebop47]

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,

[AussieMakerGeek]

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.

[bebop47]

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

[AussieMakerGeek]

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

[danielromero963]

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?

[jeffkruger]

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

[AussieMakerGeek]

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

[marcinpolniak]

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

[MattJ5]

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?

[ClaudemirC]

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