Tutorial DIY load cell brake pedal (short tuto)

Take an unused bathroom scale @10€ or buy load-cells at eBay


you got this piece :


You need 2 load-cells to make a wheatstone bridge :


Check the 3 wires to find the wire in the "middle"

check the 3 combination, you should obtain 2 equal measures and one twice bigger
2000 ohm blue - white
1000 ohm red - blue
1000 ohm red - white

hence wire the red (middle) to the Amplificator
blue Loadcell 1 with white Loadcell2

You can make a small board or use a protoboard




S is the Signal from the load cell that you need to connect to the arduino pin that will convert the Analog Tension to Digital values (DAC)

5V is taken from the arduino 5V pin to power the INA
Ground to Arduino Grond Pin: mandatory


you need
- operationnal ampli (INA122PA ou INA125) to amplify the signal 6€
- condensator to filter 100 nF
- resistor to adjust the gain 1,0 kΩ cermet 25 tours


based on the design of Andrew on www.hux.net

or you can buy a dedicated board

- derekspearedesigns
Freestanding Load Cell Amplifier with Variable Gain LCA1C à 26$
- another board 28$ chez Elane.net​

it works!

more information in french here
http://www.racingfr.com/forum/index.php?showtopic=46056

Cooool merci

no need to do that in my opinion...
I made this sketch in the french post:

you just have to use 2 different springs at different location and a hard sleeve.

first spring:
- between pedal and basement
- not connected to load-cell
- only here to find the pedal under your foot
- like in your car: first you "feel" the pedal and the first centimer you press but no pressure is applied on brake
- rather weak spring

second spring:
- this spring is between pedal and load-cell
- the pressure is converted into braking
- rather strong spring
- in your car, your foot can still go down and braking is effective

hard sleeve:
- between pedal and load-cell
- the total pressure (less the first weak spring) is full applied to the load-cell
- no pedal travel is possible
- pressure = braking at no move

Hi Racingmat,

will it work also fine with only one load cell? I will buy the leo bodnar
USB controller board, but his LC amplifier are only fixed ones and the DSD
ones are to expensive. So i'm searching for an DIY alternative!

My Plan is to use only one 150kg load cell for the brake pedal, like the HE Ultimate/Pro ones...
So my questions is if your circuit works fine for my sultan too?

Best regards

Waldemar Jost

@WowaDriver : what are the specs of your load cell?
like this one?

if so it will fit. You can see 4 wires because it is in fact 2 load cell in wheatstone bridge
It only requires to output 0-5V range (for 0 to 150 kg range)

Thanks Racingmat for your reply. Yes it would be a load cell like the one you post with the normal structure of 4 wires... And i thought the output range is between 0-5V!
Thanks a lot, so i have to experiment with it! Thanks again!

Didn't remember that there are to DMS on the stick

@RacingMat Here is the link to the loadcell i want to use... There stand Rated Output 2 mv/V do you mean this with our statement about 0-5V output range? Or do you mean the
Recommended Excitation? Than this AMP wouldn't works with this load cell i think? or i'm wrong?

http://www.mavin.cn/productsmore.aspx?CateID=168&ProductsCateID=168&ProductsID=259

If it wouldn't work fine with these loadcell, do you can change the parts for this load cell? The curcuit must be the same i think, have only to change the chip and pot i think or?

I meant the Recommended Excitation: in your case, it is 5 to 12V which is perfect

some information here: http://www.interfaceforce.com/index.php?mod=library&show=17

Just tested out right now! It works! But i have a little
sough at the beginning... is that normal?

@WowaDriver : you mean a humming sound?

@RacingMat no not a humming sound... what i mean is, when you go to the joystick settings (in my case the bonder USB board) than you will see the scale for the load cell. If the variable gain is completely of, so i mean the pot is full turn in one direction (left) to be off, the scale is still at one position. But if i adjust the variable gain with the pot, the scale work faster right, but at the beginning it doesn't be still on one position. If i can say it extremely: It jumps left right on one point.

"sough" translation was bad :-/

what amplifier are you using?

@RacingMat The INA122PA

http://www.conrad.de/ce/de/product/1010574/Linear-IC-INA122PA-PDIP-8-Texas-Instruments

You should check your soldering carefully... Maybe?

Or check your Gain pot linearity: if pot is broken, it could behave like you describe

Hmm i think the pots can't be broken... They are new ones and i tested it out with two different pots... with the one the effect was much bigger than with the other one... The soldering have to be correct... i mean it is a very simple soldering, but will check it too. When i've got news i will to inform you! May it is a prob with the load cell itself... was just an cheap alternative from amazon to test your circuit ... will test it in 4 weeks with my load cells from mavin...

please post some pictures

@RacingMat Yes sure no problem. But please don't laugh... it is only a ten minutes prototype to test ...

I tested out also a normal pot too... It didn't have the precision of the blue one, but it works too

Will this one be OK?

the HX711 works fine but you cannot read an analog value... it's serial communication: you need an arduino or any other device reading SPI communication

I gathered some information here
http://www.xsimulator.net/community/threads/working-with-load-cells.7340/

In my load cell datasheet specified Working voltage (Excitation) 9-12V. Can I connect it to HX711 directly?