Logitech G25, G27, G29, G920 load cell DIY project

[cteters]

Seeing that I haven’t completed the work, allow me to showcase the work of others in my place, as this is starting to become more of a repository of ideas on the topic.

RD member GeekyDeaks showcases a bathroom scale based load-cell project on his github
- More details about bathroom scale based loadcells

RD member Grezson built one by using a 20kg straight Bar load-cell (which are very cheap btw)
- More details about straight bar based loadcells not specific to logitech

- More general information about load-cells
- Some older research completed that is still relevant

- RD member Panicpete has also shared a lot of detail about his own project that can be found on this thread

Provided in the spoiler below is all the most pertinent information I’ve dug up on building a button load-cell for the Logitech brake pedal. Very similar in design to that of Richmotech’s model. The total cost for this project would come around $60 to $70 on the low end roughly. Be advised, I’ve not actually completed this project and ended up installing the AXC Sim brake mod in it’s place, which comes at the same cost it would take to build a button load-cell mod.

Spoiler: button based load-cell project

Parts list:

Steel Spring:
30mm Outer Diameter (This is wrong, I will updated the O.D. and I.D. later)
2.0mm Wire Diameter
50mm length
A length of 50mm is overshooting it, so the spring would need to be shorten to length with a dremel. A dual rated spring such as what is used with the nixim and gteye mod might also work, but I presume that having a combination of the spring, rubber and the load cell should provide for enough change in pressure. I've also read of some success by using a product called Real Pedal that can be found on ebay, which comes with a spring and sponge, but a bit over priced again at $30. So best to DIY this imo.

Load Cell
The load cell should have an Outer Diameter or 25mm and not likely much larger, but definitely not greater than 28mm. The Logitech housing that holds the spring assembly has an inside diameter of 30mm, so it needs to be a bit less than that. The actual rating of the load cell should be around 45kb (100lbf) and a 3 wire system that can be supplied with up to 5V.

Example load cell:
Button Load Cell (50kg) - CZL204E
FC22 Compression Load Cell (45kg)
note: This particular model would need to have the mounting brackets edged off with something like a dremel.


Amplifier:
An amplifier might not actually be needed if the supplied voltage is maintained, but they are pretty cheap and might be a good fail safe to have. An affordable standalone load cell amplifier by Leo Bodnar or maybe something like a SparkFun Load Cell Amplifier - HX711. Can't really say for sure what the best option is just yet.

OR build your own:

Spoiler

Parts:
Amplifier:
INA122PA
or
Op-Amp (Through-Hole) - LM358
Ceramic Disc Capacitor
Ceramic Disc Capacitor 10nF
Resistor
10K Ohm Trimmer Trim Pot Variable Resistor

Rubber fuel line:
This should have an outsider diameter that doesn't exceed the insider diameter of the steel spring (possibly a hair shorter to be on the safe side) and then just trim it up to fit inside the spring.

Felt or Foam:
To wrap around the load cell so as to make a more snug fit.

For assembly, simply refer to any Ricmotech style information, such as,
Sim Racing Garage review
Ricmotech assembly manual

 

[NDG]

I kinda knew that I am deforming the load cells doing that. But I'm a bit frustrated at that point, I can no longer see any flaws on the boards and components, double check, triple check and it's all really good. So I took out the cells and find a way to bend them and then they worked. The insides of the 3d prints are how they're supposed to be. I have weeks worth of reading about many peoples projects and I really can't see any flaws on my work anymore. I guess the cells are flawed from the start that started my loop of looking for flaws in the wrong places. At least one issue is solved. Gotta wait for the new ones to arrive. Thanks all you guys for the outstanding help! I'm happy for now on the PCB side.

 

[GeekyDeaks]

So I took out the cells and find a way to bend them and then they worked

My guess would be that it's an amplification issue and bending the load cells beyond their rated load is showing up as the mV change across pins 2 and 3 is much larger.

Have you tried measuring the mV change across pins 2 and 3 whilst the load cell is installed? It might give you some clues. You are looking for about 5-10mV to start with, then decreasing close to 0-2mV under load. If you find the mV going up, then you need to switch the wires on the load cell as one side will increase whilst the other will decrease.

 

[NDG]

I am happy to report that I have managed to make things work according to the designs shared on this thread. Yes it's a PITA to calibrate things. I did get it to working order, just enough to run some laps around. After a while, the brakes started to struggle to go back to zero. Then it went to a pulse of on and off state. Almost like a fast heartbeat. Mind you this is still that one load cell that I think is faulty. The new parts arrived but not the new load cells. I also needed to jam some tooth picks inside the 3d prints to make the load cell sorta *float or elevate a little. I cannot see where the thing is leaning, I can see the hollow space for the middle part of the load cells, but it's not working without the tooth picks.

Have you tried measuring the mV change across pins 2 and 3 whilst the load cell is installed? It might give you some clues. You are looking for about 5-10mV to start with, then decreasing close to 0-2mV under load. If you find the mV going up, then you need to switch the wires on the load cell as one side will increase whilst the other will decrease.

I got a new tester. I will have to do that sometime these coming days. The tester I got came with Chinese markings with English manual. Lol

May I ask what is the difference between using 0.1uf caps VS 0.22uf caps that I see on other people's design?

 

[GeekyDeaks]

May I ask what is the difference between using 0.1uf caps VS 0.22uf caps that I see on other people's design?

The capacitor acts as a filter with larger values handling lower frequencies. If noise is a big issue for a circuit you may have several at different values to filter out the expected frequencies, but in this application the noise should be minimal anyway so we are just trying to smooth out any quick dips in voltage. anything around the 0.1uF - 1.0uF is probably going to be ok

 

[NDG]

Gotcha! Thanks so much man! I'm pretty much done with the project. It took some time, mostly because of waiting for parts to arrive and a few hiccups along the way. I read your GitHub page but copied the blue PCB example on page 5 of this thread. I should've sticked with your schematics. My bad. You may be familiar with the shape of this 3d print, @GeekyDeaks , thank you so much!

The blue example on page 5 used a 10k pot for balance, oh yeah it's a PITA to calibrate, basically unusable! Your shopping list says use a 10ohm pot but I can't buy that anywhere due to availability. So I got 50ohms and 1k pots to replace the 10k pots. The 50ohms pot is easier to use finding the sweet spot, but it drops detection when pressing the brakes. It goes half full on a full pedal press then drop back down to zero. So I changed the balance pot again to 1k and it's able to hold a lot better now. Finding what works and not was a PITA, that I can say. My finished product now features two 1k pots for both balance and gain.

The 3d printed parts were also flawed. I don't have a printer so I sorted to online printing services. They didn't in-fill the plungers so it's hollow and they broke immediately. Replaced them with nuts, bolts and washers. They're good now.

I have finished two boards using old parts that I initially thought were fried. I did get the new parts but I just stuck them in my toolbox for future use if needed. I have updated this post for photos.

Thanks you all for this tutorial! It's good learning and fun DIY project. May not float everyone's boat, but it gave me a good time. Cheers to @GeekyDeaks for helping explaining stuff. I never thought I'll be in the same thread with the author of this project. Thank you all!

 

[vlad1970]

Hello everyone, I made the first scheme work with an INA122p, a 1k potentiometer and a 104 capacitor. The query is that now I have reassembled another similar one, but with an INA122PA instead of the INA122P and I don't get anything at the output no matter how much the potentiometer varies. Are there differences between the INAs? Thank you very much for your help. Greetings.

 

[GeekyDeaks]

There are some small differences to tolerance, but looking at the numbers, I would still expect the PA variant to work fine. Did you socket the chips or directly solder them? Just thinking you can do a quick test by swapping them over.

 

[Gerzson]

Not completely DIY related, but look what I found.

 

[patrickTM]

Not completely DIY related, but look what I found.

It was only a matter of time before someone cashed in on open source work. I'm not sure I like the way the force is applied to the load cell perpendicular to the angle of motion. The Fanatec CSL Elite V1s had the loadcell offset from the plunger and people complained that they had to put their foot in a specific position for the brake to work properly. I much prefer @GeekyDeaks design.

 

[JuniorBR22]

Hey guys, I managed to make the board, but the Volts out is inverted that the G29 need, the Volts out of the PCB is 0.0volts up to 3.5volts, what do I need to do for invert?

 

[GeekyDeaks]

Hey guys, I managed to make the board, but the Volts out is inverted that the G29 need, the Volts out of the PCB is 0.0volts up to 3.5volts, what do I need to do for invert?

My first guess would be that the output from the wheatstone bridge is inverted. Can you measure the voltage between pins 2 and 3 of the amplifier and post the values here for when there is no load and when you press the pedal? It'll be small, like under 10mV if memory serves, but you are looking to have it higher when there is no load and lower when you apply a load. If it's the other way around, then you probably need to take a closer look at how you have arranged the resistors to ensure the bridge is out of balance to begin with.

 

[JuniorBR22]

sorry for the delay in answering, it was because I was using another type of PCB, now I made this one on the page.
Now the volts out is always 0.49, when pressed or not pressed.
And the load cell volts out are ok, increasing by about 0.5 mvolts when you press down.

 

[GeekyDeaks]

And the load cell volts out are ok, increasing by about 0.5 mvolts when you press down.

Ok, so that is actually reversed to what you need it. The G29 has a high signal (around 3v) for the brake fully off and a low signal (I think around 1.2v) for fully on, so you need have the wheatstone bridge replicate this behaviour by arranging the resistors and altering the trim pot R_BAL1 so that the bridge is out of balance to start with and applying load brings the voltage difference closer to 0mV

 

[JuniorBR22]

Ok, so that is actually reversed to what you need it. The G29 has a high signal (around 3v) for the brake fully off and a low signal (I think around 1.2v) for fully on, so you need have the wheatstone bridge replicate this behaviour by arranging the resistors and altering the trim pot R_BAL1 so that the bridge is out of balance to start with and applying load brings the voltage difference closer to 0mV

So I need the top R_BAL1 > R1 and R2? the R1 and R2 are 1K Ohms, R_BAL1 and R_GAIN are 1K Ohms too. I don't know how to let the bridge is out of balance.

 

[GeekyDeaks]

R_BAL1 and R_GAIN are 1K Ohms too.

Oh, that is going to be tricky to get to work. The R_BAL1 trimpot is supposed to be 10 Ohm not 10k. Using a 10k can still technically work, but due to tolerances it might be really sensitive in the 0-10 Ohm region

So I need the top R_BAL1 > R1 and R2?

R_BAL1 works as a 'fine tune' of R1 so you can tweak it around the 1K Ohm point and change the resting balance of the bridge. R2 is the other side of the bridge from R1 + R_BAL1 and they all make up the bottom half of the bridge. The two resistors in the load cell make up the top of the bridge. Typically, the resistors in the load cell change about 2-3 Ohm when under load, which is why R_BAL1 only needs a small range.

Can I ask if you are familiar with how the wheatstone bridge works? Apologies if you already know, but if you don't, I'd recommend watching a quick vid on the subject as it'll make this whole process a lot lot easier and you'll probably realise how simple all this stuff really is

 

[JuniorBR22]

Oh, that is going to be tricky to get to work. The R_BAL1 trimpot is supposed to be 10 Ohm not 10k. Using a 10k can still technically work, but due to tolerances it might be really sensitive in the 0-10 Ohm region


R_BAL1 works as a 'fine tune' of R1 so you can tweak it around the 1K Ohm point and change the resting balance of the bridge. R2 is the other side of the bridge from R1 + R_BAL1 and they all make up the bottom half of the bridge. The two resistors in the load cell make up the top of the bridge. Typically, the resistors in the load cell change about 2-3 Ohm when under load, which is why R_BAL1 only needs a small range.

Can I ask if you are familiar with how the wheatstone bridge works? Apologies if you already know, but if you don't, I'd recommend watching a quick vid on the subject as it'll make this whole process a lot lot easier and you'll probably realise how simple all this stuff really is

OK, I dont know how the wheatstone bridge works, I will try to understand how it works.
thank you so much

 

[GeekyDeaks]

OK, I dont know how the wheatstone bridge works, I will try to understand how it works.
thank you so much

Cool! Once you have, let me know. There is one quick thing to try that will start you in the right direction, but I deliberately wanted to wait until you are familiar with how the bridge works first as it will make the next steps in the calibration a lot less frustrating. I suspect you might figure it out anyway

 

[GeekyDeaks]

OK, I dont know how the wheatstone bridge works, I will try to understand how it works.
thank you so much

I found this vid which explains about the measurement challenges and why a wheatstone bridge is used. It also talks about resistor tolerances which is something else that is very helpful to know and then shows a practical demonstration of the whole thing....

 

[NDG]

Hey guys! I have enjoyed this mod for more than a year now... That's sad! It means I'm still using a G27 in 2023 But, I have decided to move on and grab a DD wheel soon. I thought I'd save a buck by hanging on to these pedals using an Arduino-Logitech pedals mod. I already coded an Arduino Leonardo and made it run using the original pots of the brake. But when I replace it with the loadcell mod from this thread, it's not working unlike when it's using the G27 wheelbase. Also, the INA122 chip gets hot, like blistering hot! Am I missing something? I installed it again using the g27 wheelbase and it's still fine. I just cant make it run using the arduino leonardo. Below is the wiring I followed...

 

[GeekyDeaks]

Below is the wiring I followed...

uh-oh, I think you have +5v and GND reversed . The pedals won't mind, but the amp eventually might..... if it's all still working ok hooked up to the G29, just quickly measure the voltage to check pin 1 is GND