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

 

[Panicpete]

Ah, the G27 might run at 5v. If so, just bear in mind that the output from the amp will likely be more in the 2.5 - 5v range

If i remember correctly, the G27 doesn't have the rubber stopper in the brake pedal (like the G29/G920/G923 do), so it can travel all the way down just like the throttle and clutch pedal. In that case you should also have the entire voltage range of 0 - 5 volts. At least for my G25 that is the case.

 

[GeekyDeaks]

If i remember correctly, the G27 doesn't have the rubber stopper in the brake pedal (like the G29/G920/G923 do), so it can travel all the way down just like the throttle and clutch pedal. In that case you should also have the entire voltage range of 0 - 5 volts. At least for my G25 that is the case.

That's a very good point. Just to check though, on my G29 none of the pots in the pedals were able to turn fully from the pedal travel alone, so it would never go as low as 0v on any pedal (0.7v rings a bell, but I'd need to double check). Is that also different on the G27? At any rate, I'd recommend working out what the min/max voltage from the pedal is then taking that ratio to the voltage difference across pins 2 and 3 coming into the amp

 

[Panicpete]

I'm actually not sure what the voltage range on the original pots was, but I think the wheel should be able to detect the whole range and calibrate accordingly. However, checking the original setup beforehand and then adjusting accordingly is always sound advice

 

[Neilski]

I'm actually not sure what the voltage range on the original pots was, but I think the wheel should be able to detect the whole range and calibrate accordingly. However, checking the original setup beforehand and then adjusting accordingly is always sound advice

I dug out my measurements of 2 years ago, from before I mucked around with my own G27 pedals.
The two pedals that had the (probably correct) orientation of the pot lever (where it just gently reaches the metalwork when you release the pedal) had a swing of roughly 4.6 - 0.6 V. The third pedal with the lever displaced by one tooth (so it never reaches the metalwork) went from roughly 4.0 - 0.0 V. All three pedals basically just worked though, so the digitiser does seem to cope with whatever it gets.

 

[NDG]

I feel so bad right now. I bought two chips and enough materials for two pedals. Also 2 load cells. They are cheap though, I spent roughly US$28-35? But I have soldered and de-soldered all of them so many times that they lost their pins now and one stopped functioning already. I will reset. Please someone advise me on what PCB layout I should follow and correct parts to purchase. I'm using a g27. I already have two 3d prints for the three-wired load cell, and I would like to stay on that design. So many different layouts all over the web. I have tried 4 but nothing worked. I think I messed up on the caps and resistors ratings. So please help on that side. Next time I make the trip to the shop, I will get myself a tester too. Any advice on types and what sort of reading I should look for? The old one I have only has a clunky dial that I think is only good for testing short circuits.

 

[GeekyDeaks]

Please someone advise me on what PCB layout I should follow and correct parts to purchase. I'm using a g27

I have a spare working prototype I don't use. Can you PM me your address?

 

[NDG]

I have a spare working prototype I don't use. Can you PM me your address?

I just started a conversation with you, is that what you call PM around here? Actually my first time doing that. I never realized until now, but The 3D print I have is actually the ones from the GitHub link that you created. Thank you for your hard work mate! And for the free help you and others in this community are providing. Cheers y'all!

I will still build my own regardless if I receive your unit or not. I love DIY and it keeps me on an even keel, sometimes. You know, when I need it. Cheers all you guys!

 

[GeekyDeaks]

But I have soldered and de-soldered all of them so many times that they lost their pins now and one stopped functioning already.

I mentioned the breadboard in the PM, but I forgot to also mention that I find it's often better to get some cheap IC sockets and solder them instead of the chip. They are usually pretty cheap, although some places make you buy a tube of them e.g. https://uk.rs-online.com/web/p/dil-sockets/1831564

 

[GeekyDeaks]

I think I messed up on the caps and resistors ratings. So please help on that side

With the voltage reading you gave earlier, I think you are actually fine, but for reference, you should have the following colours on the resitors: Brown - Black - Red. You should also have a 'tolerance' band after that, which will commonly be Gold.

For the cap, it's not essential to the circuit working and is only there to eliminate noise in the circuit from causing problems with the amp (https://en.wikipedia.org/wiki/Decoupling_capacitor). I used a 1uF which has a code of 104 on the cap. If your's has a different number, let us know and we can tell you if it's close or miles out.

The old one I have only has a clunky dial that I think is only good for testing short circuits.

I have been using a cheap dial one for ages. I have a bench scope for more accurate work, but the convenience of the small handheld one makes it a lot more useful, especially for in-place troubleshooting.

 

[NDG]

@GeekyDeaks
Hey man, I have a question for you about the PCB layout you have on the GITHUB page you created. There are traces on the PCB that I cannot identify. I marked BLUE the things I think I know, correct me if I'm wrong though. But the red ones I have no idea. I already have a DIY proto board based from the layout you suggested.

 

[GeekyDeaks]

The components to the right are actually R3 and R4. It's a long story, but when I was originally playing with this, I had disconnected my clutch and moved the brake to its position. This was due to the rig having a central pilar for the wheelbase, so I could not comfortably reach the brake for any extended length of time. Unfortunately, the G29 exhibited some really, really weird behaviour when the clutch input was left hanging (like the red dial would only work intermittently), so the board was designed to have the resistors to pull the clutch high (off). You probably don't need them and it's fine to leave them off.

The 104 cap is actually to the bottom left of the amp.

The final component in purple is also a remnant of making the board dual role so that it could accept both one or two loadcells. For one loadcell, just tie the pin to ground by putting a jumper between the two pins in purple

I like the way you have etched your board. Did you do it manually or use a layout design tool?

 

[NDG]

Gotcha! Thanks again! So I guess I'll just jumper them around or make another one like the blue example on page 5.

About the PCB I etched, l'll give you my own long story if you may. I work in a PCB drilling/CNC subcontractor company here in Taiwan. I operate drilling and CNC machines and I have free access to these fancy machines and tons of scrap materials that I can turn into anything I can imagine. The holes are drilled and the lines are CNCed out on that example. I created my own protoboard design that I just edit out to any shape I need them to be. Then I just throw in some scrap PCBs on one of the machines and make these things. It's kinda off season right now so lots of machines are free or idle. When it's peak season, I won't be able to do these on the sides.

Hey man, if you have some small projects that needs a one-sided PCB that's small enough to fit in a legal envelope, just send me a PM and the design. I'll see what I can do, if I can return the favor.

 

[GeekyDeaks]

So I guess I'll just jumper them around

Just want to make sure we are on the same page... you are not planning on replacing R3 and R4 with jumpers, and just simply leaving them out, right?

 

[NDG]

Just want to make sure we are on the same page... you are not planning on replacing R3 and R4 with jumpers, and just simply leaving them out, right?

Yes leave them empty, But isn't PIN 5 of the chip supposed to be connected with the black (ground) terminal of the wheel? it is open on the board. C1 is 104 cap, jumper the purple, and PIN 5 to the ground terminal. green

 

[GeekyDeaks]

But isn't PIN 5 of the chip supposed to be connected with the black (ground) terminal of the wheel?

Yes, but it's doing that via the purple jumper. Sorry, I probably have not explained this at all well. Let me try again

Ground to the wheel base is connected to the lower pin of the 4 on the right. The one above (in green) is for a fake clutch if the pedal was disconnected. If you still have your clutch connected, you don't need to connect anything on that entire trace (3 holes).

 

[NDG]

Okay matey! Gotcha! The traces, or rather the holes I made where the ground wire from the pedals will connect is actually the traces you marked green. It's all still good if I actually understand the layout. Parts will be here this weekend, or Monday if the courier is slow. I hope my next post in this thread is a finished working product! Cheers man!

 

[NDG]

Holy s**t! I got it working! I can't believe it!

Though I am not out of the woods yet. Remember I still have some parts left from my previous attempts? So I soldered them onto the boards I made. I have a few so I thought before I throw them away, I'd give them another shot. Because I think they were fried from too much reuse and wrong wirings. But they worked! I think it's been all good from my first attempts!

The problem I have are the load cells. Before, I was testing them inside the 3d prints I got from the GitHub page but I think the 3d prints are fine. I took them out and tried pressing the load cells by hands and they worked! These load cells are weird. I got two of them. When I press the buttons (red X), or even if I step on top of them, nothing registers. But if I try to bend them from the outer part(green circle) that has more leverage for my fingers, they work!

How do these load cells work anyways? I see the inside of the 3d prints and the plunger and they are supposed to press the buttons right? We are supposed to target the buttons right?

I bought another 4 load cells, will arrive early next week. The two I got behaves the same. They also seem to hold their position, like I press one side, works, but doesn't return to zero. I need to press again on the other side to make it go back. These are very cheap btw, like 8 cents each? There are much more expensive options from other sellers but they used identical images and I don't know if they offer better quality.

 

[GeekyDeaks]

How do these load cells work anyways? I see the inside of the 3d prints and the plunger and they are supposed to press the buttons right? We are supposed to target the buttons right?

Yeah, you are supposed to press the button. In my experience, the cells behave differently when you apply load on the green part and often the resistances will change incorrectly.

Load cells are extremely simple and are just measuring the resistance across part of the cell which can deform under stress. As it deforms the resistance changes by a very small amount. For the ones I have tested, I measured about 2-3 ohm difference from rest to full load. That's about 0.3% change, which is why we use the wheatstone bridge to help measure it.

Can I just confirm that you are supporting *only* the outside of the load cell when you are applying load? The middle part under the button (and the green bit) needs to be able to move down very slightly. If you look at the model, you can see there is a fairly large, but shallow void below the where the load cell sits.

 

[GeekyDeaks]

They also seem to hold their position, like I press one side, works, but doesn't return to zero. I need to press again on the other side to make it go back.

That sounds like you are deforming the load cell in a way it was not designed for, or past its rated load. Is this when pressing on the green part?

 

[Panicpete]

The way these load cells work technically is, that under the white suff where the wires come out there is a strain gauge. Here is an explanation on how those work. So, all that matters is the deformation of the metal in that one area. The whole metal part is designed to be supported on the outer edge, and when load is applied to the button it should procude a repeatable signal taking springback of the metal into account. As @GeekyDeaks mentioned, there should be some room under the middle part for the load cell to deform, but ideally also a stop to avoid overloading the part, permanantly deforming it.