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

cteters

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Dec 4, 2018
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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.
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:

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
 
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asteroulis

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Oct 8, 2012
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FC2231-0000-0100-L
I'd say this is the one you'd like to use. It doesn't need an amplifier as well since it outputs 3 wires and 0,5-4,5V directly. Can be supplied with 3.3 or 5V . 100lbs force rating. Also , if you look closely on the picture you just posted above , the load cell is the one I posted ;-)

20Kgs are used in other applications , where the pedal arm is the load cell itself. This particular design is used as a lever , meaning that the 25lbs on the load cell you stated would need almost half the pressure on foot. So it definitely is not adequate for this use.

A great tutorial though , just update the load cell..

 

cteters

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Excellent attention to detail! I clearly overlooked that on ebay and will update the info respectively. That's crazy to think this could work without the need for an amplifier, but yeah if there is no real comparable impedance why wouldn't it? Either way, I assume having an amp should still do no harm, being that the output voltage should always be within the same rating, so maybe I'll leave some info about that up, at least for the time being until something like this is tested. Thanks for the help!
 

asteroulis

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Oct 8, 2012
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Since they use the same load cell, I'm also wondering what is this pcb ricmotec uses. Amplifier is used in load cells because they output usually mVolts. This pcb does have connectors for the logitech cabling, but I also see a chip there. Really confusing to me. It sure is not compatible with loe bodnar 4 wire amp.
But, if you end up needing a pcb, then why not go for a leo bodnar board usb board. That way you will upgrade both resolution, make them standalone, and depending on the board be able to hook up button boxes as well..
One more note here. The hx711 is used only with an arduino project. They don't do exactly the same job. I had read about it somewhere but can't recall the differences. Obviously the hx711 would be great for such a low price, but unfortunately it can't be used like that.
 

cteters

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Asteroulis, thanks for your interest and time. I haven’t honestly done any research toward the hx711, but wanted to add a footnote toward checking the sparkfun website and it was the first thing to come up. I guess I should just take that down, being that this is a live document at this point. Sorry for any confusion.


As for the ricmotec pcb, the board definitely looks like it has an amplifier on there from my guess, along with an elongated ceramic disc capacitor of some spec. So, realistically, this should still be very doable by making a basic amplifier in following the inscrutable guide. But yeah, I haven’t had any success in finding a plug and play board yet either. Having it double as a button box is a very clever idea indeed!
 

asteroulis

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Oct 8, 2012
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No, we should thank you for taking the time to write the tutorial. I've dove this research in the past but didn't share my thoughts because I hadn't tried if it works or not.
If you are willing to try it then we can say that this is a legit tutorial.
 

cteters

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Thanks, yeah, I think I'll be seeing this one through for sure! I'll be away from my rig for a month though, here in a week, so won't have any luck getting any parts before then. I guess it will give me time to hash out any final thoughts on use and implementation of an amplifier.
 

Willy Brown

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Mar 11, 2014
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With the help of the guys here and Mr Bodnar , I managed to hook up a Leo Bodnar BU-0836LC board to a button load cell inside the G25 Spring holder. I feel bad, I forgot to report back,will update soon.
Works an absolute treat and never missed beat. Had to change a resistor on the board to match my load cell. Had the opportunity to try some mid range pedals and prefer my pedal by far.
 

Panicpete

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Nov 18, 2017
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Since they use the same load cell, I'm also wondering what is this pcb ricmotec uses. Amplifier is used in load cells because they output usually mVolts. This pcb does have connectors for the logitech cabling, but I also see a chip there. Really confusing to me. It sure is not compatible with loe bodnar 4 wire amp.
But, if you end up needing a pcb, then why not go for a leo bodnar board usb board. That way you will upgrade both resolution, make them standalone, and depending on the board be able to hook up button boxes as well..
One more note here. The hx711 is used only with an arduino project. They don't do exactly the same job. I had read about it somewhere but can't recall the differences. Obviously the hx711 would be great for such a low price, but unfortunately it can't be used like that.
I also did a Load Cell Mod for my G25 Pedals some months ago, so maybe I can help here:
In the original configuration with potis the output is 5V with pedal not depressed and 0V with pedal fully depressed, so my guess is the PCB in the Ricmotech kit is there only to switch the output around (so you don't have to invert the axis in every sim).
The HX711 is not suitable if you only want to swap the poti for a load cell as it only has a digital (serial) output.

I built my amplifier based on a tutorial over at xsimulator.net, the guy also wrote a more general thread about load cells. Just be aware that with the layout given there you will end up with an inverted axis. To avoid this you need to swap the wires coming from the load cells and connect the VRef pin of the IC to +5V instead of GND (I used an INA122P).

I also got a cheap digital bathroom scale for the load cells and designed a mechanical solution to apply the brake pressure to the load cells that seemed sensible to me.

To get the right feel in the pedal I used a hydraulic shock absorber i had lying around from an old RC car. Inside the original spring housing i started with a piece of garden hose with v-shaped cutouts, which worked OK, but had a considerable hysteresis effect. Recently replaced it with a stack of disc springs and am very happy with how it works now.
The Pedal is very soft for the first ~2cm and doesn't act on the load cells at all (in a real car you also have some pedal travel with very little pedal force and no braking at all until the brake pads start touching the brake discs). As soon as the disc springs come into action the pedal gets quite stiff and the force is transmitted to the load cells.

If you are interested in my mechanical solution I should still have the SketchUp file and also some more pictures somewhere.

P1040525_.jpg P1040527_.jpg Pedal.PNG Pedal_Section.PNG
 
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Panicpete

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Nov 18, 2017
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Alright, here we go:
Parts list if you want to build it yourself:
Mechanical parts:
  • 2 load cells (Digital Bathroom Scale / eBay); mine are 24x28, I think the ones you find on eBay are a bit bigger; the bathroom scales are usually rated around 120-150 kg with 4 load cells in parallel, so each of them should be able to take about 35-40 kg assuming a slightly uneven distribution on the scale
  • flat aluminum profile 2x25 (you need about 400mm of this)
  • 30x40x6 block as spacer under load cells (I used 2x3mm MDF board)
  • 30x25x1 as way limiter between the load cells (I cut up an old DVD case)
  • dia 6x45 wooden rod as center post for disc springs (sand down one end so it can slide in the hole in the lower spring housing and cut three grooves into the other end to fit over the ribs in the upper spring housing)
  • (brazen) tube outer dia 8 inner dia 6 length 31 to replace the mounting bolt of the lower spring housing
  • some foam to put in the upper spring housing to soften the transition when the disc springs come into action; I cut up a dish washing sponge for this
  • threaded rod M4 20-25mm long (you can also use a bolt and cut off the head)
  • 2 sleeve nuts M4 with countersunk head, outer dia 6mm
  • 2 washers 18x6,4 to better distribute the force of the disc springs
  • 30 disc springs 12,5x6,2x0,5 (could be a bit softer for my liking, tried with 0,35 but they tend to "flip over" quite easily)
If you want to include the hydraulic shock absorber:
  • Hydraulic shock absorber for 1/10 scale RC cars, extended length around 90-100 mm (as the lever acting on the shock absorber is quite short you want to get the stiffest spring you can find for it)
  • shock absorber oil (I'm currently using a viscosity of 800cSt/WT61, could also be a bit higher, see above)
  • aluminum U-Profile 15x15x1 (you need about 80mm)
  • 4 countersunk bolts M4x10
  • 4 nuts M4
  • Mounting material depending on the specific shock absober you got, in my case a bolt M3x20, corresponding nut and several washers for the upper mounting and a sleeve nut M4x15 and bolt for the lower mounting
Electrical parts:
  • operational ampifier (INA122P, INA122PA or similar, make sure it supports rail-to-rail)
  • if you want you can get a socket for the operational amplifier
  • condensator 100 nF
  • precision trimmer pot 1,0 kΩ
  • some PCB, i used one with strip matrix, just be sure to remove the connection between the pins of the amplifier IC
you can solder all wires directly to the board, or get some connectors so you can unplug everything in case you need to take out the pedal
If you use this load cell amplifier and connect it to the wheel electronics make sure to always plug in the USB connection before the power connection. I had some problems in the beginning when doing it the other way round. I suspect the wheel electronics generate a voltage spike sending the IC into lockup. If you create the ground connection via USB first, that spike should not occur.

You can also omit the hydraulic shock absorber in my design, but i would suggest to use a considerably softer spring in that case (maybe the original throttle spring, it should actually fit around the stack of disc springs).

If I were to do it again, I would probably redesign the contact between the load cells and spring housing to line contacts instead of point contacts. That way the pressure applied to the load cells is distributed more consistently and not dependent on the exact positioning of the load cells under the lower spring housing mount.
Also, I found the double-sided tape i first used to put together the stack of load cells and spacers to be unsuitable. The glue used in these tapes seems to liquefy under high pressure, causing the parts to become misaligned. Just switched to superglue some days ago (except for the sticking the bottom spacer to the pedal mounting as the contact area here is way bigger). I hope it is a more durable solution, otherwise I will probably try to bolt it together.

I also had a closer look at pictures of the Ricmotech PCB. I can't make out the marking on the IC, but there seem to be four identical resistors, so it is probably indeed a unity gain differential amplifier which is used to switch around the output voltage from 0v-5v to 5v-0v (Vout = V2 – V1). This is probably required to make it compatible with consoles.

You can download the SkechUp file of my current version here (I have just updated it to include the disc springs, it also contains the old version and original setup as hidden geometry. One of the attached pictures shows the garden hose I used before).

P1040533_.jpg P1040521_.jpg P1040520_.jpg P1040517_.jpg Pedal2_Section.PNG
 
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Panicpete

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Short update on my brake pedal mod: As I thought the pedal could be a bit softer, I got some thinner disc springs (0,35 mm) and inserted them. Turns out they are too weak, because at rather low pedal forces they start to turn over / become inverted, which sounds rather unpleasant and feels even worse. I have edited the comment in my previous post accordingly. So for the moment I have just added two more discs to make it a tiny bit softer (also shortening the low-force pedal travel).

Because my pedals were also feeling a bit wobbly, I took the opportunity to use my dad's lathe during my Christmas visit and made some shiny new brass bushings to replace the plastic ones (some of them were pretty worn out already, see attached picture). Much better now :)

Pedal_Bushing.jpg
 

dirko13

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Nov 24, 2019
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Hi Panicpete, very nice project. I'd like to do something similar to my G27 pedals but with Leo Bodnar board and Hall sensors for the other two pedals. So in this case I don't need to build/buy some amplifier for the load cell am I right? I just recieved a set of 4 bathroom scales with the HX 711 amp (just for testing) and played around a bit with it yesterday but couldn't get any signal out of it. Maybe a 4.5 volt battery is not enough or I did something wrong. I connected the cells like shown in tutorial over at xsimulator.net to the HX711 but no output. :thumbsdown:
 

GeekyDeaks

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Dec 26, 2019
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Hi @dirko13 - 4.5v is fine. I have run a HX711 at 3.3v. Can I check what you are doing with the output from it? Apologies if you already know this, but the output from the HX711 is serial, so you need to clock the current sample from it with something like a microcontroller (PIC / Arduino etc..)

I have not used the Leo Bodnar board, but from looking at the image it seems to have an AD827 instrumentation amplifier, so you should not need anything else unless you want to invert the signal.

If you are using the bathroom scale load cells, then you are probably going to need to mount them somewhere. A buddy and I have been working on a similar project but you need access to a 3d printer https://github.com/GeekyDeaks/g29-load-cell
 
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dirko13

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Nov 24, 2019
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Hi GeekyDeaks, thank you very much for your reply. To be honest I thought the output will be some voltage so I tried to messure something with my voltmeter. So I didnt know that it is a serial signal. Thanks again for bringing some light in my brain.

By using the Leo Bodnar board inverting the signal can be done in their software if necessary from what I understood.

Unfortunately I don't have acces to a 3d printer but what you and your buddy did looks really pretty cool. My idea of mounting the load cells is a bit different, I want to make some sort of a seesaw o lever to bring the force on them, it's a bit hard for me to discribe , will try to upload some drawings later.
 
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Panicpete

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Nov 18, 2017
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Hi dirko13, which Bodnar Board are you planning to use? You will need one that specifically says Load Cell, the others only work with Buttons and/or Potis. So either the Load Cell Interface which supports a load cell only or the Load Cell Joystick Controller, which additionally supports buttons and analog inputs. Not sure about Hall sensors though.

If you have the (correct) Bodnar Board already, you can use it to test the load cells directly with the PC.
 

dirko13

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Nov 24, 2019
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Hi Panicpete (very nice name) thank you very much for your reply and advice.
I'm planning to use the Load Cell Interface LC-USB 16 bit from Leo Bodnar for the pedals: click
Because it's not really clear on their webpage if it supports hall sensors as well I emailed them and asked, after very fiew days they confirmed that it can be used with hall sensors. So thats great news.

I must say that your project inspired me a lot and at the beginning I wanted to do in the same way but I'm still not able to figure out how you managed to bring the force from the springhousing to the cells.:unsure:

To become a more realistic feeling to the break pedal I played around a bit with rubbers from wine bottles :geek: and additional springs from real car rear drum brakes but I was not really happy with the result. So I want to make a complete new rod with some sort of soft spring for the free travel of the pedal and some elastomer springs called fibroelast for the presure travel: check it out.
The rod will move,lets call it lever to the horizontal mounted load cells. see attached picture.
(please don't laugh I'm not good at drawing)
The idea of this I got from here

Today I was talking to a friend of mine about my project and he was very interested in it as well but he's running on a playstation 4, so I have no idea if there's any way to make or buy a controller for a load cell mod, any help here would be very apreciated as well.




LoadCell lever.jpg
 
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