G29 Load Cell Mod

WIP on replacing the stock brake pedal sensor on a Logitech G29 pedal set with a load cell

It uses one or two cheap load cells arranged in a wheatstone bridge and a small circuit using a Instrumentation Op-Amp. The cells are mounted in a 3D printed model that replaces the lower half of the brake piston assembly

Shopping list

1. 3D printer capable of 80mm x 43mm x 43mm
2. Bathroom scale type loadcells
3. PCB copper board or stripboard (35mm x 30mm)
4. INA122P instrumentation amplifier
5. 2k trim pot (dual cell)
6. 1k trim pot
7. 10ohm trim pot (single cell)
8. 2x 1k resistor (single cell)
9. 100ohm resistor (fake clutch)
10. 900ohm resistor (fake clutch)
11. M10 thread ~30mm
12. 2x Half height M10 nuts
13. M10 to 24mm washer

Software used

1. EAGLE https://www.autodesk.co.uk/products/eagle/free-download
2. Fusion 360 https://www.autodesk.com/campaigns/fusion-360-for-hobbyists

Load Cells

The load cells are cheap bathroom scale variety. I got mine from amazon:

https://www.amazon.co.uk/gp/product/B07TWLP3X8

Each cell is comprised of two resistors one which increases under load (white) and the other which decreases (black). Zero load is approx 1k, with about 2-3 Ohm change under heavy load

Wheatstone Bridge

The circuit allows for running the mod in either a dual or single load cell configuration.

For the cells above there are three wires

1. white - increasing resistance under load
2. red - common
3. black - decreasing resistance under load

Single Load Cell

In a single load cell configuration, the load cell is used as the top portion of the wheatstone bridge. R1, R2 and R_BAL1 need to be populated in order to construct the lower half of the bridge and R_REF should be replaced with a link to GND to run the amp in positive gain mode

The 1k resistors for R1 and R2 should be measured and the larger of the two should be used for R1 to allow R_BAL1 to adjust the link with R2 and force the bridge out of blance with no load

Dual Load Cell

If two load cells are used then R1, R2 and R_BAL1 should be left empty and R_REF should be populated with a 2k pot. R_REF will allow the INA122P to be run in reverse gain mode by ensuring that the higher side of the bridge is connected to v- on the LOAD_CELL connector (J1)

Amplifier Schematic

Since we need to output a higher voltage with no load on the cells we have two choices:

1. unblance the bridge so that load brings it back into balance
2. invert the gain

Unblancing the bridge is the intended approach for a single load cell configuration and involves tweaking one side of the bridge by a very small resistance via R_BAL1.

Inverting the gain is intended for the dual load cell configuation. Fortunately, the INA122 is rail to rail, so you can have Vout go all the way to V+ by tying ref to V+ and ensuring the higher potential side of the bridge is connected to V-in on the amp

For our purposes, we actually want zero load to be a little less than V+, so we use another trimpot to bring ref down to reduce the upper deadzone. Unfortunately, this causes the common mode rejection to degrade as we have a high impedence on ref. As per the datasheet, we should really put a unity gain op-amp in place to act as a buffer, but since we do not require high precision, we can simply reduce ref lower as required and not worry too much about it's precise value

Clutch

The circuit also allows for simulating a clutch, allowing it to be removed if desired. The G29 has been known to glitch if the clutch is disconnected and the leads left hanging.

If you do not need this feature, you can leave R3 and R4 unpopulated on the board

PCB

STL models

Single Load Cell

The single load cell holder should fit in the standard pedal case stl/single_holder.stl

Dual Load Cell

There are two separate bodies for the dual confifguration. stl/dual_holder.stl contains the model for the load cell holder that mounts into the pedal frame and stl/dual_spacer.stl contains the model for the small spacer that sits between the two cells.

NOTE: this model will not fit inside the stadard pedal case

G29 brake levels

From reading around it seems that there is not an agreed set of values for what represents min and max braking. Below is a table I took from attaching an AVO directly to the middle pin of the brake potentiometer on my set and monitoring the output on the PS4:

Level	Voltage
0%	3.0
25%	2.5
50%	2.2
75%	2.0
100%	1.9

Pedal Pin Outs

-----------------------
\  1   2   3   4   5  /
 \   6   7   8   9   /
   -----------------

Pin	Role	Colour
1	Gnd	Black
2	Throttle	Orange
3	Brake	White
4	Clutch	Green
5	NC
6	+3.3v	Red
7	NC
8	NC
9	+3.3v	Red

The above values were taken by removing the wiring loom from the pedal set and measuring at the spade connectors. Note that these pin outs are different to those reported by others for the G27 with +3.3v and Gnd switched

Inspiration

This is a popular mod and others have come before with interesting designs that you may also want to take a look at:

• https://www.reddit.com/r/simracing/comments/92exmk/diy_load_cell_brake_for_g29_with_native_ps4/
• https://www.racedepartment.com/threads/low-budget-load-cell-mod-for-logitech-g25-g27-g29.167760/
• https://imgur.com/gallery/gOjAf#SXX5zrY
• https://www.racedepartment.com/threads/logitech-g25-g27-g29-g920-load-cell-diy-project.174623/