Logitech Pedals Load Cell Mod

I recently installed a load cell on the brake pedal of my Logitech G27 Pedal.
Had to google around a bit to get all the information I needed so I tought making an Instructables page might be a good idea.

The brake pedal now feels more like the real deal with only having a short throw and after that more pressure translates into more braking power.

Lets get started...

Parts

• INA122PA Instrumentation amplifier
• 0.1uF capacitor (K104K10X7RF5UH5)
• 1K multiturn trimpot (67WR1KLF)
• 4x4cm proto board
• 2 x 50kg load cell (half bridge)
• 3mm aluminium plate
• M5x25 washer
• M8 threaded rod - 45mm
• Rubber vacuum cap or something similar (must fits over the M8 threaded rod)

Tools

• CA Glue
• Coarse sandpaper
• 7mm drill
• M8 tap

Open up the body by first removing the pedals (each have two internal hex screws).
Then remove all the silver screws from the bottom, two are hidden behind the carpet gripper.
Fold the gripper out and you will see the two screws in the gap that opens up.
When lifting the top cover, remove the two screws holding the cable on the inside, now your are left with the base plate and pedals.

Disconnect the wire spade connectors from the brake pedal.
Now remove all pedals by removing all the black screws from the underside of the bottom plate.

Sand the area around the center back hole of the brake pedal.
This hole can be used to mount the pedal set to a plate, but only the ones of the outer pedals are mostly used for this purpos.
We will be reusing this hole as a holder for our load cell.
Sand the washer and glue the washer concentric with the hole using CA glue.

We will be making a full bride load cell out of two half cell ones.
Make a spacers out of the aluminum to fit in between the two load cells.
Make sure the center hole is large enough so the flexing part of the load cell does not touch the spacer.
Sand the mating surfaces and glue the package together with CA glue, the nipples on the load cells should be facing away from each other.

Build the load cell amplifier on the proto board according to the schematic.
Try to use the side of the potentiometer that increases in resistance when turning it clockwise.

Connect the supply wire (white one mine) to the ground wire of the second one (black on mine).
Do the same for the other pair.
On pair is connected to the 5V power rail, the other to ground.
The signal wires of the load cells must be connected to the inputs of the load cell amplifiers.
You might have to change the signal wires around if there is no output on the load cell amplifier when pressing on the load cells.

You can test this using a 5V supply and a multimeter connected to the output of the amplifier.

Remove the spring mechanism of the brake pedal by removing the top inner hex screws.
Drill the center hole out with a 7mm drill bit and tap the hole with an M8 tap.
Push the rubber vacuum cap over the piece of M8 threaded rod and thread it in the hole :).

The height can be adjusted to alter the stop position of the brake pedal.
You may want a second bigger cap to make the stop a bit more flexible, but I like it this way.

I also drilled new mounting holes for the bottom part of the mechanism to sit lower and further to the front, you can see it in the last image.
This way you don't have to trim as much on the top cover due to the pedal sitting higher.

Refit the spring and reassemble the mechanism.

Fit the load cell under the brake pedal an refit the screws holding the pedal through the bottom plate.
Only lightly tighten the screw as to not put to much pressure on the load cell when no pressure is applied to the pedal.
You can add some loctite to the screws to make sure they do nut vibrate loose.

Use small spade connectors or solder the load cell amplifier to the signal wires that came off the potentiometer.

• White: Signal
• Red: 5V
• Black: Ground

Refit the other two pedals to the bottom plate.
You may need to trim the top cover on the places indicated in the images to get it to fit.
The brake pedal sits a bit higher now due to the load cell being under it.
I also drilled a hole in the top plate to be able to tune the potentiometer.
You can remove the spacer of the brake pedal to make it sit at the same depth as the clutch pedal.

That's it.
You will have to reverse an recalibrate the brake pedal signal in your sim of choice.
It's also possible to reverse it globally in the Logitech software.
Adjusting the potentiometer will correspond to having to press less or more in order to max out the braking signal.