G25 modification for pedals

Or how to replace spiky potmeters by

contact less optical potmeters

Contents

1. Introduction .......................................................................................................................................... 3

2. Electrical schemes ................................................................................................................................. 6

3. Installation procedure ........................................................................................................................... 9

4. If the setup doesn’t work… ................................................................................................................. 15

5. Components ........................................................................................................................................ 16

6. Afterthought… ..................................................................................................................................... 18

Figure list

Figure 1: Logitech control panel ................................................................................................................... 3

Figure 2: Electrical scheme of G25 pedal ...................................................................................................... 6

Figure 3: Electrical scheme of modified G25 pedal ...................................................................................... 6

Figure 4: Working of linear polarizers ........................................................................................................... 7

Figure 5: Hardware modification, quasi 3D .................................................................................................. 7

Figure 6: Side and top view of modification ................................................................................................. 8

Figure 7: G25 pedals ..................................................................................................................................... 9

Figure 8: G25 pedals upside down. ............................................................................................................... 9

Figure 9: Pedal case opened like an oister .................................................................................................. 10

Figure 10: Gas pedal, indicating the potmeter, the three contacts pointing to the right .......................... 11

Figure 11: Installation ................................................................................................................................. 12

Figure 12: Light on LDR when pedal is pressed .......................................................................................... 13

Figure 13: Screen shot of GT Legends controller settings .......................................................................... 14

Figure 14: Resistor color coding .................................................................................................................. 17

Figure 15: Electronic components .............................................................................................................. 17

1. Introduction

The Logitech G25 pedals, though mechanically very sturdy, have an Achilles heel. To measure how far a

pedal is pressed a variable resistor or potmeter is used. When the pedal is not pressed the resistor value

is low. By pressing the pedal the value of the resistor increases up to 10 kΩ. However, this potmeter

uses mechanical contacts, and due to ‘wear and tear’ the contacts degrade over time. This results in

‘spiky’ behavior. This can be observed easily at several places. Open the control panel and go to the

properties of the G25 wheel (see Figure 1).

Figure 1: Logitech control panel

Slowly press the accelerator pedal and watch the green bar increase. If the green bar slowly and steadily

increases as you press the pedal, then nothing is wrong. If, however, small changes in pedal pressure

causes the green bar to jump up and down or it vibrates heavily in a steady position then you probably

have a problem. Of course, you can also startup you r favorite racing game and check out the controller

settings there.

Small vibrations are probably not a big deal. The reason is that a car has a lot of mass and reacts only

slowly to high frequency variations of the gas or brake pedal. If the vibrations are big enough then in a

racing game, you can see the needle of the speedometer vibrate. This happened to me, and doing time

trials I estimated that the time loss on a given circuit was about 0.7 seconds (which on a total lap time of

about 1.5 minute is quite a lot). After implementation of the modification as described here, the loss

was gone.

So, now you know how to check if you have a problem and what the possible consequences might be.

Possible solutions for this problem are:

1. If your setup is still under warranty, then contact the supplier and ask for a replacement. Beware

that any hardware modification voids your warranty!

2. Clean the potmeters. There are sprays available which improves the contact between the

‘needle’ and the variable resistor.

a. Advantage: cheap. Probably works.

b. Disadvantage: this procedure has to be repeated every now and then. On the internet I

found a comment from a guy who cleaned the potmeters every 2 to 3 months!! I prefer

permanent solutions…

3. Get spare parts from Logitech

a. Advantage: after replacement, the pedals are probably in the same shape as when you

bought them.

b. Disadvantage: it is not certain if and when you could get these spare parts. And these

need to be installed as well.

4. Replace the potmeters with ‘non contact potmeters’ or Hall sensors.

a. Advantage: the best possible solution.

b. Disadvantage: these are quite expensive… about 30 to 35 Euros per potmeter. Total

price tag is over 100 Euro (excluding post and package, I imagine).

5. Buy a new controller: a new G25 or G27 or even better: a Porsche wheel (Fanatec).

a. Advantage: you’ll get a working product

b. Disadvantage: price!!! Also, the G27 is no better than the G25 regarding the sensors in

the pedals. So this will solve the problem for about 1 or 2 years (depending on usage)

and then you’ll be needing this document again…

6. Change the mechanical potmeters for ‘optical’ potmeters. And that’s what this document is

about.

a. Advantage: for about 20 Euros you can fix the all three potmeters permanently.

b. Disadvantage: you have to be a bit handy and creative…

Disclaimer and advice

I cannot be held responsible to any damage (material, financial or health) resulting from reading and

using this document to modify the pedals (or any other device) that you may modify with this

knowledge.

Read the whole document (yeah, I know it is tough…) and balance the problem that you have against

the possible risks when implementing this solution. However: if you are about to buy a new set, then a

small investment might prolong the life time of your current setup.

To give an idea about how much work is involved: after getting all the components, it took me one

afternoon to install everything. Writing this document took longer…

For those who are more interested in flight simulators: the ‘optical potmeter’ as described here can also

be built inside a joystick. I did that a couple of years back when I bought a new (Logitech) joystick as a

replacement of an older one which potmeters left much to be desired. Much to my chagrin in a couple

of months also the new one showed a spiky behavior. Therefore – as an experiment - I took the old one

apart and replaced the most critical mechanical potmeter for an optical one. From then on it

outperformed my newer joystick greatly: it was even better than brand new! Later, when I got into

racing games I found out that here the pedals suffer from the same issues…

There is good stuff out there, but often it is difficult to find the information. However, if a supplier does

not state very clearly that he is using Hall sensors or other non contact sensors in some particular game

controller then that game controller most probably features common potmeters.

Good luck with the assembly and happy gaming!

Tokin

Version 1.0

2009 – 09 – 22

2. Electrical schemes

The following plot (Figure 2) shows the electrical setup of the G25 as it was originally made. When a

pedal is not pressed, the needle at the green dot points to the higher end of the resistor and the input

signal is a resistance of about 10 kΩ. Pressing the pedal will move the needle downwards, and the

resulting resistance decreases down to about 0 kΩ. Note that Logitech choose to have a lower resistance

– and therefore a lower output voltage – when pressing the pedal.

Figure 2: Electrical scheme of G25 pedal

The next plot (Figure 3) shows the electrical setup of the modification. An LED or LED’ or Light Emitting

Diode is used to illuminate an LDR or Light Dependent Resistor. The input signal is now the voltage over

the fixed resistor of 1 kΩ. As more light falls on the ‘LDR’ or, its resistance decreases and therefore the

voltage over the fixed resistor increases1.

Figure 3: Electrical scheme of modified G25 pedal

The problem now is how to regulate the amount of light on the LDR when pressing the pedal. To do that,

two linear polarizers are needed: one is kept in a fixed position, and the other one rotates when

pressing a pedal.

1 It may seem more logical (in Figure 3) to place the fixed resistor on the position of the LDR and vice versa.

However, the calibration became more difficult and the ‘Deadzone’ increased substantially. Therefore this setup was chosen. Of course, the position of the LED and the resistor of 47 Ω can be exchanged without consequence.

Input signal

5 Volt

New schedule

47 Ω

1kΩLED

LDR

In case you don’t know, a linear polarizer is a sheet of plastic which transmits light with one polarization

direction, as indicated by the arrow (see Figure 4). If two polarizers are placed on top of each other and

the polarization directions are the same, then the total transmission is high (see the overlap of the two

polarizers in the left plot). However, if one of the polarizers is rotated then less light passes through the

region where both polarizers overlap (see overlapping area in center plot). In the right plot, the

polarization directions are perpendicular, and therefore hardly any light passes the area where both

polarizers overlap.

Figure 4: Working of linear polarizers

So, in order to change the amount of light on the LDR, a fixed polarizer is placed in front of the detector,

and one is placed in front of the LED which rotates when pressing a pedal. Figure 5 shows a possible

implementation of the circuit shown in Figure 3.

Figure 5: Hardware modification, quasi 3D

The G25 potmeter has a small extension which can be used. On this shaft a small tube or plastic cylinder

is clamped. In this cylinder an LED is placed. Small holes provide for access to the electrical leads. The

cylinder is closed with a linear polarizer. The LED produces light with all kinds of polarizations, but after

passing through the first polarizer only one polarization direction remains. In Figure 5 the polarizer

before the LDR is in the same position as the polarizer in front of the LED. This means that lots of light

falls on the LDR, which results in a low resistor value. When pressing the pedal, the shaft of the

potmeter rotates, and so does the tube containing the LED and polarizer (less and less light passes

through the polarizers and reaches the LDR, see also Figure 4). In this way the amount of light falling on

the LDR is regulated.

Figure 6: Side and top view of modification

Figure 6 shows the same idea, but the connecting wires are omitted to draw a more clear picture. Some

hints:

- You can use an insulator sticky tape to fix the LDR on the metal frame. By bending the leads the

position of the LDR can be tweaked to get it precisely in front of the LED.

- Use VERY flexible wires to connect the LED. Beware that the tube in which it is mounted rotates

when the pedal is pressed. Therefore the wires need to be very flexible and of sufficient length.

Note that there are two kinds of wires: there are the ones which have a fixed core, and the ones

which consist of a bundle of small wires. The latter are more flexible than the former.

- The final orientation of the polarizer seen in the side view is fixed during the calibration of the

setup.

3. Installation procedure

- Preparation:

o Disconnect the G25 from the computer. It is always a good idea to work on electronic

equipment when it is unplugged.

o Using Allen keys (also Hex, zeta, Unbrako or Inbus keys), remove the heavy metal plates

from all pedal (cf. Figure 7).

Figure 7: G25 pedals

o Turn the pedals upside down and remove the chromium colored screws around the

edge of the box (all 14 of them) see circled locations in Figure 8.

Figure 8: G25 pedals upside down.

o Turn the pedals back into their normal position and lift the top cover, see Figure 9. Be

careful not to overstretch the wire which is entering the rear part of the pedal set. Now

you have access to the hardware.

Figure 9: Pedal case opened like an oister

- Disconnect the wires from the potmeter, see Figure 10. These are made of metal shoes which

are slid over a contact point. With help of a small screwdriver you can remove them. Take

special note of the colors of the wires and write them down (in case for whatever reason you

cannot get the setup to work you need to be able to go back to the previous setup):

o Red wire is +5 Volt

o Black wire is 0 Volt (ground signal, or ‘GND’)

o The signal wires have a unique color for each pedal.

Figure 10: Gas pedal, indicating the potmeter, the three contacts pointing to the right

- Get small tubes which fit reasonably tightly on the potmeter shaft.

- Drill a hole through the tube.

- Take note of the shorter lead of the LED that side needs to be connected to 0 V (GND). The

longer lead needs to be connected to the resistor of 47 Ω. Place the LED inside the tube in such a

way that the leads are sticking out through the holes in the side. Make sure that in each hole,

there is only one lead (this avoids a short circuit). Mark the hole from which the longer lead is

sticking out with a ‘+’ sign, and the hole where the shorter lead is coming from with a ‘-‘ sign.

- After shortening the leads, connect the resistor to the ‘+’ end of the LED.

- Close the tube at the LED side with a small piece of linear polarizer (cut to size). Now the LED

and resistor may be a bit loose in the tube. You should wrap a piece of insulator tape around it

to fix their position.

- Connect the flexible wires (I used yellow ones, since these were the most flexible I had at hand):

one to the LED (preferably black wire) and one to the resistor (preferably red wire). For those

not so experienced, keeping a color code for wires is highly recommended. The chance on errors

is reduced.

- Place the tube on top of the shaft.

- For working on the metal frame of the pedal, it is handy to remove it from the black plastic

frame. With a Philips screw driver you can remove the 4 black screws on the bottom of the

plastic pedal plate see red square boxes in Figure 8.

- Cover a large part of the left side of the metal frame with insulating sticky tape.

- Bend the leads of the LDR as indicated in the previous picture and use sticky tape to fix it on the

frame.

- Attach the resistor of 1 kΩ to one lead.

- Attaching the remaining wires concludes the installation. The only thing that needs to be

installed now is the polarizer on the left side of the frame. The whole setup should something

like Figure 11.

Figure 11: Installation

Before switching on the computer or hooking up the pedals to the computer again:

- Check all the wires: make sure that there are no short circuits!

- Test the system by pressing the pedal and verify that all wires are neither too long nor too short.

The tube should be able to rotate in such a way that the wires move virtually without stress.

Switch on the computer, and verify that the LED is burning! Go to the G25 properties (as indicated

Figure 1). Because the second polarizer has not yet been installed a lot of light falls on the LDR and

therefore its resistor value is low, so the pedal is indicated as not being pressed. Place the second

polarizer between the LDR and the LED. Depending on the orientation of the polarizer, you can tweak

the amount of light on the LDR. To set the polarizer in the correct position, execute the following steps:

1. Press the pedal you modified.

2. Position the polarizer in such a way that the control panel indicates that indeed the pedal is

pressed completely. This will correspond with little light passing through the system. The

differences should look something like Figure 12.

3. Fix the polarizer in this position (eg. Using sticky tape).

4. When releasing the pedal the control panel should indicate that the pedal is indeed released.

Figure 12: Light on LDR when pedal is pressed

By pressing and releasing the pedal slowly, you should see the green and red bar steadily move up and

down smoothly. If this is the case, then it is “Congratulations! Mission accomplished!”

Now start your racing game and go to the controller setup. An example is shown in Figure 13. There you

may need to make a few changes to get the controller set up properly.

IMPORTANT NOTE REGARDING CALIBRATION:

1. The G25 potmeter will have (almost) no resistor value when you don’t press the pedal. Because

now an LDR is used in front of which two polarizers are mounted, the lowest resistor value will

probably be something like 1 kΩ. In other words, for the accelerator and brake pedal you will

see a small green and red bar respectively. This is not a problem, and can be solved in the racing

game by applying a small ‘dead zone’.

2. Also, because the configuration stacks two non linear response curves (the LDR does not

respond linearly when it receives twice the amount of light, and also changing the angle

between the two polarizers results in a non linear response) the sensitivity curve in a game (eg.

GT Legends) needs to be adapted.

In Figure 13 you can see a correct setup. When not pressing the pedals, the progress indicators are

empty. Also note that the ‘Deadzones’ may not be the same for all pedals. The electronic components

and hardware setups are not identical, and therefore small differences may be present. Also, during the

first days in usage, you may need to tweak or adjust the ‘Deadzone’ a little.

Figure 13: Screen shot of GT Legends controller settings

4. If the setup doesn’t work…

- Check that the LED is shining brightly. If it doesn’t:

o Wrong resistor in series with LED? Check color code or resistor value.

o Error in applying voltage? If you apply the voltage in the wrong direction the LED will not

work.

- Check that the amount of light really does change when changing the orientation of the

polarizer.

- Put the multimeter in ‘Voltage measuring mode’. Place the multimeter on the ‘+’ and ‘-‘ spot on

the LDR (cf. Figure 12) and verify that the voltage changes as you press and release the pedal.

For example, the voltage could change from 1.7 V to 3.6 V. From this you can calculate the

resistor value as follows:

With Rfixed = 1 kΩ, and Vmeasured = 1.7 Volt, the resistance of the LDR is 515 Ω.

With Rfixed = 1 kΩ, and Vmeasured = 3.6 Volt, the resistance of the LDR is 2571 Ω or about 2.57 kΩ.

If the variation in voltage that you measure is very small, and the fixed resistor have been

chosen incorrectly. Ideally, the fixed resistor should be the average of the lowest and highest

resistor value that you have calculated. In this example, the ideal resistor value is (the average

of 515 Ω and 2.57 kΩ = ) 1.5 kΩ. The chosen 1 kΩ works nicely, but 100 Ω would have been

much too low, and 10 kΩ would have been a bit too high.

- Test the LDR when it is not connected to anything: measure the resistor value when it doesn’t

receive light and when it does receive light (hold it close to a lamp). In the latter case the

resistance should be well below 5 kΩ.

- If the calibration proves troublesome, then it may be a good idea to start your favorite racing

game and tweak the orientation of the fixed polarizer there. Press the pedal, and tweak the

orientation of the polarizer in such a way that the progress indicator on the screen indeed

shows that the pedal is fully pressed. Then fix the polarizer.

- Make sure that the original potmeter of the G25 is not connected! At the very least the center

connection needs to be removed.

If nothing works, you may need to rewire the pedals back as you bought the G25… Mission failed.

5. Components

Here is a basic check list of what you need to complete the job.

Hardware:

- Multimeter: highly recommended! This will allow you to test the components and help you to

avoid errors.

- Soldering station. You need to connect wires. I don’t recommend ‘twisting wires’ to make a

connection.

- Insulating sticky tape. To fix the LDR to the metal frame, and to cover open connections.

- Small drill to make holes in the tubes which are to be fitted on the potmeter shaft.

- Philips screwdriver to take the G25 apart.

Electronic components you need for each individual pedal:

- 1 LED. There are many different ones. Use a type that has a clear bulb (you should be able to

look through it; it should not be diffuse) either white or red.

- 1 resistor in series with the LED, typically 47 Ω, which ensures about 20 mA through the LED and

cause it to shine brightly. If you don’t own a ‘multimeter’ or other measuring device take note of

the color coding. The colors on the resistor will tell you the resistor value. A so called ‘metal film

resistor’ will do fine.

- 1 LDR. This is the tricky part. You need one which in the dark has a resistor value of more than

10 kΩ, say between 15 and 20 kΩ. The diameter should be slightly larger than the LED. It is

meant to receive the cone of light which is emitted by the LED.

- 1 resistor of 1 kΩ. This is a non critical part. Note that the color coding of this resistor is different

from the resistor of 47 Ω. See for an example of color coding Figure 14. The ideal value is the

average of the LDR in the setup when it receives the least and most amount of light. It is not

possible to predict the best value up front. A small simple potmeter of 10 kΩ can be used which

gives some freedom of adjustment. Since this potmeter will be placed in one position and never

touched again it will not show ‘wear and tear’ problems.

Advice: go to a local shop which sells electronic components. Especially when you don’t have much

experience with making simple electronic circuits the guys over there might give you some valuable

hints. If – for example – they sell LED’s which require a different series resistor, they can recommend the

correct item. At the very least, they should understand the scheme as shown in Figure 3. Figure 15

shows you what the components are supposed to look like. Of course they also have wires, insulating

sticky tape, soldering stations (for this simple work you don’t need something expensive) etc.

-

Figure 14: Resistor color coding

Figure 15: Electronic components

Last, but not least, you also need a linear polarizer. Per pedal you need:

1. A polarizer in front of the LDR, covering the hole in the metal frame: a square piece of about 15

mm x 15 mm.

2. A polarizer in front of the LED. The size depends on the tube you are using, but should be less

than 10 mm x 10 mm

Linear polarizers are more abundant than you may be aware off… Every LCD display has one! There are

two options get them:

- Expensive option: go to a shop where they sell sunglasses and get the cheapest model that

polarizes the light. Beware: lots of sunglasses are just colored pieces of plastic and don’t polarize

the light. To test it: hold one in front of you eye and look through a second one. If you can block

the light by rotating the second sun glasses you know that both use polarizers as required here.

If nothing happens then either one or both are just plain pieces of colored plastic. Perhaps you

can scavenge at a local optician where they make sun glasses for people who wear regular

glasses. They may have small pieces left when they cut sun glasses to size for a regular

customer.

- Cheap option. Any LCD display uses polarized light… I bought a small alarm clock for 4 Euros, and

took it apart. You can get lucky in that the polarizer is just a loose piece of plastic on top of the

glass panel containing the liquid crystals. I was not lucky; I had to use a sharp knife (be careful!)

to peel off the top plastic layer which was the polarizer. Be careful when removing the

remaining glue: the polarizing sheet must remain crystal clear and not become diffuse.

Advice: make sure that you know where to get ALL the items for buying stuff. Because: if a critical

component is missing, you’re stuck!

6. Afterthought…

So why didn’t Logitech choose this option? There are several reasons:

- Potmeters are dirt cheap! Hall sensors can be made into a contact less potmeter with excellent

performance, but they come at some price. On the other hand, Logitech used to have them in

an almost entry level joystick a long time ago, but this type is sadly not supported by windows

XP… In joysticks spikes are more critical than with pedals. Imagine flying in a tight turn hot on a

bogies tail, needing to pull just a little bit further on the stick to get your guns to bear on him.

And now imagine a spiky potmeter in your joystick… Any small adjustment will throw off your

aim, or worse: it can cause you to stall. That is why in the better joysticks they use these Hall

sensors.

- Calibration is easy! Largely because the response of the potmeter scales linearly with the angle

of the potmeter shaft. The LDR has a non linear response with respect to the amount of light

falling on it. Also, by rotating a polarizer in front of another, the total transmission is also not

linear with respect to the rotation angle. Add them both up and you get a strange transfer

curve. However, by adjusting the sensitivity a perfectly good setup can be made.