revtor
07-14-2008, 05:01 PM
Update: The article below may still be of use, but I wrote new more detailed articles regarding this and related modifications. Here are the links to the updated versions:
• HVAC control unit roller wheel illumination (http://e31wiki.org/wiki/HVAC_control_unit_roller_wheel_illumination): Fix for the roller wheel illumination.
• HVAC control unit button illumination (http://e31wiki.org/wiki/HVAC_control_unit_button_illumination): This article shows a very simple method to increase the brightness of the button background illumination. This may be desired after pumping the roller wheel brightness.
• HVAC control unit button indicator illumination (http://e31wiki.org/wiki/HVAC_control_unit_button_indicator_illumination): The HVAC control unit has green and yellow status indicators in the buttons. Some may prefer an all orange look. This article describes how to replace the original LEDs with BMW orange LEDs.
(Some pictures of this mod were already posted several months ago, but I completely forgot to make a write-up for the forums. I made a complete and detailed Dutch article for ClubE31 Netherlands' club magazine. This is a rush translation from that article, so I hope there aren't too much typos and mistranslations. I will post this article at a few E31 communities)
HVAC/IHKA controller roller wheel illumination mod
Whoever drove an 8 series at night probably noticed the inconsistency in illumination of the controls. Especially the roller wheels for temperature and fan speed on the HVAC/IHKA controller are barely illuminated. Whether or not you find this an issue depends from person to person, but some people strive for perfection. This article is meant as a guide for people who wish to fix the bad roller wheel illumination but don't know yet what to expect and where to begin.
The article is written into detail so everyone should be able to understand it and attempt the mod for themselves if interested - even people with little technical skills. Seasoned users may want to skip large blocks of text or even only refer to the pictures as a guide.
Removing the HVAC/IHKA control unit
Unlike the radio and OBC/MID, the HVAC/IHKA control unit cannot be removed from the front. This means the center console will have to be removed. This sounds worse than it actually is - if you're not too clumsy it should take less than ten minutes.
To begin the radio and OBC/MID must be taken out. The radio comes first. It would lead us too far to discuss this into detail because of all the different mounting systems - if you don't know how to remove the radio, follow the instructions from the manufacturer. Alternatively you can search for experiences and tips from other owners on the various E31 communities the Internet has to offer.
Once the radio is out, you can easily remove the OBC/MID from the center console by reaching your hand in the opening left by the radio, pressing the tab on the bottom of the OBC/MID and meanwhile pushing the unit towards yourself.
The next step is revealing the screws that hold the center console in place. Pull up the shifter's leather boot. It just clicks softly in the center console and should come off quite easily. Then grab the shift knob with both hands and pull hard and straight up. This will require some force. Avoid accidently hitting something when the shift knob comes off the shift lever. The metal shift lever has some sharp edges - it's recommended to wrap a soft towel around it to avoid damage to the center console if you accidently touch it while taking it out. Depending on the model you'll spot one or two screws in the opening of the shifter that hold the center console.
The other screws are on the top side of the center console. On a plastic center console two caps above the OBC/MID hide the screws. Be careful when removing these caps to avoid tool damage on the plastic. On a wooden center console three screws can be found on top of the OBC/MID opening. Remove all screws.
The center console is now loose. If the car is equipped with a hands-free mobile phone you must disconnect the microphone now. The microphone is the small grill left from the OBC/MID. Reach in through the OBC/MID opening, push the microphone out of the center console and disconnect it. All connectors inside the center console will fit in one direction only and the shape or color makes clear where they should connect to. Thus there's no real need to label all connectors although that never hurts.
There are two ways to proceed. Remove the center console completely or just flip it until you can reach the HVAC/IHKA controller's rear side. If you remove the center console, you must disconnect all switches left and right from the shifter. You must press the small tabs on left and right sides of these connectors to unlock them. The HVAC/IHKA control unit has two connectors: A large one with a locking handle and above that a small one - both on the left side of the unit.
Now you have the center console out of the car or flipped in a way you can reach the HVAC/IHKA control unit's rear side, remove it from the center console by removing the four screws that hold it (two on the bottom and one on each side).
Dismantling the control unit
Once the HVAC/IHKA control unit is out of the car, it's time to take a closer look at the failing illumination. This means dismantling the unit completely. Even if the cause can't interest you, the fix will require you to take it completely apart.
Always put the unit on a clean and soft towel when you work on it. The front scratches very easily. Always work very carefully. If something seems to be stuck, check twice if you overlooked anything instead of using brute force. The risks involved with this mod are very low, but a careless person will manage to break everything...
Begin with the removal of the buttons. The best and safest way to do this is by using a key removal tool. This is a simple tool which hooks on both sides behind the key so you can pull it off very gently. One that works for a computer keyboard should also work for the HVAC/IHKA control unit. Alternatively you can use a small screwdriver or knife.
http://tweakers.net/ext/f/H6lnaAlSyIVoyKPVYIGOL3c0/full.jpg
On the unit's bottom you can see the three axles around which the roller wheels turn. Each axle has a small lip. Turn all three roller wheels to their center position - all lips will now point to the front. Remember this as it will aid the assembling later on. Grab the lip with a pair of long nose pliers, push the it slightly towards the center of the axle and then pull straight up with a bit force to remove the axle. Remove all three axles.
Remove the connector from the fan and remove the four screws on the rear side. Take the backside off. Be careful when handling the unit at this point. The roller wheels are now loose, but they can't be removed yet due to the existing illumination. If you press the wheels you may damage the illumination.
http://tweakers.net/ext/f/3qHdIFMC9lBsVHW68Dn3T2mo/full.jpg
On one side of the printed circuit board (PCB) is a dashboard filament bulb. Turn the bulb a quarter counterclockwise and pull it out. The PCB is mounted with two screws. Remove these and lift the board. Removal may be stiff and possibly you must bend the wide white flatcable a little to aid. Below the board is yet another PCB, also mounted with two screws which you must remove. Take both circuit boards out of the unit. On the bottom PCB you'll notice a black plastic tube - the car's interior temperature sensor. It may have collected a lot of dust over the years so now may be a good time to clean it with compressed air. Be careful though, as the wires from the thermocouple are very thin and you don't want to damage these.
With the circuit boards removed you can see the cause of the bad roller wheel illumination. The bulb's light is distributed to the wheels by a three legged plastic lightbar. That's a very optimistic construction and practice learns us that it doesn't work. Remove the two screws that hold the lightbar. Now you can carefully remove the roller wheels along with the lightbar.
http://tweakers.net/ext/f/e2uWwvWtZihIve9yuYjzgHmQ/full.jpg
In theory the lightbar could work, but it would require a much brighter light source. There's simply too much light lost in the construction. However, a much brighter filament bulb might melt the plastic.
Solutions
The goal is to get more light into the roller wheels. A few options are possible. A simple yet effective solution to the problem comes from Wuffer - a well-respected member from the Canadian and North American E31 communities. Wuffer cut off the bulb holder from the lightbar and attached one high brightness LED on each remaining bar. LEDs are able to produce a lot of light in a small angle without the disadvantage of heat generation as with filament bulbs. This way enough light can be distributed to the roller wheels. The drawback is that you must damage the lightbar. My preference goes out to non-destructive mods.
One of the nicest approaches is presented by Martin - an electronics wizard from the Swiss and German E31 communities. Martin developed a printed circuit board with LEDs and brightness control which fits exactly in place of the lightbar - plug-and-play. For those who don't have the means to develop their own circuit boards, Martin also sells it for a reasonable price.
The solution presented in this article is some sort of golden mean between the forementioned approaches: A non-destructive solution built with simple hobby material. Just as with Martin's solution a brightness control is provided to adjust the light output exactly to the rest of the HVAC/IHKA controller's illumination.
Choosing the right LEDs
LEDs are available in all dimensions, shapes, colors, brightnesses,... In other words it's important to make the right choice. In this mod the LEDs will be placed inside the roller wheels. This means very little light will be lost - the LEDs don't have to be very bright. In fact, about 20 millicandela is already sufficient. Modern high brightness LEDs produce several thousands millicandela, so 20mcd shouldn't be a problem at all. The brightness control will allow you to adjust the amount of produced light exactly to your needs, so the exact brightness specification of the LED isn't crucial. What is important is the LEDs color. To match BMW's traditional orange the LEDs must feature a wavelength between 605 and 610 nanometer. LEDs with shorter or longer wavelengths will turn out too yellow or too red.
There are several constructions possible to mount the LEDs inside the roller wheels. This article describes two. The most simple one is to use standard round LEDs looking down into the roller wheels. For this setup the LEDs must suffice following requirements:
• Shape: standard round LED
• Diameter: maximum 3mm
• Viewing angle: 80° or better
• Brightness: 20mcd or better
• Wavelength: 605nm to 610nm (BMW orange)
The other construction uses small to very small surface mount (SMD) LEDs which look forward from within the roller wheel. These LEDs are only recommended if you have experence with soldering small components. Otherwise soldering may turn out in frustration. To get good results, the LEDs should meet following requirements:
• Shape: SMD
• Diameter: maximum 3mm in all directions
• Viewing angle: 120° or better
• Brightness: 15mcd or better
• Wavelength: 605nm to 610nm (BMW orange)
Small SMD LEDs often don't produce enough light. A simple fix to this is to use two LEDs in parallel for each roller wheel. In that case don't forget to double the current through in the calculations later on.
It's very unlikely the LEDs will have the correct brightness out-of-the-box. We need a brightness control.
Brightness control
The brightness control is simple and fits on a very small piece of prototyping board. This is a pre-manufactured printed circuit board with holes/islands in a fixed grid so you can easily solder components and connect islands the way you want without going through the hassle of developing your own PCB. If you restrict yourself to the board dimensions in this article, the dimmer can be mounted inside the HVAC/IHKA control unit and you will be able to adjust the brightness from the rear through the lamp hole.
Most LEDs have an operating voltage below 3.5 volt - orange LEDs usually 2 to 2.2V. You could put three LEDs in series - one for each roller wheel - without requiring more than the battery voltage. Putting the LEDs in series helps to keep the heat dissipation down in the resistor and transistor. After all, the more voltage is over the LEDs, the less is left for the resistor and transistor. The resistor limits the current through the LEDs. It should never be omitted! In other words, if the battery is 12V and the LEDs 2V each, never put 6 LEDs in series without resistor to get up to 12V. It's better to have two branches of 3 LEDs in series with a resistor for each branch. It is recommended to leave at least a few volts margin between the LEDs and the battery voltage. This avoids black-outs when the battery voltage takes a dip - for example when starting the engine. Another thing you must keep in mind is that a car's battery voltage isn't 12V as printed on it, but actually closer to 13.8V with - in worst case scenario - peaks up to 15V in a running car. If you calculate resistors with 12V as battery voltage you may run into a problem later on. You best always calculate the resistors keeping the worst case scenario in mind - in a car that's 15V.
http://tweakers.net/ext/f/Iw27v0UAXLkOp6GdKScttSuu/full.jpg
Components
• Prototyping board board with holes/islands (0.1" pitch)
• P1: Trimmer potentiometer 100K Linear (top adjustment)
• T1: Transistor BC547 or equivalent
• R1: See text
• D1...D3: See text
The schematic shows a simple emitter follower circuit. In an emitter follower the voltage over the emitter (E) always equals the voltage over the base (B) minus a small voltage drop over the base-emitter. When trimmer potentiometer P1 is turned, transistor T1 automatically adjusts the current through collector (C) and emitter until the voltage over the emitter is back equal to the voltage over the base. Ohm's Law in action. This is certainly not the best way to dim LEDs, but it's simple and small. Exactly what we need.
The value of the current limiting resistor R1 isn't critical in this circuit's case because the current can be adjusted with trimpot P1. For safety, turn the trimpot all the way to minimum (counterclockwise) before you first turn on power. If the resistor value turns out to be too low, you will not blow the LEDs.
Resistor R1's value can be calculated as follows. The example uses LEDs with an operating voltage of 2.2V and a current of 20 milliampere. Because of the voltage drop over base and emitter you can calculate with 14V battery voltage instead of the worst case scenario.
http://tweakers.net/ext/f/ci3AZYdBhzEN5gwXvkh03z8L/full.gif
The calculated resistor value of 370 ohm does not exist in the popular E12 series. You can get resistor values outside the E12 series but these will be more expensive. So its best to stick to E12 values. The E12 series consist out of following base values: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. Unless the calculated value is very close to a lower E12 value, you should always pick the next higher value from the E12 series. In this case that would be 390 ohm.
It's important to determine the amount of power that will be dissipated by the resistor because standard resistors can handle up to 0.25 watt only.
http://tweakers.net/ext/f/qZeykOXmZuGTxQmGW7K5UNhK/full.gif
http://tweakers.net/ext/f/73zveTIOVgp7GTjXKzBi9hwv/full.gif
The power in the resistor is well below the maximum allowed value. There are no problems to be expected from this side. You should now have gathered all information required to build the brightness control, so let's get started...
It's best to start with cutting the circuit board to the correct dimensions. The material can easily be sawn with an iron saw. The rough sides can be worked away with a smooth file. The final board should be 3 x 9 islands large. Next drill a 2.5mm hole for the mounting. Take a look at the print layout for its exact location. Not too close to the side but certainly not too close to the trimpot. You must avoid the screw's head can touch and short circuit the trimpot's connection leads. If uncertain you can always put a small piece of isolating plastic foil between the screw and trimpot.
Put the components on the PCB as shown in the components view. Note that a standard trimpot has a length of five islands while the print layout only has room for four islands. You must bend two connection leads a bit so it fits on four islands. This way the distance between the connection leads and the screw is increased. Moving all components one island to the right wouldn't work. Once you put the HVAC/IHKA controller back together you'll see why. The available space is limited.
Solder the components and lay the traces as shown in the print layout view. Important: Both components view and print layout are shown as seen from the comonents side. If you work on the solder side you must mirror the print layout! You can make traces by connecting islands with large amounts of solder, but it's a lot easier, less messy and with less chance upon unwanted short-circuits if you first lay the traces with naked copper wire or the remains of the components' connection leads.
http://tweakers.net/ext/f/RK2HqL7kawdBFyvqYRHH6xFD/full.jpg
The circuit is simple so it's virtually impossible to mess up. The usage of the brightness control is of course not limited to the HVAC/IHKA control unit. You could use it everywhere you want to adjust the brightness of a couple of LEDs. For larger quantities of LEDs you better look out for a real LED dimmer that works with puls code modulation (PCM) - much more efficient.
• HVAC control unit roller wheel illumination (http://e31wiki.org/wiki/HVAC_control_unit_roller_wheel_illumination): Fix for the roller wheel illumination.
• HVAC control unit button illumination (http://e31wiki.org/wiki/HVAC_control_unit_button_illumination): This article shows a very simple method to increase the brightness of the button background illumination. This may be desired after pumping the roller wheel brightness.
• HVAC control unit button indicator illumination (http://e31wiki.org/wiki/HVAC_control_unit_button_indicator_illumination): The HVAC control unit has green and yellow status indicators in the buttons. Some may prefer an all orange look. This article describes how to replace the original LEDs with BMW orange LEDs.
(Some pictures of this mod were already posted several months ago, but I completely forgot to make a write-up for the forums. I made a complete and detailed Dutch article for ClubE31 Netherlands' club magazine. This is a rush translation from that article, so I hope there aren't too much typos and mistranslations. I will post this article at a few E31 communities)
HVAC/IHKA controller roller wheel illumination mod
Whoever drove an 8 series at night probably noticed the inconsistency in illumination of the controls. Especially the roller wheels for temperature and fan speed on the HVAC/IHKA controller are barely illuminated. Whether or not you find this an issue depends from person to person, but some people strive for perfection. This article is meant as a guide for people who wish to fix the bad roller wheel illumination but don't know yet what to expect and where to begin.
The article is written into detail so everyone should be able to understand it and attempt the mod for themselves if interested - even people with little technical skills. Seasoned users may want to skip large blocks of text or even only refer to the pictures as a guide.
Removing the HVAC/IHKA control unit
Unlike the radio and OBC/MID, the HVAC/IHKA control unit cannot be removed from the front. This means the center console will have to be removed. This sounds worse than it actually is - if you're not too clumsy it should take less than ten minutes.
To begin the radio and OBC/MID must be taken out. The radio comes first. It would lead us too far to discuss this into detail because of all the different mounting systems - if you don't know how to remove the radio, follow the instructions from the manufacturer. Alternatively you can search for experiences and tips from other owners on the various E31 communities the Internet has to offer.
Once the radio is out, you can easily remove the OBC/MID from the center console by reaching your hand in the opening left by the radio, pressing the tab on the bottom of the OBC/MID and meanwhile pushing the unit towards yourself.
The next step is revealing the screws that hold the center console in place. Pull up the shifter's leather boot. It just clicks softly in the center console and should come off quite easily. Then grab the shift knob with both hands and pull hard and straight up. This will require some force. Avoid accidently hitting something when the shift knob comes off the shift lever. The metal shift lever has some sharp edges - it's recommended to wrap a soft towel around it to avoid damage to the center console if you accidently touch it while taking it out. Depending on the model you'll spot one or two screws in the opening of the shifter that hold the center console.
The other screws are on the top side of the center console. On a plastic center console two caps above the OBC/MID hide the screws. Be careful when removing these caps to avoid tool damage on the plastic. On a wooden center console three screws can be found on top of the OBC/MID opening. Remove all screws.
The center console is now loose. If the car is equipped with a hands-free mobile phone you must disconnect the microphone now. The microphone is the small grill left from the OBC/MID. Reach in through the OBC/MID opening, push the microphone out of the center console and disconnect it. All connectors inside the center console will fit in one direction only and the shape or color makes clear where they should connect to. Thus there's no real need to label all connectors although that never hurts.
There are two ways to proceed. Remove the center console completely or just flip it until you can reach the HVAC/IHKA controller's rear side. If you remove the center console, you must disconnect all switches left and right from the shifter. You must press the small tabs on left and right sides of these connectors to unlock them. The HVAC/IHKA control unit has two connectors: A large one with a locking handle and above that a small one - both on the left side of the unit.
Now you have the center console out of the car or flipped in a way you can reach the HVAC/IHKA control unit's rear side, remove it from the center console by removing the four screws that hold it (two on the bottom and one on each side).
Dismantling the control unit
Once the HVAC/IHKA control unit is out of the car, it's time to take a closer look at the failing illumination. This means dismantling the unit completely. Even if the cause can't interest you, the fix will require you to take it completely apart.
Always put the unit on a clean and soft towel when you work on it. The front scratches very easily. Always work very carefully. If something seems to be stuck, check twice if you overlooked anything instead of using brute force. The risks involved with this mod are very low, but a careless person will manage to break everything...
Begin with the removal of the buttons. The best and safest way to do this is by using a key removal tool. This is a simple tool which hooks on both sides behind the key so you can pull it off very gently. One that works for a computer keyboard should also work for the HVAC/IHKA control unit. Alternatively you can use a small screwdriver or knife.
http://tweakers.net/ext/f/H6lnaAlSyIVoyKPVYIGOL3c0/full.jpg
On the unit's bottom you can see the three axles around which the roller wheels turn. Each axle has a small lip. Turn all three roller wheels to their center position - all lips will now point to the front. Remember this as it will aid the assembling later on. Grab the lip with a pair of long nose pliers, push the it slightly towards the center of the axle and then pull straight up with a bit force to remove the axle. Remove all three axles.
Remove the connector from the fan and remove the four screws on the rear side. Take the backside off. Be careful when handling the unit at this point. The roller wheels are now loose, but they can't be removed yet due to the existing illumination. If you press the wheels you may damage the illumination.
http://tweakers.net/ext/f/3qHdIFMC9lBsVHW68Dn3T2mo/full.jpg
On one side of the printed circuit board (PCB) is a dashboard filament bulb. Turn the bulb a quarter counterclockwise and pull it out. The PCB is mounted with two screws. Remove these and lift the board. Removal may be stiff and possibly you must bend the wide white flatcable a little to aid. Below the board is yet another PCB, also mounted with two screws which you must remove. Take both circuit boards out of the unit. On the bottom PCB you'll notice a black plastic tube - the car's interior temperature sensor. It may have collected a lot of dust over the years so now may be a good time to clean it with compressed air. Be careful though, as the wires from the thermocouple are very thin and you don't want to damage these.
With the circuit boards removed you can see the cause of the bad roller wheel illumination. The bulb's light is distributed to the wheels by a three legged plastic lightbar. That's a very optimistic construction and practice learns us that it doesn't work. Remove the two screws that hold the lightbar. Now you can carefully remove the roller wheels along with the lightbar.
http://tweakers.net/ext/f/e2uWwvWtZihIve9yuYjzgHmQ/full.jpg
In theory the lightbar could work, but it would require a much brighter light source. There's simply too much light lost in the construction. However, a much brighter filament bulb might melt the plastic.
Solutions
The goal is to get more light into the roller wheels. A few options are possible. A simple yet effective solution to the problem comes from Wuffer - a well-respected member from the Canadian and North American E31 communities. Wuffer cut off the bulb holder from the lightbar and attached one high brightness LED on each remaining bar. LEDs are able to produce a lot of light in a small angle without the disadvantage of heat generation as with filament bulbs. This way enough light can be distributed to the roller wheels. The drawback is that you must damage the lightbar. My preference goes out to non-destructive mods.
One of the nicest approaches is presented by Martin - an electronics wizard from the Swiss and German E31 communities. Martin developed a printed circuit board with LEDs and brightness control which fits exactly in place of the lightbar - plug-and-play. For those who don't have the means to develop their own circuit boards, Martin also sells it for a reasonable price.
The solution presented in this article is some sort of golden mean between the forementioned approaches: A non-destructive solution built with simple hobby material. Just as with Martin's solution a brightness control is provided to adjust the light output exactly to the rest of the HVAC/IHKA controller's illumination.
Choosing the right LEDs
LEDs are available in all dimensions, shapes, colors, brightnesses,... In other words it's important to make the right choice. In this mod the LEDs will be placed inside the roller wheels. This means very little light will be lost - the LEDs don't have to be very bright. In fact, about 20 millicandela is already sufficient. Modern high brightness LEDs produce several thousands millicandela, so 20mcd shouldn't be a problem at all. The brightness control will allow you to adjust the amount of produced light exactly to your needs, so the exact brightness specification of the LED isn't crucial. What is important is the LEDs color. To match BMW's traditional orange the LEDs must feature a wavelength between 605 and 610 nanometer. LEDs with shorter or longer wavelengths will turn out too yellow or too red.
There are several constructions possible to mount the LEDs inside the roller wheels. This article describes two. The most simple one is to use standard round LEDs looking down into the roller wheels. For this setup the LEDs must suffice following requirements:
• Shape: standard round LED
• Diameter: maximum 3mm
• Viewing angle: 80° or better
• Brightness: 20mcd or better
• Wavelength: 605nm to 610nm (BMW orange)
The other construction uses small to very small surface mount (SMD) LEDs which look forward from within the roller wheel. These LEDs are only recommended if you have experence with soldering small components. Otherwise soldering may turn out in frustration. To get good results, the LEDs should meet following requirements:
• Shape: SMD
• Diameter: maximum 3mm in all directions
• Viewing angle: 120° or better
• Brightness: 15mcd or better
• Wavelength: 605nm to 610nm (BMW orange)
Small SMD LEDs often don't produce enough light. A simple fix to this is to use two LEDs in parallel for each roller wheel. In that case don't forget to double the current through in the calculations later on.
It's very unlikely the LEDs will have the correct brightness out-of-the-box. We need a brightness control.
Brightness control
The brightness control is simple and fits on a very small piece of prototyping board. This is a pre-manufactured printed circuit board with holes/islands in a fixed grid so you can easily solder components and connect islands the way you want without going through the hassle of developing your own PCB. If you restrict yourself to the board dimensions in this article, the dimmer can be mounted inside the HVAC/IHKA control unit and you will be able to adjust the brightness from the rear through the lamp hole.
Most LEDs have an operating voltage below 3.5 volt - orange LEDs usually 2 to 2.2V. You could put three LEDs in series - one for each roller wheel - without requiring more than the battery voltage. Putting the LEDs in series helps to keep the heat dissipation down in the resistor and transistor. After all, the more voltage is over the LEDs, the less is left for the resistor and transistor. The resistor limits the current through the LEDs. It should never be omitted! In other words, if the battery is 12V and the LEDs 2V each, never put 6 LEDs in series without resistor to get up to 12V. It's better to have two branches of 3 LEDs in series with a resistor for each branch. It is recommended to leave at least a few volts margin between the LEDs and the battery voltage. This avoids black-outs when the battery voltage takes a dip - for example when starting the engine. Another thing you must keep in mind is that a car's battery voltage isn't 12V as printed on it, but actually closer to 13.8V with - in worst case scenario - peaks up to 15V in a running car. If you calculate resistors with 12V as battery voltage you may run into a problem later on. You best always calculate the resistors keeping the worst case scenario in mind - in a car that's 15V.
http://tweakers.net/ext/f/Iw27v0UAXLkOp6GdKScttSuu/full.jpg
Components
• Prototyping board board with holes/islands (0.1" pitch)
• P1: Trimmer potentiometer 100K Linear (top adjustment)
• T1: Transistor BC547 or equivalent
• R1: See text
• D1...D3: See text
The schematic shows a simple emitter follower circuit. In an emitter follower the voltage over the emitter (E) always equals the voltage over the base (B) minus a small voltage drop over the base-emitter. When trimmer potentiometer P1 is turned, transistor T1 automatically adjusts the current through collector (C) and emitter until the voltage over the emitter is back equal to the voltage over the base. Ohm's Law in action. This is certainly not the best way to dim LEDs, but it's simple and small. Exactly what we need.
The value of the current limiting resistor R1 isn't critical in this circuit's case because the current can be adjusted with trimpot P1. For safety, turn the trimpot all the way to minimum (counterclockwise) before you first turn on power. If the resistor value turns out to be too low, you will not blow the LEDs.
Resistor R1's value can be calculated as follows. The example uses LEDs with an operating voltage of 2.2V and a current of 20 milliampere. Because of the voltage drop over base and emitter you can calculate with 14V battery voltage instead of the worst case scenario.
http://tweakers.net/ext/f/ci3AZYdBhzEN5gwXvkh03z8L/full.gif
The calculated resistor value of 370 ohm does not exist in the popular E12 series. You can get resistor values outside the E12 series but these will be more expensive. So its best to stick to E12 values. The E12 series consist out of following base values: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. Unless the calculated value is very close to a lower E12 value, you should always pick the next higher value from the E12 series. In this case that would be 390 ohm.
It's important to determine the amount of power that will be dissipated by the resistor because standard resistors can handle up to 0.25 watt only.
http://tweakers.net/ext/f/qZeykOXmZuGTxQmGW7K5UNhK/full.gif
http://tweakers.net/ext/f/73zveTIOVgp7GTjXKzBi9hwv/full.gif
The power in the resistor is well below the maximum allowed value. There are no problems to be expected from this side. You should now have gathered all information required to build the brightness control, so let's get started...
It's best to start with cutting the circuit board to the correct dimensions. The material can easily be sawn with an iron saw. The rough sides can be worked away with a smooth file. The final board should be 3 x 9 islands large. Next drill a 2.5mm hole for the mounting. Take a look at the print layout for its exact location. Not too close to the side but certainly not too close to the trimpot. You must avoid the screw's head can touch and short circuit the trimpot's connection leads. If uncertain you can always put a small piece of isolating plastic foil between the screw and trimpot.
Put the components on the PCB as shown in the components view. Note that a standard trimpot has a length of five islands while the print layout only has room for four islands. You must bend two connection leads a bit so it fits on four islands. This way the distance between the connection leads and the screw is increased. Moving all components one island to the right wouldn't work. Once you put the HVAC/IHKA controller back together you'll see why. The available space is limited.
Solder the components and lay the traces as shown in the print layout view. Important: Both components view and print layout are shown as seen from the comonents side. If you work on the solder side you must mirror the print layout! You can make traces by connecting islands with large amounts of solder, but it's a lot easier, less messy and with less chance upon unwanted short-circuits if you first lay the traces with naked copper wire or the remains of the components' connection leads.
http://tweakers.net/ext/f/RK2HqL7kawdBFyvqYRHH6xFD/full.jpg
The circuit is simple so it's virtually impossible to mess up. The usage of the brightness control is of course not limited to the HVAC/IHKA control unit. You could use it everywhere you want to adjust the brightness of a couple of LEDs. For larger quantities of LEDs you better look out for a real LED dimmer that works with puls code modulation (PCM) - much more efficient.