AILD-1 Automotive Instrumentation Lighting Dimmer: A Solution to Dimming After Upgrading Instrumentation Lighting to LEDs
Background: The Problem
Upgrading instrumentation lighting in your older car from using the original incandescent bulbs to instead using LED-based bulbs is becoming increasingly popular
for a number of reasons, among them:
increased choices for a wide range of drop-in bulb replacements, in many cases allowing every single instrumentation bulb to be replaced with a suitable LED
improved light appearance, in brightness and/or color
reduced energy usage, and consequently less heat generation
faster response times
increased bulb lifetime
Despite the advantages of LEDs over incandescent bulbs in such applications, though, there is often one problem that's not so easy to overcome: preserving the
functionality of the instrumentation dimmer control. In a great number of cars prior to the widespread use of LED lighting technology, the illumnation level of
the incandescent bulbs used for dashboard illumination was controlled by a simple rheostat "dimmer" control. This was based on the simple principle that the
illumination level of incandescent bulbs (in the range needed) is a fairly linear function of the voltage across the bulb's filament. Thus, a rheostat—a simple
variable resistor—was sufficient for controlling the illumination level.
LEDs, however, are not so simply controlled; their illumination level does not respond linearly with voltage, and various LEDs generally have different responses
to changes in voltage, and these responses are very different than those exhibited by incandescent bulbs. The dimming solution used for LEDs (also compatible with
incandescent bulbs) is pulse-width modulation (PWM).
So... the problem is that we'd like to preserve the functionality of our existing dimmer control—the exact rheostat already present in our car—but somehow adapt
it so that it is providing a PWM-based signal to the instrumentation lighting rather than the original simple voltage level.
A Simplified Overview of Traditional Instrumentation Lighting Circuits
A simplified schematic diagram of typical traditional incandescent automotive instrumentation lighting might be instructive.
When the instrumentation lighting is on (switched on by switch "S1" in this diagram; this is often actually part of a multi-purpose headlight switch), power is
applied through the rheostat to all the instrumentation lights, which are wired in parallel. The various instrumentation lights often use separate grounds:
often screws or bolts attached to the car's metal chassis.
What You Might Have Already Discovered
If you've discovered this page, you may very well have been on the hunt for products that solve the problem described above.
You have likely discovered the following:
There are many, many (hundreds, thousands?) of PWM-based, LED dimmer products available.
Most of them employ a low side driver; these switch the "low side" (more negative voltage) of the power to
the load (lighting circuitry). If you experiment with these, you will discover that (if they have two output terminals), you will be unable to connect the
negative (−) side of the input/supply to the negative (−) side of the output/load. Thus, these are not suitable for use in your common-ground car,
where it is unlikely that you'll be able to easily rewire all the places where instrumentation lighting is connected to the chassis ground.
Even if you do find a PWM-based dimmer that employs a high side driver, you will
likely discover that it doesn't provide any means of controlling it using your car's existing dimmer rheostat. Dimmer rheostats typically have a very low
resistance range (e.g., 0 Ω to 6 Ω). Most dimmers that support an external control will specify either a 10 kΩ rheostat/potentiometer or a 0 V to 10 V
control voltage; neither of these is anywhere near a good match for a low-resistance rheostat.
Our Solution: The AILD-1
The AILD-1 was designed from the ground up to solve exactly the problems described in the previous sections, along with some other problems
present in the automotive environment.
simple installation: wired between existing dimmer rheostat and the wires originally connected to that rheostat, along with one new ground wire
anti-noise rheostat input filter (implemented in software; eliminates or reduces the effect of the "high resistance" spots while moving the rheostat control)
illumination level fader (up/down adjustments are smoothed)
after a brief delay allowing further adjustment, illumination level is locked to avoid jitter (hysteresis implemented in software)
output short-circuit protection with visual fault indicator and automatic reset
reversed input power polarity protection
two indicator lamps (LEDs):
red indicator lamp for faults and other status (solidly lit indicates output short circuit or current limiter hit; 1 Hz blinking indicates that rheostat is disconnected;
4 Hz blinking indicates that rheostat profiling—system training—is enabled; 10 Hz blinking continuously for 5 s indicates resetting to factory
configuration; 10 Hz blinking and also on/off at 1 Hz rate indicates incomplete or invalid configuration; rapid flickering during firmware programming)
blue indicator lamp for monitoring output; illuminates at same level as lights connected to output; also blinks (along with any lights connected to
the output) for 2 s at 4 Hz to indicate that rheostat profiling/training has completed
defaults to 50% illumination level if rheostat is disconnected
pre-configured for 6 Ω rheostat; can be trained to work with rheostats in the range 4 Ω to 10 kΩ, and can also be trained to invert the operation of the
rheostat (turn opposite way to increase illumination)
for those who want the ultimate in control or are interested in future potential developments: in the event new software or different configuration
parameters are desired, a single, socketed 8-pin DIP IC containing the software can be replaced or the software and/or configuration can be programmed
via the 6-pin AVR ISP header (an AVR programmer such as the Atmel AVRISP mkII or the
Tiny AVR Programmer would be required if you ever wanted to experiment with this)
How The AILD-1 Is Wired Into The Car's Instrumentation Lighting Circuit
You remove the wires to your car's rheostat, connect them to two terminals on the AILD-1, and then connect two other terminals on the AILD-1 to the rheostat.
And then add a ground wire to the AILD-1. That's it.
This is the revised electrical diagram; the red lines show the original wiring from the original diagram that has been altered.
This diagram shows as an example that the AILD-1 allows mixing LED-based bulbs (the first two example lamps) with incandescent bulbs (the last lamp shown).
While it is often desirable to use LED-based lamps everywhere, sometimes it's not practical.
(Also note that in this diagram, the current limiting resistors that are actually integrated into typical LED-based lamp assemblies are omitted to
keep the diagram simple.)
12 V DC nominal voltage (typical automotive voltages of 14 V DC are acceptable and voltage spikes are suppressed)
3 A maximum output current for output/load
125 Hz PWM frequency for output/load
AILD-1 main unit dimensions: 84 mm × 54 mm × 31 mm (3.3 in. × 2.1 in. × 1.2 in.)