Dimmer circuit for Omori gauge
As mentioned in the intro the Omori gauge uses LEDs for
white for the face and red for the needle. When connected to +12V
the gauge is VERY bright - enough to see the illumination even on a
sunny day. This is clearly way too bright at night, and in fact
proved to be somewhat of a distraction to night driving. The next
problem to be tackled was to bring the gauge illumination down to the
level of the rest of the cluster.
My first attempt to reduce the brightness was only moderately
successful, and was simply a resistor in series with the illumination
wire. This served to reduce the current flow and therefore
brightness of the backlight LEDs. Before I go further, a little
LEDs, unlike incandescent globes, maintain a relatively constant
forward voltage irrespective of the current flow through them.
Brightness is controlled by varying the current, and is usually set by
a series resistor. The value of this series resistor is
calculated using Ohm's law, with the voltage derived from the
difference between the total supply voltage and the LED's forward
voltage. When two or more LEDs are connected in series, the
forward voltages of the LEDs are added together for the calculation.
this example (at left), two LEDs with a forward voltage of 3.5V each
are connected in series to a 12V power supply, via a resistor to limit
the current. The voltage across the resistor will be 5V as
shown. If we require a forward current of 20mA, the resistor
value can be calculated using Ohm's law, as follows:
R=V/I (R is resistance in ohms, V is volts and I is current in
=5/0.02 (20mA = 0.02A)
I did not trace the backlight circuit in the Omori gauge, but from
observation of the effect of fitting a resistor in series with the
illumination wire, it uses two or more circuits similar to the example
above. The resistor effectively reduces the total voltage
available to the backlight circuits, reducing the voltage across the
internal series resistors, and hence the current through the LEDs.
problem with this arrangement is that the Omori gauge uses red and
white LEDs, which typically have different forward voltages. Red
LEDs are usually around 2V but white are 3V or more. When
dropping the total supply voltage to the backlight circuits, a point is
reached where the red LED circuit's resistor still has voltage across
it and the LEDs illuminate, but the higher voltage requirement of the
white LEDs is no longer met, and the resistor voltage drops to
zero. Just above this point, the white LEDs dim to a significant
degree but the red LEDs are still bright resulting in a red cast across
the entire gauge face and red halos around the needle boss and surround
(see photo). The backlight also becomes very sensitive to voltage
fluctuations which frequently occur, for example each time the brake
lights operate the white backlight dims momentarily.
In order to dim the white and red LEDs evenly and maintain the proper
look of the gauge, a technique different than simple voltage dropping
was needed, and that technique is pulse width modulation or PWM.
Due to its efficiency, PWM is in very common use in power supplies for
all kinds of applications, from mobile phone chargers to air
conditioners to PCs. Rather than reducing voltage, in PWM systems
the full supply voltage is applied for a controlled period, then
removed for a further period. The duty cycle is the ratio of "on"
time to the total "on" plus "off" time or cycle period, and is
proportional to the actual power applied to the connected electrical
load. The cycle period is very short, usually running at several
thousand cycles per second depending on the application.
PWM to power the Omori backlight means that rather than dimming the
LEDs, they are operated alternately at full brightness and off.
LEDs are able to switch on and off very quickly, and at a sufficient
cycle rate our eyes perceive this as a reduction in brightness rather
than flashing. Because the red and white LEDs switch on and off
simultaneously, they appear to dim evenly. The circuit shown here
runs at around 140 cycles per second - more than enough for a flicker
free result. Voltage fluctuations have virtually no effect,
because there is still a significant voltage across the current
limiting resistors in the gauge. As can be seen in the photo, the
red cast and halos are not evident.
It may sound complex, but the PWM circuit is actually very simple -
only eight components are required, and all are cheap and readily
available. Below is a circuit diagram and brief description of
the circuit operation.
is a timer which derives its period by charging and discharging a
capacitor via resistors between 1/3 and 2/3 of supply voltage.
The capacitor and resistor values determine the actual timing
periods. In this circuit, the output (pin 3) is initially high
(+12V), powering the LED backlights and charging the 0.1uF capacitor
via diode 1N4001 and 5K trimpot. When the capacitor reaches 2/3
(around 8V) supply voltage, the LM555 output (pin 3) switches low (0V)
and pin 7 goes low. The capacitor then discharges via the 100K
resistor and 5K trimpot. Once the capacitor discharges to 1/3
(around 4V) supply voltage the output switches high again, pin 7 goes
open and the cycle repeats.
Adjusting the 5K trimpot allows variation of almost 0% to around 5%
duty cycle. In practice the Omori gauge closely matches the
brightness of the rest of the cluster at around 2% duty cycle, so the
adjustment range is ample.
None of the components in this design are critical and substitutes may
be used. For example the 1N4001 diodes could be replaced with
1N4004, I used greencaps for the two capacitors on the right side of
the diagram, but ceramics could be used. The 100uF capacitor is
only for filtering and anything from 47uF to 470uF could be used.
There is a small PCB below the tacho in the MR2 cluster which is
actually a signal conditioner for the factory boost gauge and may be
removed. I used this space to advantage and built a small PCB
with the dimmer circuit to take its place. Pictured below are the
original PCB (left) and the new dimmer (right).
And here is a solder side view:
The threaded lugs on the bottom of the board connect to the
same positions on the cluster as the original board, and I have used
these to carry the electrical connections to the new Omori gauge.
The unsoldered pads on the left side connect to the Omori gauge loom to
complete the connections. The full details of these connections
are explained in the Electrical Wiring In
Here is the PCB fitted to the cluster and wired to the gauge:
There is no reason for this circuit to be built exactly the way I have,
but if you'd like to build the PCB to fit as above, the solder side
photo should show enough detail to replicate my design. The PCB
is 27 x 50 mm, and just copy the spacing of the threaded lugs from the
original board. I used some screw terminals off old speakers to
make the threaded lugs - just had to reshape them a bit and cut off
some tabs. You could desolder the lugs from the original PCB and
use them - I didn't because I wanted to keep the original board
intact. If making a PCB is not practical, the circuit could quite
easily be built on veroboard or the like, and it certainly doesn't have
to be fitted inside the cluster.
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