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How-to: Dimming Aftermarket Gauges With Dash Lights


81CJH

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This is a How-To guide for getting your aftermarket gauges to dim automatically, in sequence with your dash lights.

The circuit described below may also be useful if you have an aftermarket head unit or LCD screen(s) installed that supports display dimming, rather than just day or night mode.

Background:

Previously, the way most people dimmed their gauges was by using a 1k ohm potentiometer (or pot, aka rheostat) connected to the +12V illumination wire from the cigarette lighter or traction control switch. If you connect the Blue with White stripe wire from the cig. bulb to the left pin, and gauge illumination wire to the middle pin, you can adjust the brightness of your gauges just by turning the dial.

The down side to this is that you need to put the potentiometer somewhere accessible, yet out of the way, so that you can adjust the gauge brightness when required.

How it should be:

Ideally, your shiny new gauges should be the same brightness as your dash, and dim along with all your other dash lights using the same button.

GOOD NEWS! This is exactly what my new transistor circuit does. Better yet, it's dead simple to build, costs less than $2.50 and can be installed in roughly 30 minutes. :spoton:

post-7377-1190620551_thumb.jpg post-7377-1190620612_thumb.jpg

Parts list (Jaycar):

NPN Transistor: ZT2285 - $1.20 (TIP31)

Base resistor (RB): 10k ohm RR0596 - $0.38 (1% 1/2 Watt, Pack of 8)

Collector resistor (RC): 300 ohm RR0559 - $0.38 (1% 1/2 Watt, Pack of 8)

Emitter resistor (RE): 110 ohm RR0549 - $0.38 (1% 1/2 Watt, Pack of 8)

*Note: Collector & Emitter resistor values are correct for use with a pair of Autometer Cobalt series gauges. Different brands of gauges or dimming more than two gauges will require different values. The 10k ohm Base resistor should be universally correct.

Pin out for transistor (writing side up):

Pin 1 (Left) = Base

Pin 2 (Middle) = Collector

Pin 3 (Right) = Emitter

Pin 4 (tab/casing) also = Collector.

post-7377-1190620682_thumb.jpg

Construction Notes:

Over stating the obvious here; Soldering irons get hot. They melt solder pretty well, so don't be surprised if your finger gets melted if you touch the hot bit. Surprisingly, the wires & components that you're soldering also get hot - while you're still holding them. :buttrock:

See where I'm going with this? Good, let's get on with it...

Shorten the legs on the transistor & on one end of the resistors, leaving enough length to be able to bend the resistors up and over the transistor (see photo).

Solder the short ends of the resistors onto the transistor legs, double checking that the correct value resistor is attached to each leg (See circuit diagram, & use a multimeter if necessary).

This part is best done on the bench with a small hobby vice or a pair of pliers to aid alignment. You can do it in the car, but resistors are quite small and like to fall in hard to reach places when you drop them because "OWW, I just burnt my finger". :pinch:

post-7377-1190621255_thumb.jpg post-7377-1190621270_thumb.jpg

This is what your circuit should look like. Leave bending the resistors up & over 'till later. I ended up bending them back straight anyway to make the in-car soldering easier.

Note: If possible, use 2 to 3cm lengths of heat shrink to insulate your soldered connections. This is the best way to prevent short circuits from wires or component leads touching each other. Electrical tape will suffice if you don't have heat shrink, but is quite fiddly.

Solder an extension wire (about 10cm long) to the thick black earth wire from the cigarette lighter socket.

Solder the extension to the 110 ohm resistor connected to pin 3 of the transistor like in the circuit diagram.

Now, cut the two wires to the cigarette lighter bulb holder and strip the ends, leaving a little wire on the socket for reconnection at a later date (or just bin it).

Solder the Blue with Red stripe wire to the 10k ohm resistor connected to Pin 1 of the transistor.

Solder the Blue with White stripe wire to the 300 ohm resistor connected to Pin 2 of the transistor.

Solder the illumination wire for the gauges straight onto the top tag of the transistor.

post-7377-1190621174_thumb.jpg post-7377-1190621194_thumb.jpg

With all your wires soldered on & heat shrink applied & shrunk, bend the resistors up and over the front of the transistor so they lie flat on the top of the transistor.

Use electrical tape and wrap around the package (to insulate the metal tab & rear casing), then tape the package to the wiring loom for the cigarette lighter. This makes for a nice neat package that is fairly solid and well protected.

It's as easy as that! :roll:

Some more info for those interested:

Unfortunately there doesn't seem to be a wire that you can just hook your gauges up to & have them dim properly right out of the box.

This is because of the way Ford dim their dash lights.

The dash lights are not dimmed by a variable positive voltage, as you would expect. Let's look at the cigarette lighter illumination bulb, since it's just an ordinary light bulb and is simple to imagine.

When the park/head lights are on a constant +12V is applied to one side of the bulb, regardless of the dimmer setting. The brightness is actually controlled by raising the potential of the "earth" wire using Pulse Width Modulation.

When the dash lights are at their dimmest (1st setting), you have your +12V on one side of the bulb, but the "Earth" wire is actually reading +8.45V, with respect to earth. This means there is only 3.55V across the bulb, which makes it dim.

As you increase the brightness setting of the dash lights, the voltage of the "Earth" wire progressively drops, getting closer to earth potential and increasing the voltage across the bulb.

When the dash lights are brightest (4th setting), the "Earth" wire reads +0.265V, creating almost a full +12V across the bulb, which = bright.

Pulse Width Modulated ground wire voltages:

Dash lights dimmest (1): 8.45V

(2): 5.70V

(3): 3.01V

Dash lights brightest (4): 0.26V

Our aftermarket gauges are expecting to be fed with a variable positive voltage, and don't allow for this method of dimming. We're given one wire for voltage input, and the other side of the gauge bulbs (or leds) is grounded inside the gauge.

So you can see that neither the +12V wire or the PWM'ed earth wire is any good to us by itself for the purpose of dimming our aftermarket gauges.

If you were to hook up the +12V wire from your cigarette lighter bulb to the gauge illumination input, it wouldn't dim at all. If you hooked up the "Earth" wire, the gauges would dim, but they'd get dimmer as the dash lights got brighter.

What I've done, is use the PWM'ed earth wire to drive an inverting transistor circuit, which gets the dimming happening "Right Way Up".

When the dash lights are dimmest (PWM = 8.45V), the transistor is turned fully on and draws the supply voltage down closer to earth, making the gauges dim.

As the dash lights get brighter, the PWM signal gets less and less, turning the transistor further off, allowing the output voltage to rise closer to the suppply voltage.

When the dash lights are brightest, the transistor is fully off and you're essentially feeding the gauges directly through the 300 ohm Rc resistor.

Well I think that's more than covered it. If you have any questions or comments, feel free to reply here or PM me.

Enjoy.

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  • Still have a turbo, it's just on a diesel.
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Great work, may have to fire up the soldering Iron this week end.

Will this work with three gauges or do some of the values have to change.

Edited by seventytwo
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You'll probably have to adjust the values a bit. Best bet is to use a multimeter to measure the current drawn by the gauges, then use ohm's law to calculate the value of the resistors.

Bear in mind you may need to switch up to 1 Watt resistors depending on the extra current required.

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  • Resident golf buggy drift champion
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I didnt use any pots etc just soldered the fukkers on (to traction control) and its been 18 months and all good.....seems like overkill to me?

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  • Resident golf buggy drift champion
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They dim very well and at the same level as the speedo....which wire....id have to check its been 18 months :)

to tell the truth I rarely dim them anyway.

Ill have a look at which wire and let you know

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  • <put funny sh*t here>
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You'll probably have to adjust the values a bit. Best bet is to use a multimeter to measure the current drawn by the gauges, then use ohm's law to calculate the value of the resistors.

Bear in mind you may need to switch up to 1 Watt resistors depending on the extra current required.

I'm in the same boat as seventytwo. I'll be using this setup with 3 x Autometer Cobalt Gauges.

So how exactly do we do this? Any chance of explaining it to us?

Your saying to measure the current drawn by the gauges? Do we just hook up 12V to the 3 gauges in series, and then using a multimeter in series too measure the current drawn by the gauges? (im guessing the senders and all that dont have to be hooked up properly yet).

Then using R = 12V / X amps we can determine the resistance of something, but im not sure which resistor this is calculating the value of.. since theres two which might need to change - the one on the emitter and the collector.

Sorry for the noob questions!

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  • Member For: 18y 6m 11d
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Well ideally, grab a 1k ohm pot & hook it up so you can dim the gauges manually. If possible do it in a darkened garage so you can compare the brightness of the gauges to the rest of the dash.

I found that on my gauges that at the highest brightness I wanted I was only feeding about 5.3V into the illumination wire.

Once you have the gauges at the maximum brightness you require, you can turn off the lights, disconnect the pot without moving the knob and measure the resistance across the two legs you had connected. The value you get is the resistance you should use on the collector. Nice & simple, no maths.

*Note: The collector resistor determines the maximum brightness of the gauges, while the emitter resistor determines minimum brightness.

If you don't have a 1k ohm pot, just stick +12V into the gauges through a multimeter (in series) to measure the DC current being drawn. That will give you the maximum current required by the gauges and you can work out your resistor values from there.

Example: If your gauges require 50mA (0.050 Amps), then 12V / 0.050A = 240 ohms. Once again, this is the collector resistor. Also bear in mind that if you need more power you need to drive your gauges, you may need 1 Watt resistors, not 1/2 Watt. Power = Current x Voltage. Remember to use the voltage drop across the resistor to calculate the power, not the full 12V.

As for the emitter resistor, I'd have a guess at keeping it around 1/3 of your collector resistor value. I had a 300 ohm collector resistor & a 110 ohm emitter resistor, and the ratio seems about right.

Remember, all we're doing here is supplying for more current to feed the extra gauge. The proportions should remain the same.

Good luck!

Edited by 81CJH
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