I have 3 autometer gauges on my A-pillar pod which are just too bright at night. I have the light power source connected to the hot lead going to the headlight switch.

If I use the headlight switch to dim my dash lights - my gauge lights dim too...but at a different rate. I like my dash lights all the way up - but my gauge lights need to go down. My dash lights are dim by the time my gauge lights are at an appropriate brightness.

So my idea was to take the headlight dimmer switch out of my old beater - and wire it inline with the power lead and use it as a variable resistor tucked under the dash. Well I just hooked it up and it doesn't dim the lights.

I checked my haynes manual and they're the headlight dimmer wires...hooked up a meter on the ohm scale and can see it vary from .5 to 10.5 on the resistance - and the unit worked in my old truck - so I don't get why it won't dim the lights....any suggestions?

 

As long as the dimmer switch had two connectors and didn't need a ground, I'm not sure why it's not working. Are the pillar gauges on full blast now?

 

I saw about the same post over in the Electrical Forums about 4 months ago.
What it said was that the lights in the autometer gauges were LED’s and they have almost no load, so you old pot is not dropping any voltage.
You need to put a load resistor on the wire that is going to the gauges then your old pot will work.

 

Yeah they have LEDs which don't draw much (yes they're on full blast).

Where can I get a load resistor? How do I know which one to get?

Maybe I'll call autometer too....

 

Here is the Thread I was talking about, you might want to read it.

https://www.ford-trucks.com/forums/s...&highlight=LED


Good Luck

 

That is true. I had a feeling that the little load had something to do with it, because when we put dimmer switches in houses, we have a Min/Max wattage load that has to be present in order for the lights to dim. Wasn't sure if dimming a D/C system was similar to dimming an A/C system like I'm used to working with.

 

Very helpful link...

But it doesn't have the solution and I bet I'll have to call autometer for their advice...they should know.

 

I talked to autometer...they have a dimmer switch setup for $30 that will dim up to 7 gauges.

It's that....or I find another way. For $30...I'll probably just get it...

 

LED's can be dimmed two ways:

1. reducing the current by putting a resistor in series with lead that powers them. Since they are powered by 12V, I bet they have a 1.2k or 1.5K resistor already inside. Insert a 470ohm resistor in series with that lead, and they'll dim about 25-30%. Increase the value to dim more, decrease the value to increase brightness.

the 0-10 ohm dimmer thing you have doesn't reduce the current enough for LED's. But it does just dandy for light bulbs because the current drawn by the lightbulbs is quite high.



2. create a simple "chopper" circuit that pulses them at something above audio frequency (40khz or so). Then, vary the duty cycle of the pulses. The wider the pulses, the brighter the LED's. The narrower the pulses, the dimmer the LED's. Like so:
http://ourworld.compuserve.com/homep...htm#dimmer.gif

years ago I used a circuit like that with much larger transistors to control the brightness of my roll bar mounted off-road lights. I could change the brightness from 5% to 95% power with a ****, and a switch to turn them off. I did this because there were instances where I wanted the lights on, but didn't want 600W of light blasting at oncoming traffic. I had them pointing as an arc, so I could see deer on the side of the road as well as covering the area next to what the high beams illuminate. Essentially I had light from straight ahead, to out the driver and passenger windows. All on a dimmer. Anyway, if you click the above link, since you're driving LED's, you can replace the 2N3055 transistor with a 2N2222 transistor which is about 20 cents instead of a few bucks. I'm referring to the schematic on the right.

But like you said, 30 bucks and it's over with if you want to buy something.

 

I was hoping you'd reply.

What I don't understand...is that my gauges will dim about 60% with the headlight dimmer already on the truck (that dims the stock dash gauges). I have pretty much the same switch but off the 89 instead of a 95...and for whatever reason it didn't work. It's just that it takes 95% dimming of the dash lights to reach 60% dimming of the LEDs and it doesn't really work.


I think since it's only $30 i'll probably just get the dimmer from Autometer. If it was more money it would be worth the effort to do something else...

 

I wonder where they measured the current in the other thread. Did they measure between the dimmer and the gauge pod, or the hot and the dimmer?

I'm not sure what size bulb the dash uses, but if you add them up, it will give you an idea of how much more resistance you want to add to your new dimmer.

roughly 6-8 bulbs, 1 watt each (?) 8watts/12 volts=.6A include the radio, the ac controls, and the ash tray. Maybe triple that, so 1.5Amps.

You could buy a potentiometer (variable resistor with a ****) with the range you need and not worry about hooking up the extra resistor to the dimmer. (ahh covered in the other thread too)

The benefit of the chopper/pulse width modulation (PWM) circuit is that LED's have much tighter power requirement than regular lights. Once the voltage goes so low, they shut off, effectively only dimming 70-100% (depending on the led and the built in power circuit). Switching it on and off fast is more effective. (most leds are powered by 4.5volts so they gauges will probably have built in regulator circuits)

Since it runs off 12V, you should be able to use almost any device designed to run in a computer too. I have seen a lot of fan control plans that use the same chopper circuit. PWM fan control plans are all over the net, and in most computer stores/websites.

Wow, ok heres what i found real quick
NXP-305 $30/35 shipped
This one has 3 fan/light controls 'for up to 18 watts' so probably 6 watts each, and a sound activated light control and 2 blue 12" cold cathode lights for that pimped out ride look. And comes in silver, or black. Did I mention it includes lights.
or the lower model for $13/18shipped
NXP-301
does not have the light controls.
Hmmm, so this one is 5.25" wide instead of 3.5"

There is plans all over to depending on how much soldering you want to do.
pwm theory

 

You can always email me. I try to get to FTE often but I'm not succeeding in doing that, lately.

I'll explain. First you need a math formula for calculating LED current.

LED current = (supply voltage - voltage drop across LED) / limiting resistance

20ma = (14V - 1V) / R

R = 600 ohms. Since LED"s are quite bright, and most LED-backlit gauges have a plastic surround that transmits light very well, often they use a 1,000 ohm resistor, so the LED runs at a little more than half brightness.

Your dashpot gives you 10-15 ohms of resistance at the dimmest setting, and 0 ohms at full brightness.

So with the dimmer in series... you have 1,000 ohms plus 0-15 ohms in series with the LED. As you can see the dimmer isn't changing the resistance enough to get full brightness to full off.

The dimmer works great with lightbulbs, because the current draw of the bulbs is much higher, and, you have how many bulbs? 6 on the cluster, two or three in the radiio, one in the ash tray, and probably a few more. The dimmer is a wire-wound variable resistor, capable of passing about 4-5 amps no problem. So because of the current draw, 0-15 ohms will make the bulbs go from full off to full on.

Also, the dimmer, if you leave it at the most dimming setting, gets freaking hot.

I ran into this myself actually when I put in the thunderbird digital cluster - it has eight bulbs, and they are brighter bulbs than what is in the original f-series cluster. Probably because they have to make the LCD visible even in daylight. Anyway, the dimmer switch saw 10A of draw, and the wires and dimmer got way to hot for my tastes. So I made a simple circuit that replaced the backlight light bulbs with super bright LED's, and lots of them. Now the backlight draws 3A and it's just as bright as with the bulbs, and then some. The dimmer circuit I made actually measures the current going through the light dimmer switch to the light bulbs in the radio, ash tray, and so forth. Since it's "following", the only problem I have is if the light bulbs in the radio and ash try burn out - then there is no current passing (obviously), and the dash LED's will then "follow" and not light.

So if my thunderbird cluster ever stops being visible, I know to replace the bulb in the ash tray But that's how I eliminated have a seperate **** for the brightness of the cluster, and able to retain the factory ford dimmer for the time being.

Eventually the dimming functions of anything that dims will be controlled by the in-dash PC, so these types of things will be touchscreen activated. I just haven't gotten around to it since I had to take the homemade dash out since it cracked on the driver's side