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I am looking for input from the electrical brains. I am going to be installing Speedhut gauges in a friends car this weekend. I need to know where I can tap into the dash lights for power. I don't use it on my car but she for some reason drives at night and is visually challenged.

I know where to get power at the BCM fuse block but need advice on where to connect to the dash lights that hopefully does not involve taking the whole car apart.

Thanks much
 

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I assume they will be LED illuminated which means even a small wire will do. What about taking power from on of the button illumination wires at the traction control or hazard button?
 

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Power for the gauge is easy. There is a spare location in the BCM fuse block that works great. The lights need to be connected to the car dash lights so they will adjust with the rest of the dash lights.
 

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It looks like the easiest place to get the light power is Fuse 28 in the BCM. It is a 2A fuse that feeds the PWM voltage to the steering wheel control switch backlights.
 

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Not sure what you mean by "sense the changing voltage". That fuse is downstream from the PWM device that does the dimming, so a bulb connected to it should dim with the instrument lights. Depending on the characteristic of the bulb in the new gauge it could still be too bright or dim, of course.
 

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Awesome response John.

I'd ask if the Speedhut gauges expect a PWM signal for dimming (likely), or if they need an analog input. ?
 

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They heve two inputs for lights. One is directly connected, the second is through an inverter. Both connect to the dimmer side of the dash power. There is another pair to power the gaige
 

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Awesome response John.

I'd ask if the Speedhut gauges expect a PWM signal for dimming (likely), or if they need an analog input. ?
I think that the PWM signal is considered analog, a little choppy, but effectively analog.
In other words, if you put a voltmeter on it you will get a 0-12V reading.
 

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I think that the PWM signal is considered analog, a little choppy, but effectively analog.
In other words, if you put a voltmeter on it you will get a 0-12V reading.
Not really. Depending on the receiving device, anyway. Toggling 0-12V on and off at a 50% duty cycle will never appear as 6V. If you were sending this to a device that expected a max 6V input signal, the 12V 50% PWM could very well fry the thing.

I found a nice paper here, though, with some interesting filter designs:

https://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Method-for-Converting-a-PWM-Output-to-an-Analog-Output-When-Using-Hall-Effect-Sensor-ICs.aspx

As far as I can tell, the idea is to use a cap (or an RC filter) to smooth out the pulses. A cap that's not being charged will discharge and produce a decaying signal. If it's charging and discharging at the correct frequency, the output could be made to look like a ramp.

Fun problem, though. An op-amp, a cap and a resistor could clean that right up.
 

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Not really. Depending on the receiving device, anyway. Toggling 0-12V on and off at a 50% duty cycle will never appear as 6V. If you were sending this to a device that expected a max 6V input signal, the 12V 50% PWM could very well fry the thing.

I found a nice paper here, though, with some interesting filter designs:

https://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Method-for-Converting-a-PWM-Output-to-an-Analog-Output-When-Using-Hall-Effect-Sensor-ICs.aspx

As far as I can tell, the idea is to use a cap (or an RC filter) to smooth out the pulses. A cap that's not being charged will discharge and produce a decaying signal. If it's charging and discharging at the correct frequency, the output could be made to look like a ramp.

Fun problem, though. An op-amp, a cap and a resistor could clean that right up.
If the PWM output is being sent to something with a high enough input impedance you are correct, but let's imagine that we are supplying it to some LEDs. (Which is what we are doing, oddly enough.) They will glow with varying brightness as the duty cycle changes, and a voltmeter will display a varying voltage. It is how the instrument lights are dimmed, and it is how the courtesy lights in the mirror are dimmed. Don't take my word for it, read the description in the service manual. It is also how I am dimming the airbag on/off indicator lights with my seat sensor emulator. All I am using for that is a transistor triggered by a PWM trigger from an arduino. No capacitor needed.
 

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Fuse 28 does provide a varying voltage as the instrument lights are dimmed. I tested with the engine off, so the source voltage is around 12V. It may be more with the generator supplying 14V, but I didn't check. The 10V reading is at maximum brightness, the 2V is somewhere near minimum. My test light was also connected and went from bright to barely glowing.
 

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I tried to get my Speedhut Gauges to be the same brightness as dash gauges but unsuccessful.

I got power from one of the wires going to the gauge cluster but the dimming was not linear.

I ended up getting unregulated power from the line feeding the gauges and wired in a dimmer.

I leave the dash cluster light at full brightness and adjust the dimmer so the Speedhut gauges are close in intensity and color. I rarely drive at night so it's not that big of a deal.

The dimmer I used is a Blue Seas 7506 Deckhand Dimmer. It's made for marine use so $$$. I had one laying around. I'm sure you can find a less expensive 12 volt dimmer at Newark/MCM Electronics or web.
 

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forgot to mention, the added LEDs are dimmable. Notice the extra dimmer dial in the lower right of the picture. Needed that at night when they are too bright.

Bill.
 

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I'd like to see some gauge faces with white text on black background, lit by UV lights in front. My C5 had a similar setup and it looked amazing.
 
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