How To: LED Backlighting a real gauge

[KLaFaille]

Hey there, here's a quick step by step of a way to LED backlight real instruments that start out as white incandescent, in this case a Percent RPM gauge.

 

Gauge Backlighting (3 of 10).jpg by KLaFaille, on Flickr

 

1. Here's the gauge assembly removed from it's can. Next to it you can see the standard incandescent T1-3/4 lamp which is very common in aircraft components, and the hollow cylinder next to it is the housing the lamp sits in.

 

 

Gauge Backlighting (1 of 10).jpg by KLaFaille, on Flickr

 

2. Here's the internals of the gauge. You can see towards the face of the gauge the rear of the lamp, the housing, and the copper plate that contacts the bottom of the bulb. There are three lamps wired in parallel so that if a bulb burns out the other two still work. One half of the lighting circuit goes directly to the gauge body itself, ground, while the other half goes to each of the three copper plates.

 

 

Gauge Backlighting (4 of 10).jpg by KLaFaille, on Flickr

 

3. The lamp, housing, and LED we are going to replace the lamp with. Because the LED is smaller than the housing, we are going to use the leads of the LED to keep it in place in the housing and give us a good electrical contact.

 

 

Gauge Backlighting (5 of 10).jpg by KLaFaille, on Flickr

 

4. Step one is to take the cathode (negative) leg and bend it flat against the base and back towards the front of the LED.

 

 

Gauge Backlighting (6 of 10).jpg by KLaFaille, on Flickr

 

5. Next we wrap the cathode around the body of the LED. This is so the LED will sit snuggly inside the housing and you get a good electrical connection to ground. You may have to trim off a bit of length so that you don't have any of the cathode overlapping onto itself. After we wrap the lead, the wrapped end of the cathode is soldered to itself. At this time you can better round out the cathode around the LED body and make sure it fits snug in the housing.

 

 

Gauge Backlighting (7 of 10).jpg by KLaFaille, on Flickr

 

6. I used hot glue to "pot" the LED securely into the housing, both in the rear and around the front of the LED also. While the glue is cooling make sure the LED sits centered in the housing.

 

 

Gauge Backlighting (8 of 10).jpg by KLaFaille, on Flickr

 

7. Take the anode (positive leg) of the LED and bend it 90 degrees flat against the back of the housing. Then take and bend it back over itself so you end up with something like in the picture.

 

 

Gauge Backlighting (9 of 10).jpg by KLaFaille, on Flickr

 

8. Fill the gap left in the anode with solder. We don't have to go crazy but want enough so that we end up with a decent sized ball. Trim off the excess anode and file smooth any sharp edges left by trimming.

 

We have basically made an LED lamp at this point that works on the principal electrically as the incandescents we removed. The cathode is touching the housing which goes to ground, and the anode is touching the copper plates supplying the positive voltage. Depending on the voltage you choose to power your lighting and the voltage drop of the LED itself you will need a resistor to soak up the excess voltage.

 

In the case of the LEDs I used, they have a forward voltage of 3.7V, which is a bit odd for a green LED but that is what it is, and I will be supplying them with a 5V power supply. 5V - 3.7V means we need a resistor that will soak up about 1.3V so the LEDs don't go poof. I ended up using a 22 Ohm resistor which resulted with about 3.65V going to each LED.

 

A caveat regarding LEDs in parallel. Generally the way the LEDs are wired in the gauge now isn't the preferred method. Ideally we would like to calculate the required resistance needed for each LED and put a resistor in series with each one. Wiring LEDs in parallel with only one resistor requires that all the LEDs have a very close voltage drop to each other otherwise a single LED may end up pulling more current than the others which could lead to a failure. I did a simple swap of incandescent to LED, and incandescent lamps don't care about voltage the way an LED does and given the limited room (and some laziness on my part) I simply kept the wiring the same.

 

The end result:

 

Gauge Backlighting (10 of 10).jpg by KLaFaille, on Flickr

 

Hope this was helpful,

 

Red

[Deadman]

It may be easier to use this method incase there is a failer with the LED

http://www.viperpits.org/smf/index.php?topic=3437.0

[KLaFaille]

True true. I did try something like that at first, but had no luck with it. I will do that method for the emergency panel, landing gear panel, and others with similar indicators.

 

That's a good link to have in any case.

[Deadman]

no problem the only problem I have found with some cheep hot glue is it remelts with heat can make a mess in some things all of a sudden

[PanelBuilder]

Very nice indeed. You couldn't post some more pics of the inards of that gauge, could you?

 

Cheers,

Colin

[KLaFaille]

@Deadman, yeah you have to be wary of that "cold melt" hot glue sometimes. With these type LED, they don't drive that much current and only use about .074 Watts (Power = Voltage * Current or 3.7V * .02A = .074W) so they don't get too hot. Incandescents and the like though I'd probably stick with the take the flesh off your finger hot glue for sure.

 

Oh hey, I snagged a proper real Oil Pressure gauge last night for $26, too bad that other % RPM went way too high for my liking.

 

@Colin - I'll see what I can do. Once again it is amazing that something that seems so delicate and precise can handle the abuse that aircraft put them through.

 

Red

[STP Dragon]

Nice pictures! What camera/objective did you use?

[KLaFaille]

Thanks,

 

Canon 5D w/ 35mm f/1.4L anywhere from wide open to about 5.6 on one or two I guess, generally around 2.0 - 2.8 though.

 

K

[STP Dragon]

Canon 5D w/ 35mm f/1.4L

WOW, very nice combination :thumbup:

[PanelBuilder]

Nice pictures! What camera/objective did you use?

 

One trick that works pretty well for an object with little depth, is to put it on a flat bed scanner.

 

I first saw this in a photography magazine; an archaeologist was using it for arrow heads.

 

A six inch steel ruler doesn't hurt the composition either.

 

Cheers,

Colin

[PanelBuilder]

[quote=KLaFaille;1290333@Colin - I'll see what I can do. Once again it is amazing that something that seems so delicate and precise can handle the abuse that aircraft put them through.

Red

 

Thanks Red. One question: what does the "Ng" on the face, up by the 20, represent?

 

Colin

[Sid6dot7]

It is what the gauge indicates: the gas generator speed/rpm or the working turbine's rpm. Mainly used in turboprops. The propeller's/free turbine's rpm thus are named "Np".

Edited September 13, 2011 by Sid6dot7

[KLaFaille]

Yes, technically that Ng should go away as the gauge is implemented in the A-10. A bit of paint maybe, or I'll just be lazy and leave it.

 

Red

[PanelBuilder]

Yes, technically that Ng should go away as the gauge is implemented in the A-10. A bit of paint maybe, or I'll just be lazy and leave it.

 

Red

 

Cool. One other question for you Red, if you don't mind. It's just that sometimes I don't understand the simplest things. Like how is the bezel attached to the body on these things?

 

I've got the milspec for these style gauges right in front of me and I still can't figure it out.

 

Thanks,

Colin

[KLaFaille]

The bezel appears to be brazed or silver soldered onto the front of the instrument can. There is a spacer used inside between the glass and the face of the instrument to keep the gauge assembly in place. The rear cover containing the the receptacle connector is high temp silver soldered into place, so you need a propane torch when opening these style of instruments.

 

I'll see about getting some pics eventually.

 

Red

[PanelBuilder]

Good man. Thank you!

 

Colin