Resistance Box (Decade Box) (for Fault Finding)

So I have been a Aircraft mecani..... mekhani.... a plane fixer for close to 10 years. Recently I had to test and recalibrate temperature gauges and probes to their gauges. This is usually done by a Avionic tech, but the closest one to me is about 120km away that means on a good day around a hour+ of driving for either of us. Luckily they are good friends of mine and usually explain things for me over the phone to save us the trouble and get the plane out of our hangar.

I've seen after the first few times of trying to fault find and troubleshoot probes, that they use this fancy box with nothing more than a stack of rotary switches, but it sets a certain resistance quite accurately. So being the aspiring tool collector I am, I googled one of them. It turns out it's a expensive piece of kit, with cheap ones around R990 (around $60) for the same thing.

After chewing on this idea of the tool for awhile and seeing if my budget will allow it, the avionic tech said he build their travel decade box. Their lab tools should all be calibrated, but as soon as they leave the shop they needed a travel calibration certificate which can take up to a month to get. He sent me a schematic that I promptly lost and had to redesign this whole thing again, over the phone with them. Luckily these guys are patient, they have to be they install avionics.

Ok, story before the recipe over, time to get building.

All parts I got was from a local supplier, but they are a international company so I'll add links. If you want to build this in South Africa, try these guys as well. They are my go too when I'm operating from home soil.

Parts:

• 1 x Enclosure (50mm(H)x80mm(L)x150mm(W))
• 6 x 12 way, 1 Pole rotary switch
• 6 x Knobs for a 6.35mm shaft (If you don't have a 3D printer)
• 2 x Banana Socket (one red, one black if you want to be fancy) (Black)
• 2 x Banana Plugs (Again one red and one black for extra fancy) (Black)
• 10 each of the following resistors: 1Ω, 10Ω, 100Ω, 1kΩ, 10kΩ and 100kΩ (This is a kit I happen to have)
• 2 x Alligator clips (Red) (Black)
• Some wire as test leads (Or just some old multi meter test leads)
• Here are the leads I built that you can buy pre made (Red) (Black)
• 2-3 x Hookup wires that you'll cut up, so don't use your fancy ones that you got in your Arduino kit.
• The link above is a roll of the wire, good to have in your shop.
• Adessive back paper A4/A5(The kind they use to make labels)
• Clear Vinyl film
• One cup of tea. This is for you to enjoy while building this box.

Tools:

• Soldering iron (Butane) (Station) (Normal AC) (Battery)
• Wire cutter/side cutter.
• Step drill bit. (I never knew my real drill bit)
• Multimeter (the one I have) (Had to borrow a fluke because I was not at my home shop)
• Electric or battery powered drill
• Sandpaper
• Printer (Sticker or normal)
• Pair of scissors
• That amazing brain of yours (One tool everyone can borrow but can't keep)

Nice to haves:

• A kind of roller or soft tip scraper.
• A "exacto" knife or scalpel
• De-burring tools
• Lead sucker
• Flux capacitor

As we go through the steps you can tweak the parts needed, this list is not law and you can add or remove what you think is overkill. For instance the step dril. That is a expensive bit, but you'll be fine just drilling with normal drill bits.

Now lets make like Sir Brian May and rock and roll

This is your project to make and this is only a guide from me to you. That said, you'll be working with power tools and a very hot soldering iron so please be careful.

If you doubt in anyway, ask for help. That's how we get better, failing and asking for help to get back on track.

Onward to the build.

I did all my designing on fusion 360, that I'm still learning to use, to plan out everything.

The rotary switches is placed on the top (or bottom) of the enclosure. Note on them the indexing pin and keep that in mind.(There is a tip later on in the steps to help you make a locating hole). I made a sticker that fits on top for all the values this can put out. I'll add it for you to download if you have the same box.

The "circuit" is just the resistors in series on the switch with a bridge wire connecting every switch to each other and the connections to the banana socket.

I tried to use the circuit design function in fusion for the first time. You are allowed to judge my efforts, as I myself wa shocked. I think after some practice I'll get the hang of using it.

The cad design of the box is bellow. Like I said, I'm still learning Fusion so don't expect export quality CADs.

So you would think we are starting to build this box, but there is still somethings to do. First we need to make a sticker that we can both use as a drill guide and the final label on the box.

For this I used adessive back paper that allow you too print onto, peel of the backing and stick. Think they call it label paper. I copied my box top to inkscape to not have to figure out where my holes where all over again. I printed two, one to act as a template to accurately drill the holes for the rotary switches and a second one that will be the actual label. This means is can completely ruin the first one with the drill, peel it off and just replace it. Professional look? Check.

I also made a drill template for my banana sockets on the side. More hours spent on design means less hours in the workshop drilling, more time to drink tea.

Too make the label that will stay on the box, I took clear vinyl covering and stuck it over my sticker before cutting. Here is where patience is key too prevent air bubbles on the label. If you do get air bubbles that is not excessively large you can poke a hole in it with your scalpel and use a hairdryer or a heat gun to soften the vinyl and push it down. i used a soft tip scraper and worked from one side to the other in small diagonal movements. Key takeoff here is to take your time.

After the vinyl is on your label, go ahead and cut it out.

Now I can see a lot of comments about how I copied the box sketch to inkscape from fusion. I had to call a guy who cuts our window gaskets for help. The simplest way he found was to create a new sketch in Fusion and project the top of the box and the side with the plugs onto that sketch, right click and save as DXF.

I then open this DXF file in inkscape and only set the manual scale factor to one(1). Next Shift+Ctrl+D will open the document settings, almost at the bottom at the scale tab (Scale x:) add a 1 to the end of all the zeros. This worked fine for me. (Thanks Greg)

The label is below if you want too use it as well. I'll include the svg file too if you want to cut it on your laser cutter/vinyl cutter.

After I printed everything I realized that I did not add a 0 to the scale. You get the fancy new label.

We'll drill now, promise.

After reading through all that I presume you are vibrating with anticipation to drill the holes in this box. So without further adieu let get to drilling.

I used a centerpunch on all the holes for the step drill to get a good starting point. If you are using normal spiral drill, maybe do the same, as well as pilot drilling to keep the hole centered in all axis. I went up to 8mm with my step drill and finished off with a 10mm spiral drill for the final fit of the switches. I repeated the same process on the sides for the banana plugs.

The box was deburred using a fancy deburring tool that I have, but sandpaper works just as well to break the chips from the holes.

Peel of the template and you are done with the box, for now.

Here is the step that took a lot of my time and concentration. Don't rush this anymore than is needed. If you are new to soldering electronic components, I suggest this instructable that helped me a lot when I started with electronics.

Start by tinning all your points on the switches. this saves you a lot of effort when adding the resistors. I also marked my nr.1 pin with a red dot to know where I should start. I also marked the switch to remind me which I am busy with. Now half one leg of the resistor and solder it to pin one. Repeat the step to all other pins leaving the other leg of the resistor unsoldered. Bend the resistors to each other and soldering it to the one next to it, making sure you solder it before the resistor body and pin. Daisy chain them together continually making sure you are using the correct value resistor.(Keep 1Ω and 10Ω apart for instance)

Solder a piece of hookup wire to the center/input pin. This wire will be soldered to the output/pin 1 of the next switch but don't do that now.

Test your switches one by one and measure the resistance to double check you done it correctly. Remember that the switches themselves may have internal resistance, in my case 0.3Ω per switch, measure your multi meter's internal resistance as well. If you have the same switches as I do, you'll notice some of them don't turn the full twelve steps. Remove the fixture nut and washer, below them is another washer with a tab, just turn that to the eleventh hole. this will give a hard stop on the 10th click. (I returned my switches as I thought they are faulty and they sent it back calling me a idiot)

Next solder a piece of hookup wire (of your desired length) to our banana socket.

we're almost done soldering, the last bit of soldering will be done after most of the assembly is done. Speaking of assembly, lets do that now.

Now that we have prepped everything that can go into the box we can start assembly.

Mark in the inside of the box the switch position corresponding with the label we made. Remember it' s going to be inverted so if your label goes low to high, from left to right, on the underside the low will be on the right.

As soon as you poke your switch through the hole, you'll notice the indexing pin. I shortened mine by half as my box wall thickness is around 2-3mm. To index them I took a paint marker, dabbed a few drops on the pin and pushed the switch through. this marked where I should drill a shallow hole. If you feel that the switch doesn't require a lot of resistance to turn you can cut the indexing pin off completely and solely rely on the fixture nut's tension to stop it from rotating.

Fasten your switches, shorten their shafts with your wire cutter and install the knobs. Set your knobs on 0 and tighten the grub screw.

Fit your banana sockets to the side in the same manner.

Congratulations, you are done. No wait we have still some soldering left.

Now the daisy chain between the switches are made. Solder the wire from the red banana socket to the input pin of the first switch.(That's usually the pin in the middle) Link this switches' output wire you soldered, to the input of the next. Repeat this until all your switches is connected to each other. Finally solder the wire from the black banana socket to the no.11 pin of your final switch. Done!

Do a final test of the system before closing up the box.

You have build yourself a resistance substitute box/Decade box. I'm so proud.

Most temperature sensors work by lowering their resistance as they get warmer and warmer. The hotter they are the lower the resistance.

We start by setting a max and a min with another gauge. Next we set a resistance per increment on the gauge and record that.

Out of frame is a calibrated multimeter to double check the values.

After that we move to the aircraft and double check the values we found on the test gauge and see if it matches on the installed gauge. If they do we'll most likely test the probe to see if it's resistance matches at certain temperatures.

This narrows down what we need to replace and saves us and our client money.

A thing or two to remember about this box.

• It's not to be used with high current. I used 1/4 watt resistors, but I'll substitute them for 1W in my next iteration
• Don't rely on its face value alone for accuracy. These resistors have a 1% accuracy but sill there are internal resistance from the wire and the switches. I use my multimeter to double check the setting I'm on.
• The 1Ω interval switch will be out of reading since it is adding very low values, but that said mine was out of set value by 0.3-0.4 solely because of the switch. This diminish on the higher readings.
• Don't build a bomb with this idea please. I know you may be tempted, but please, no.
• Feel free to modify this idea and posting your mods/hacks below. maybe I'll add it to mine.

This is not the only build out there and if you feel mine is missing something, search for the other. Here is one on Instructables that I came across.

Hope you enjoyed this.

W.Bean

• After I built everything I realized that my label was missing a 0 and the divisions where not the same as the rotary switch's clicks. I found a program that makes the making of fronts for these projects much easier. I'll link them here.

• The fact that my rotary switches have a 0.3Ω internal resistance. This added to over 1.8Ω of total resistance in the system making the x1Ω switch in error. The other where fine overall. I did not use the x1Ω as yet.

• This project was made out of necessity. I was troubleshooting and was not at my home base. All tools I had here, as well as some parts. This made (in my opinion)a sub par Instructable.

• You'll see some of the photos have a reddish hew to them, My RGB light seems to put out more R than the G or B. I switched to normal white for the rest

• I hope you'll leave a comment for me to improve on. Improvement is what separates us from animals