Saturday, October 21, 2006


Coping with the AS5035 shaft encoder...

I'm able to take most of the day off today to work on Tommelise, so I decided to grit my teeth while my hand is really, really steady, which it is during the mornings and build up a few shaft encoders for my pseudostepper motors. To that end I'm going to document how I wire the surface mount AS5035 magnetic shaft encoder chip with ordinary tools so if anyone else decides to do this they won't have to invent a way to get the soldering done.

To bring you up to date the AS5035 is not a DIP chip so you have to deal very differently with it. Ordinarily, you'd need a $1,000+ workstation do be able to work successfully with surface mount chips. I'm going to show you how to do it with a $15 Radio Shack soldering jig and an ordinary, low power $15 soldering iron.
Here's the jig, especially made for old guys like me who wear reading glasses. It has two swivel mounted alligator clips under a fair sized magnifying glass and a holder for my cheap Radio Shack soldering iron.
Here's a very nice closeup that Adrian made some time ago of what the leads that you're going to have to solder onto the AS5035 look like. The only problem with this pic is that it tends to play with the heads of people like myself who perpetually live in hope simply because it is such a good closeup. You get the idea that the chip is a lot bigger than it is and forget that those stripboard holes are really only 2.54 mm apart.

Once you have your AS5035 in your hand, you really understand that this chip is about the same size as a sequin and you'd better not have hay fever because you can sneeze it away.
Here's what the chip looks like under the magnifying glass in my soldering jig. That's an ordinary sized alligator clip you're looking at.

Now the soldering jig has two swivel-mounted alligator clips the idea being to, say, hold the board you're working on in one and the wire you're trying to solder in the other. You can't use ordinary insulated wires to solder to the feet of this chip very easily. They're simply too big. You need very fine gauge wire, something that is rather difficult to come by in a hobby shop or ordinary electronics store. You don't need more than a few inches of it, either. He're how I got my filament wire. I bought a roll of multicore copper wire and then stripped the insulation off of several inches of it. Presto, you have filament wire in it's own very handy jig holder, viz, the rest of the coil of wire.

I tried to use the alligator clip to hold individual pieces of this fine gauge wire first time I tried to do this. It's better just to leave the filaments attached to the spool after stripping the insulation and do one side of the chip's soldering at a time.

First thing you need to do is get an old sponge that you're ready to throw away, wet it and squeeze it out thoroughly. Use the sponge to wipe excess solder off of your soldering iron after it's heated up. Be sure that you sponge doesn't have holes in it. You don't want to lay a finger on the business end of your soldering iron.

Once that's done use a very thin bladed screwdriver to slightly splay the AS5035's pins so that you can deal with them. You want the one that you're working on a short distance away from the others so that you won't make a solder bridge between pins. Accept that you're going to make solder bridges and you're going to get the filament wire connected to the wrong pin more often than not.

To deal with a solder bridge, use the damp sponge to clean your soldering iron hot tip and then stroke it across to solder bridge until the solder is absorbed onto the surface of your soldering iron. This may take several strokes. You should wipe your soldering iron after each stroke to get the bit of solder that you extracted from the pins off of it. If you don't get rid of the excess solder off of your iron you WILL get it back on the pins and maybe even make another bridge. Trust me on this.

Ideally you should wet the end of the filament with solder, just enough to make it silvery. No lumps please. Once you've done this put it on the target pin and tap the filament with the CLEAN soldering iron tip. That will weld it to the pin. Mind, this sometimes happens after 2-3 tries. I'd have loved to have photographed this process for you, but I've only got two hands and a limited amount of light to work in. :-(

Here is the first filament successfully attached to the pin.

One of the annoying aspects of doing this kind of work is soldering two filaments to pins that are side by side. You'll often overheat the filament weld of the pin already done while trying to attach the second one beside it. That's a pain.

Here's all of the filaments connected. It took me about 5-10 minutes once I set the job up. I'm not going to do another one this morning, though. My hands are shaking a bit and that makes doing the job very, very difficult. Oh yeah, you can see the damp sponge that I was using to clean my soldering iron tip in this photo.

The problem now is that you have a surface mount chip with uninsulated filaments hanging off of it like a daddy long legs spider. Adrian solved the problem by drilling a hole in a piece of stripboard and soldering the filaments to the strips. It is easy then to make other connections.

I took another approach. I used a standard silicone sealant like this...

...and mounted my chip to a piece of wood like this.
You can secure it to the mount that you will place close to the shaft with the magnet attached. Daubing the silicone over the filaments insulates them quite nicely and secures the chip to the wood. The nice part about this approach is that you can peel the chip and filaments off of the wood later if you need to and clean the chip and filament without having to use heat. If you're careful you won't even pull any of the filaments off of their pins. I have done this and I'm a clumsy oaf.

You'll notice that when I'm working I tend to print out a photograph of what I am trying to achieve in large format so that I'll have something to refer to. I also printed out the schematic so that I can cross check the photo from Adrian's design page in the wiki.

That's all for the moment. This is a work in progress. I'll be posting the rest of the process as I get it done and documented. I'm going to take a little break, take a bath and go buy a few capacitors. BBL.

I like the irony. To use modern surface mount parts, we must borrow a page from prehistoric days when cavemen mounted components on wood.

I'll stop mangling metaphors now. That's a great guide. I always, always, always forget to take photos during the process, you showed us the whole work.
I wonder if varnish lacquer or nail polish or something would help? Get one lead on correctly, then lacquer it so you can't make accidental soder bridges with it...
***I wonder if varnish lacquer or nail polish or something would help?***

Given the distance between the pins you'd need a very steady hand and a really fine little brush.
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