Prototyping puzzle - Page 1
Forum section: electronics / beginners

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 08:22:01 pm
content: Hi everyone,I have a rather open question that arises while I am trying to make a small prototype board, but is not limited to this instance. I have done many prototypes or one-shot builds using perfboards and stripboards (both veroboards and cluster-of-3-pads style), without much problems until now. Yeah, sometimes it got a little hairy, but the wiring involved has always been manageable.Today, I wanted to give a shot to the MAX7219CNG 8-digits LED display driver in order to build a 2x4 digits daughter board for a project I am working on, in order to simply use an SPI attachment between the microcontroller and the display. Seems very straightforward at first glance. Here is an idea of the daughther board with SIL headers for SPI/clock/power/contrast, 8 digits and the I.C.:The major wiring work is to connect each common cathode pin of the 7-segments displays to the relevant digit pin of the MAX7219, and to connect together all anode pins of a particular segment across all displays and then to the relevant segment pin of the MAX7219. This is where my troubles begins. In order to do that on a reasonably sized perfboard (4x6 cm in my case) that fits a little demo box to contain the project, the wiring is very tight. Unfortunately, I am out of AWG30 / AWG26 wire and of single-strand wire altogether (waiting for delivery), so I did with multi-strand ~AWG 22.My problem: I cannot do it. Well, I can, but it takes so much time and effort that it is just not worth it. I doubt that it will be much easier with AWG30. Here is how it looks with the first two digits wired (bad choice of wire colors, red are the cathodes and each black wire jumps from anode pin to anode pin on the digits):A closeup on the wiring:Now for the question:How wouldyoudo it ?The most important is not really finding a clever way for this particular project (although I would be interested in what you can come up with ;-) ), the point for me is to decide how I should handle prototyping in the future.Should I start making my own PCBs ? I did that a looong time ago, and refrained so far from setting up an etching station in my home lab these days because I do not like the idea of stocking the etchant chemicals in the same apartment as my baby girl. But I guess that can be done safely, the only thing is that I am not even sure I could home print and etch a single layer board of that size that would work for this circuit. However, this is the solution I am the most attracted to today. What is the state of the art in home etching these days ? Is it economically competitive with seeed or the other over-the-mail PCB prototyping services ? Not that this would be the most important point, I use that kind of service sometimes, but here I am talking about waking up with an idea and going to bed the same day with a working prototype, so I rule them out.I have also given some thought to going back to the old days of wrapping (used that a lot 15 years ago when I was in telco). Well, in the case at hand, it would surely be faster and cleaner, that's for sure. But this is not an all-around system, I could use it as an extra tool in my toolbox at most.But maybe I have missed some perf/vero/strip board techniques that could have helped me in that case and make prototyping easier for me in the future ? What do you use ? How do you do it ? Especially with ICs when you cannot use bent components legs as tracks.And BTW, because I can see the point coming up: I do own a few breadboards, I use them all the time for testing stuff that requires experimentation and tweaking. But in that particular case I am talking about prototypes that can fit in a box, be self-contained, can be demoed as a black box, etc. And the same circuit on a breadboard would probably look like a spaghetti plate anyway ;-)

author: IanB
author_rank: Super Contributor
timestamp: August 06, 2012, 08:29:19 pm
content: One thought that comes to mind is to use ribbon cable or flat flex cable rather than individual wires. You could solder one end to a row of connectors or pins, then twist/bend the cable to the right orientation and solder the other end?

author: madires
author_rank: Super Contributor
timestamp: August 06, 2012, 08:44:52 pm
content: There are special lab prototype PCBs for multiple ICs similar to breadboards. For example:http://www.reichelt.de/Laborkarten/UP-941EP/3//index.html?ACTION=3&GROUPID=3374&ARTICLE=23970

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 06, 2012, 08:49:21 pm
content: I think I'd use some kind of bus structure for the anode's, like 7 tracks across the PCB. It would not save time but it would make it easier to wire and to find the one wrong wireIt would make the PCB bigger though... The best thing would be to make/have made a dedicated PCB for this, shouldn't be too expensive seeing other posts.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 06, 2012, 09:01:14 pm
content: I use self fluxing wire - bought a couple of reels years ago from RS which I still have. Can't find it on the RS site butGreenweldseem to sell the stuff.For this type of application you can run it point to point. It's easiest to just tack one or two pins on the 7-segment displays, then loop the fine wire around a pin and on to the next or poke a loop down through the PCB hole so that it just, but only just comes out the component side.. Then solder all in one go - you can get very high wiring densities this way. Keep out of the way of the fumes though.

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 09:54:49 pm
content: Quote from: IanB on August 06, 2012, 08:29:19 pmMmmhh. Can't see that working well, given the scale and the multiple bends ribbon no1 would have to make to be woven under ribbon no2. Or it would take up a lot of thickness, but with arbitrary thickness, long wires work fine too, only they take up a lot of room. Thanks for sharing that tough though, I thought of something similar myself but concluded it would be difficult.

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 09:55:54 pm
content: Quote from: madires on August 06, 2012, 08:44:52 pm<nod>, I have been using something similar in the past, with separate strip islands. Those are indeed handy. Does not solve the anode connections in that case though.

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 10:02:01 pm
content: Quote from: PA0PBZ on August 06, 2012, 08:49:21 pmYes, I wanted to do that initially. But space constraints in my prototype box and a limited stock of perfboard sizes made me dump the idea. But now that you are mentioning it again, I wonder if either using a secondary board (grand-daughter board anyone :-) ? ) just for the 7 lanes, or using thick wire (think 0 Ohms straps) + heatshrink tube would work better. Imagine something like this:'= '-> heatshrink'-' -> non isolated soldering points left free for wires to the anodes========-========-========-========-========-========Mmmhh... might actually work. Definitely, the bus idea is a 'clean' one, just needs to find a suitable implementation for this use-case with that board size..

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 10:11:07 pm
content: Quote from: grumpydoc on August 06, 2012, 09:01:14 pmThat actually is very close to what I had in mind when I mentioned wrapping techniques!We also used to use that kind of enamel wire. However, it wasn't fluxing, you had to scrap the enamel at the connection points (well, the wrapping tool would do it for you :-) ).So, if I understand correctly the process with these: enamel gives you insulation, but when heated, it melts, leaving the wire free to make contact, and releases flux ? Is that correct ?If this is how it works, this thing look excellent ! I need to try it.Actually, I thought of doing this with regular wire-wrap wire (scraping enamel by hand) or kynar wire (melting the insulation away without cutting the wire at each intermediate anode), but both methods seemed messy.

author: madworm
author_rank: Frequent Contributor
timestamp: August 06, 2012, 10:12:23 pm
content: The last time I was confronted with a 7221, I ended up with a terrible mess of wires. I will NOT do something like that again.

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 10:17:52 pm
content: Quote from: madworm on August 06, 2012, 10:12:23 pmWeird to say so, but I feel better knowing it's not just me :-)Also the 7219/7221 have completely nonsensical pinouts. I mean, at first I thought the pinout was weird but optimized for some sort of board layout, now I think it is just random ;-)Edit: Plus they are darn expensive! However I think there are cheap (like half the price) functionally equivalent alternatives (no, not a bunch of shift registers wired in series). I just always forget their model numbers ;-)

author: pickle9000
author_rank: Super Contributor
timestamp: August 06, 2012, 10:32:37 pm
content: Kynar wire wrap wire is the best....mike

author: mariush
author_rank: Super Contributor
timestamp: August 06, 2012, 10:59:02 pm
content: My first thought would be to use thin transformer wire that's enameled by default. Wrap wire (after removing the insulation) around each pin of the digits and solder the wires right at the base of the pins with a bit of solder, then route the thin wires to 1-2 of those round pads so that the wires will now be in the back. and then solder each digit to the PCB.I could then route the wires somewhere on the PCB where I have more room, group them together as needed for the multiplexing and so on.

author: baljemmett
author_rank: Unknown
timestamp: August 06, 2012, 11:36:18 pm
content: Quote from: tramjoe on August 06, 2012, 08:22:01 pmThe fine, self-fluxing wire, as mentioned -- see also systems like RoadRunner and Verowire. I used one to wire 96 LEDs and associated resistors to a DIN41612 connector on a Eurocard (and a whole pile of shift registers on a corresponding driver board!) for instance, which although rather labour-intensive was a damn sight more pleasant than trying to do it with individual lengths of chunkier wire would have been...However, in this instance I'd look at whether there might be an easier route -- e.g. could you use a pair of Avago HDSP-B03E or similar modules? Used one of those in a prototype recently, much nicer to wire up as you just have the eight segment anodes and four common cathodes brought out. Unfortunately that particular model is a clock display, so instead of having a DP segment after each digit it has a colon in the middle, but it did the job -- models with four decimal points do exist but I couldn't source one in a hurry!

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 11:43:18 pm
content: Quote from: baljemmett on August 06, 2012, 11:36:18 pmI'll look those two up, thanks for the pointer!QuoteYeah, of course that's the easy road :-) But you're cheating a bit there ;-). I would normally have done so, using 4-digits modules is what I would do anyway if building this thing for real. But for this prototype, I used basically what I had in stock. Also, with 4-digits modules, digits alignment and spacing is perfect, and the BOM is cheaper. No reason to use discrete 7-segment digits in anything that should be done in more than 1 copy.

author: tramjoe
author_rank: Regular Contributor
timestamp: August 06, 2012, 11:58:18 pm
content: Look at that, I love it:Apparently done with a vero-wire (now defunct system if I understand correctly) type system: the guy built his own enamel coated wire dispenser penhimself.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: August 07, 2012, 12:47:36 am
content: Wire wrap. Still have my wire-wrap tool, plenty of spools of wire and a box of assorted WW sockets for just this thing.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 07, 2012, 09:58:06 am
content: QuoteBasically, yes. It works best with pad board or tri-pad, stripboard is OK but not really much better than just fine wire. Not sure how well the coating actually acts as a flux but it does melt when it gets to soldering temperature so you don't have to scrape it away beforehand. Watch the fumes though as it releases an isocynate when heated.For long runs I tack the wire to the board with superglue, if you're careful you can do very neat busses with all the lines parallel and flat on the board. Even if you're not especially neat then the wiring takes no more space than the 2mm or so that you're going to clip the component leads to above the solder side of the board so it doesn't increase the overall thickness.

author: T4P
author_rank: Super Contributor
timestamp: August 07, 2012, 02:05:08 pm
content: I parallel stuff with a bare copper wire/ tinned or untinned in a straight row if possibleBut if THERE's a opposite voltage pin on the same row it's easier to use seeed but don't screw upYour screwups can take 12-30 days to arrive depending on what delivery method you chose