My 4 Digit 7 Segment display multi-communication schematic thoughts?

[corrado33]

So, I've had a crack at designing a schematic.  I've taken all that I know (not much) into doing this.

Basically my idea is to have a small PCB that I can simply provide communication and power to and have it display stuff.  I've based it on the ATTiny4313 so that if, let's say I wanted it to have some special function, it could.  If I wanted to link a couple together to make an 8digit display, I could. See where I'm going with this? 

Sure, I could probably buy something similar to what I'm doing, but I wanted to try to design this before I tackled my main robot PCB.  And since I've never made a PCB before, I wanted to do something small/cheap first. 

Here's the schematic.  I haven't made a board yet because I don't know if I did anything/everything right.

I'm assuming that this will get regulated 5V from whatever main board it's connected to, but I added a decoupling cap... because I thought it'd be a good idea? 

Click for full size.

And I've attached the eagle file for anyone who has it and wants to look at it that way.

EDIT:  Also, I've set up four 3 pin headers for the communication wires.  1 for USART, 1 for UART (probably could have been combined), and one for two wire SPI and one for 3 wire SPI (also could have been combined). 

Also I just remembered that I forgot to make headers/connector for power.  Ooops!


Well, I got bored, and I started designing a one-sided board.  It's REALLY fun!  It's kinda like a puzzle. 

[corrado33]

I know, double post... I'm getting terrible at that.

But I made a board.  I think it's pretty good.

No outside right angles, no acid traps, only 4 jumpers, nice ground plane ect.  (I left the ground plane out so you could see the traces better.  The top traces (red) will be jumpers, so I just let eagle autoroute them since I can put the wire wherever I want.)

Tell me what you think!
Click for full size.


It's a hundred times better than my first board, where I had to use .001 in. traces. Yeah, that was probably bad.  All of the traces on this board are 16 mils or larger.  Most of them are 24 mils, only between pads did I use 16 mils. 

[corrado33]

Ok, well, I know I'm making a blog here, but I made another board.

Added a .1uF cap by the VCC pin, and added a couple extra pins on the other end of the MCU in case I want to add more later.

I've also trimmed the board down.

[Graynomad]

Any reason you went for the 10-pin ISP header rather than the 6-pin version (which I think is more common).

I assume the display is common anode?

______
Rob

[corrado33]

Any reason you went for the 10-pin ISP header rather than the 6-pin version (which I think is more common).

I assume the display is common anode?

______
Rob

I smoked the 6 pin circuit on my programmer.     I'm serious though, my 6 pin one doesn't work, while my 10 pin one does.  lol

Yes, common anode. 

[corrado33]

Well, I'm making the PCB now!  I figured I'd make it just for practice, and even if I missed something I can make it again.

In other thoughts, I used the backing from a shipping label at work and the paper came RIGHT off of the board, I didn't even have to scrub.  It literally came off in less than 30 seconds.  I just peeled it off. lol

The board is sitting in the FeCl3 now.  I didn't heat it because I wanted it to go slowly, so hopefully I'll have a working board soon!

EDIT:  The ONLY trace I saw that was questionable was the one going through the pins of the 10 pin programmer.  That one was very close to a pad, but not touching.  We'll see how the etchant does.  If anything I can separate it with a razor. 

EDIT2:  Ok, the board is made and tinned.  Only ONE trace didn't come out.  It's the one leading into the left side of the hundreds place "8" on the picture.  It was all by itself in the middle of two large areas that were being etched and it got etched too.  I don't know if I was too rough with my little sponge brush, or if the toner just didn't stick well enough, or I left in in too long.  But I'd say that's not bad.  I'm just going to try to jumper it (it's only like a 3mm gap).  So, note to self, don't put smallish traces in the middle of large areas being tinned.  Oh, and I did end up heating the etchant.  I got impatient.     Overall, I'm happy with my first ever PCB.  OH and the toner I used SUCKED!!!  None of the printers at work have a "darker" feature.  They just print the same every time.  Some of the large areas look "speckled".  If I would have had more toner it would have worked better.  I tried copying it on the darkest setting, but it didn't like to print on my backer paper thing. 

[waltr]

That's great. As you do more boards you'll get better on knowing how to run traces and what might not work when looking at the toner deposits.  I've used model enamel paint and a small brush to touch up a trace before etching where the toner wasn't heavy enough.

Keep us posted.

[corrado33]

That's great. As you do more boards you'll get better on knowing how to run traces and what might not work when looking at the toner deposits.  I've used model enamel paint and a small brush to touch up a trace before etching where the toner wasn't heavy enough.

Keep us posted.

REALLY?  Model paint?  I have a ton of that (I loved building models when I was younger).  I'll have to do that next time. 

[Soeren]

Hi,

A Sharpie or similar is much easier to use than paint... And very easy to clean off.

You should try photo transfer. An average laser printer and the right "film" (I use drafting film, but have used baking paper when out of film), an UV source and some pre-sensitized PCB and you can make traces down to 8 or 10 mils, with no need for retouching.

[Joker94]

@corrado33, I assume you are using Eagle to deign the circuit board?

If so, what is the part you are using for the 4 Digit display? the only parts I could find in eagle are displays with 8 pins.

Cheers,

Joker94

[corrado33]

Hi,

A Sharpie or similar is much easier to use than paint... And very easy to clean off.

You should try photo transfer. An average laser printer and the right "film" (I use drafting film, but have used baking paper when out of film), an UV source and some pre-sensitized PCB and you can make traces down to 8 or 10 mils, with no need for retouching.

I might try that.  We did photo etching in school when we did microfluidics.  It was a PITA to get the time right.  Too little and it didn't work, too much and the stuff under the mask started to harden too.  (It was a resin that hardened with UV).

@corrado33, I assume you are using Eagle to deign the circuit board?

If so, what is the part you are using for the 4 Digit display? the only parts I could find in eagle are displays with 8 pins.

Joker, I had to design my own part.  I have a 12 Pin 4 Digit 8 segment display.  It's weird because I have exactly the same looking display, but one is a 12 pin and one is a... uh... 36 pin?  The grounds for all of the digits are already tied together just like you'd have to do to multi-plex them.  

EDIT:  I don't have a tiny drill bit nor a drill press.  So I'm buying both tomorrow .  I know you don't need a press for it but I've been looking to buy one for a while.  Now I just have to figure out how to put a tiny tiny bit into a 1/2 in chuck (or whatever is on the press)

[Soeren]

Hi,

I might try that.  We did photo etching in school when we did microfluidics.  It was a PITA to get the time right.  Too little and it didn't work, too much and the stuff under the mask started to harden too.  (It was a resin that hardened with UV).

Before doing any PCB, you make a test strip with a narrow strip of the pre-sensitized* PCB material and a piece of tin or similar non UV translucent material.
Cover all but 5mm of the strip, expose for 30 seconds, move the cover 5mm further and expose for 30 seconds, move, expose, etc until you reach a total of 5 minutes (with an average UV source (like 2..4 30W tubes at a 1m distance) or adjust as you see fit.
When you develop the strip, it will be easy to see what time works best for a sharp ad well defined edge.

Etching shouldn't be neither to fast nor too slow. If too slow, your traces will "under cut", which is a real disaster with narrow traces - another good reason to keep traces as wide as the circuit and PCB allows.
Too fast OTOH, will usually result in areas that aren't etched completely through - if you own (or have access to) a microscope, try studying the edges of the traces, to see if they get under cut or if they're sharp and well defined.


*You can coat raw PCB material with photo resist yourself, but for the occasional one-off PCB, the pre-sensitized is a better choice.

[corrado33]

Thanks again soeren.

Yeah we had trouble with exposure times because we were using a VERY powerful UV source inside a special UV filtering cabinent and our exposure time was around 10 seconds.    But, I really like your idea.  If we would have had time, we would have done something similar.

I did read about undercutting.  That's why I wanted to do it without heating the FeCl3 so I could just check it every minute or two to see if it was done.  In the end, I ended up using a sponge brush to "expedite" the process in the big areas that were etched as they were the last parts of copper left.  

On another note, I just bought a drill press.  A nice powerful 16 speed floor model off of craigslist.  (For really cheap)  Then I proceeded to try to carry it downstairs.  THAT was an experience.  I almost died/crushed my dogs/broke a bunch of stuff.  That said, it's safe down there now lol.  Those things are REALLY heavy, and I even took part of it apart.  

EDIT:  OH I didn't buy a huge drill press just to drill circuit boards,  do a bunch of other stuff too (and some that requires a good bit of power).  I've been working around ways to drill straight for years.  lol

[Graynomad]

We used to use a router for drilling PCBs, they are very high speed which is better for those tiny drill bits.

______
Rob

[Soeren]

Hi,

On another note, I just bought a drill press.  A nice powerful 16 speed floor model off of craigslist.  (For really cheap)  Then I proceeded to try to carry it downstairs.  THAT was an experience.  I almost died/crushed my dogs/broke a bunch of stuff.  That said, it's safe down there now lol.  Those things are REALLY heavy, and I even took part of it apart.  

Ouch! *Softly whisteling "That's what friends are for"* 

EDIT:  OH I didn't buy a huge drill press just to drill circuit boards,  do a bunch of other stuff too (and some that requires a good bit of power).  I've been working around ways to drill straight for years.  lol

It's a very good think to have a drill press. I don't know how I'd get along without one for the large work and a couple of smaller ones for PCB and routing work (A Proxon like in the pic and a home built one).


As Graynomad mentions: If you use carbide drills (which is the only ones that will make clean edges), you should drill at really high RPMs for best result. Some manufacturers say "30,000 to 100,000 RPM or above" for small diameters, but that's usually not available to the amateur, since such speeds would vibrate an average hobby drill to death in no time, but at least go as fast as possible.

[corrado33]

That little thing is COOL!!  I can't see it being useful for making anything other than very tiny holes though.  (And how often do I do that?  )

[Soeren]

Hi,

That little thing is COOL!!  I can't see it being useful for making anything other than very tiny holes though.  (And how often do I do that?  )

It drills to 6mm max. but I seldom use it for anything but PCB's and such like and nothing above ~3mm, but it's very precise, compared to eg. a Dremel clone mounted in a drill stand.

[corrado33]

I swear you know a little bit (ok a lot-a bit) of everything don't you?  

Anyway, I'm ordering some bits today.  Mcmaster is weird, I search by metric size (1 mm) and it brings me to 1 product in "micro bits", but if I click on solid carbide, I get a couple different variations of a 1 mm bit.

But regardless they have a "circuit board bit" section, so I'll most likely order from there.  

I'll probably order 3-4 1mm bits, three .8 or .6mm bits, and two 1.#mm bits.  

Unfortunately my newly acquired press only goes to 4300 RPM.  So... that's interesting.  If it doesn't work, I'll just zip tie my dremel to the chuck of the drill press and use it that way.    (With the drill press off of course).  That'd be an interesting (and dangerous) sight if both were on.  

EDIT:  HMMMM... there are much cheaper sites to buy drill bits than mcmaster....  At mcmaster a 1.0mm bit would be $4.## bucks.  At drillbitcity it's $1.50 a piece...  Also, these bits have shank diameters of 1/8 inch, which is good and bad.  Good cause I can put it in my press, and bad cause I can't fit it in my dremel. 

[Soeren]

Hi,

I swear you know a little bit (ok a lot-a bit) of everything don't you?  

I do read and do a lot and have always been nosy (and had too many years to do it ), but there's plenty of stuff I either don't know the first thing about or really suck at, dislike or simply don't care about - I try to avoid such things though, as there's already too little time for stuff that I do care about.

I'll probably order 3-4 1mm bits, three .8 or .6mm bits, and two 1.#mm bits.  

I guess the last one was 1.3mm - sounds like a reasonable selection (but skip the 0.6mm for now, they shatter if you look hard at them and you don't really need them for normal PCB work).
Most of the (PCB) holes I drill are 0.8mm.

Unfortunately my newly acquired press only goes to 4300 RPM.  So... that's interesting.  If it doesn't work, I'll just zip tie my dremel to the chuck of the drill press and use it that way.    (With the drill press off of course).  That'd be an interesting (and dangerous) sight if both were on.  

As much as I'd like to see it, I wouldn't like to be anywhere near when it goes down, but perhaps you could make some nice circles   

EDIT:  HMMMM... there are much cheaper sites to buy drill bits than mcmaster....  At mcmaster a 1.0mm bit would be $4.## bucks.  At drillbitcity it's $1.50 a piece...  Also, these bits have shank diameters of 1/8 inch, which is good and bad.  Good cause I can put it in my press, and bad cause I can't fit it in my dremel. 

There are much cheaper sites than McMaster - period.
While they have a nice selection of everything, you can usually find what you need cheaper somewhere else.
Back in the seventies I had to pay a bit more than $11 a pop though 

Recently I stumbled over a Chinese site that did nothing but carbide drills, but I bought a box of resharpened 0.8mm bits for next to nothing a couple of years ago and I've only ruined two so far (it pays off to get industrial quality).

The shanked types should be able to go in a Dremel, if you use the right size inner "claws".
Don't ever use them hand-held - and if you do, don't use the shanked types, they'll break where the shank meets the drill proper.

And when you break them, save the shanks. they can be ground to a sharp point and mounted in a metal tube for a long lasting metal scribe.

No matter the price, carbide will outlast HSS steel a thousand times - and stay sharp. A HSS drill will ruin the edges of a hole after about 5..10 holes.

[corrado33]

Meh, I don't feel like multi quoting you so I'll just answer you the old fashioned way.  

Yeah I don't know what I was thinking saying that the bits wouldn't fit in my dremel.  1/8in shanks are perfect for them.    Maybe I was crazy last night.  Also the pictures of the bits make them look A LOT bigger than they probably are.  

The site I was on also had resharpened bits, for about $0.84 a piece (for 1mm).  Maybe I'll try those.  What's the chance I'll break 10 of them?  (They sell them in packs of 10).    

So I'll probably buy some 0.8s 1.0s and 1.3s in decreasing order.  (I actually did mean 1.#, meaning something greater than 1.0, but 1.3 will prob. work)

Another annoying thing is that I can't find (Ok I haven't looked that hard) a chuck key for my press.  It didn't come with one (people lost it), and the company that made it's website doesn't work anymore.... so I gotta figure something out.  Sure I could loosen the chuck with two pairs of pliers but that'd get annoying, and I'd mess up the chuck.  I did some research and it's not a normal chuck key, it's some British Japanese Imperial Standard or something crazy like that, but I'm just trying to find one that'll fit.  From research I found online it's a 6mm pin and it has 12 or 13 teeth and it's a 5/8in chuck... I'll find it eventually.  I'll probably head to lowes or home depot this weekend and buy the biggest chuck key they have.  (I tried the biggest at ace hardware, didn't work).  .

Hmmm... I searched for the writing on the chuck instead of the brand of press and found out that JT3 is Jacobs Taper #3.  I should be able to find that.  

Off to order some stuff---    (Man it's so much easier to do this stuff when you have a job and stable income  ).


EDIT:  Wow....  WOW... I went back to look at my board today to fix that one trace that was in the middle of nowhere and got etched.  Well, this board would have never have worked.  My MCU ground wasn't even connected!!!!!  Well, it was my fault as I didn't check to see if my pathways for the ground plane were still intact after I had upped the exclusion area.  (And added some text).  But, I fixed it, changed a few things around (especially the power bus, since it had a tiny trace like area for the ground pathway.  It was pinched between two pins, so it had to go small to fit through.)  But I switched the position of the GND and +5V so now the ground has a nice wide open pathway to... uh... make a potential difference?

[Soeren]

Hi,

What's the chance I'll break 10 of them?  (They sell them in packs of 10).    

In how short a time?   

Sure I could loosen the chuck with two pairs of pliers but that'd get annoying, and I'd mess up the chuck.  I did some research and it's not a normal chuck key, it's some British Japanese Imperial Standard or something crazy like that,

If you cannot get the chuck off to bring it with you, you can make a mold by using silly putty or similar and then use that to make a "copy" in Plaster of Paris (you only need a partial copy).

There's no such thing as a standard chuck key. I have 7..8 different keys for my various drill equipment, where no two keys are the same (well, perhaps my 2 hammer drills can share the same key, but they're rarely at the same physical location).

On my hand held power drills, I duct tape the key to the cable, at a distance from the drill, so they can still be used.
On stationary drills, a strong magnet on the column or the top shield is my favorite - good for holding drills as well (although magnetic drills shouldn't be used for steel and other magnetic material).

When I mislay a key, I use the blunt end of a drill that fits the holes for leverage and a large screwdriver to tighten/loosen - ugly, but it works - and the key will turn up another day 

In the unlikely case that you cannot find (or make) a key, you might consider a new (or used) chuck for replacement - preferably a key-less model.

[...] But I switched the position of the GND and +5V so now the ground has a nice wide open pathway to... uh... make a potential difference?

Hehe, let's hope so... Or not.