[kipman725]

Hello I need ot produce a prototype pcb. I think two at the most unless anyone has a great desire to build an 8x7 animated LED display. I have looked around and all the PCB prototyping companies are charging... well well above my means I think at most I can budget £25 for this project. I have access to lazer printers but have no idea where to get hold of the chemicals needed for etching ... anyone any idea how I can get cheap PCB's? (they don't need to be high quality just usable).

 

[Pars]

Olimex is probably the cheapest, although they are priced per panel (100 x 160mm):

OLIMEX Ltd. - Electronic Design and PCB sub-contract assembly OEM/ODM service

Or, make your own. Look here. Note for etching solution all you need is hydrogen peroxide and Muriatic Acid, which is used to clean swimming pools, etc. and should be available cheap in hardware stores. This is mixed 2 parts hydrogen peroxide to 1 part muriatic acid. Watch the vapors, as they are somewhat nasty (do this in a well ventilated area). This stuff works much much better than normal ferric chloride etchant. Gloves and goggles also recommended. It really isn't very hard to do, even double sided boards.

 

[error401]

BatchPCB is cheap for one-offs. They charge a bit more than Olimex per board, but you're charged by sq. in. so it often ends up being cheaper. Turnaround time is pretty bad though. Barebones PCB (by Advanced Circuits) is also quite cheap, but doesn't include solder mask or silkscreen, which is probably acceptable for this application. Turn time for these is really quick.

 

[Joshatdot]

BatchPCB is decent pricing, they could compete with Olimex! PatchPCB's cost is per square inch no matter how many you order. Olimex's cost is how many PCB's could be made from 6.3 x 3.9 panel or 12.6 x 7.8 panel, and how many panels you want to order.

 

[Joshatdot]

I just checked what it would cost my Joshatdot v1.03 CMoy's with BatchPCB. My board is about 1.5 in^2, which is $5 per board. So 12 would cost me $60 + Actual S&H + $2...and I think $10 "setup fee"

I'll give them a try on my next CMoy order.

 

[error401]

Quote:

Originally Posted by Joshatdot /img/forum/go_quote.gif I just checked what it would cost my Joshatdot v1.03 CMoy's with BatchPCB. My board is about 1.5 in^2, which is $5 per board. So 12 would cost me $60 + Actual S&H + $2...and I think $10 "setup fee" I'll give them a try on my next CMoy order.

The problem for small boards is that the size is in 'whole' square inches, so in your case if the board is 1.5" x 1.5" = 2.25sq. in., you'd be charged for 3 sq. in. per board. If you want to get the most out of it, it requires planning of your board size, just like Olimex.

My last order was with them on Jan 4 and the boards haven't yet reached me. 'Shipping and handling' was $15.56, which includes the $10 setup fee. The quality is apparently good, and you get solder-side silkscreen too, but the turnaround time is awful. As far as I can tell from their order tracking, the board to be panelized hasn't even reached the US relay shipper yet, and it's still a week or two to get to me here in Canada. That's over a month!

Their online DRC and order submission is pretty nice though, at least compared to Olimex. You get an e-mail with DRC results when you upload a design, including images of how the board will be fabricated. Nice to verify that it looks right with their tools before you finalize the order. Everything is all stored online, including the board images, and you can go back and order more boards easily. I must say I think it's quite well done; many of the bigger board houses don't even do things like that, and this is a tiny hobbyist shop.

 

[tangent]

Quote:

Originally Posted by kipman725 /img/forum/go_quote.gif I have access to lazer printers but have no idea where to get hold of the chemicals needed for etching ... anyone any idea how I can get cheap PCB's? (they don't need to be high quality just usable).

I'd definitely have the boards professionally made for something like this. The end product won't look good if all the LEDs don't line up crisply. Even if you're so good in a machine shop you can drill all those holes accurately, the task will drive you batty. That's got to be worth a few pounds to you...

 

[Joshatdot]

error401 .. where does BatchPCB actually make them? I thought they were a US company. Makes me wonder why the long turn around.

I ordered my PCB's from Olimex Jan 8th, they were shipped the 15th, and arrived today.

 

[FallenAngel]

Quote:

Originally Posted by Joshatdot /img/forum/go_quote.gif error401 .. where does BatchPCB actually make them? I thought they were a US company. Makes me wonder why the long turn around. I ordered my PCB's from Olimex Jan 8th, they were shipped the 15th, and arrived today.

BatchPCB has them made in China by Gold Phoenix

 

[kipman725]

Hello Thanks for all the advice. My board looks to need to have two sides due to the number of crossing connections so I will not be able to produce it myself. Anyway I have been fiddling around with Eagle and have made scematics and a board layout. I will have undoubtedly made errors as I have never routed a board before. If anyone has any free time could they have a quick glance over the files to make sure I haven't made an embarasing mess?
board.jpg
schematic.jpg
disp.sch

*The holes are ment to be M3 sized.. I don't know if drill holes are mesured in radius or diameter
*I am unsure which layer comes out on the silk screen so I placed the text on the dimensions layer.

 

[tangent]

Can you regenerate the board graphic without the silk layers turned on, so we can study the traces more easily?

Consider using a more modern PIC, like the 16F628. They'll have more I/O per $. If you go high enough up in their line, you can probably avoid the need for the external EEPROM.

Also, check your current drive limits. A PIC can probably only drive a few of these LEDs at once, so you'll have to strobe them rapidly to give the illusion of constant illumination. Depending on what you want to do, you might need external drive.

On that same note, you should thicken up those LED drive traces. There's a nontrivial amount of current here...not exactly a "power" circuit, but...

 

[kipman725]

I aggree with you on the need for more available drive current for the LED's I have previously constructed simlar circuits but they had fewer LED's per coloum. The PIC output due to the display strobing only drives one LED at a time however the multiplexer has to sink the current of upto 7 LED's which it's not capable of. Therfore I will have to put in 8 transistors to help it out!

As for using another PIC, the current one has the the exact number of needed I/O and I am very familier with it. Looking at microchips website there are no PIC's with large enough EEPROMS as I want to display 24FPS animations which require at least 1536Bits/s of animation (hence the rather large eeprom). Also using cheap EEPROM's allows changing of the animation by just plugging a different EEPROM in

Anyway thanks for the comments I shall return with another itiration soon.*

*I may switch to a PIC with an internal oscilator as that will reduce costs and board space.

 

[error401]

Quote:

As for using another PIC, the current one has the the exact number of needed I/O and I am very familier with it. Looking at microchips website there are no PIC's with large enough EEPROMS as I want to display 24FPS animations which require at least 1536Bits/s of animation (hence the rather large eeprom). Also using cheap EEPROM's allows changing of the animation by just plugging a different EEPROM in

It's expensive and obsolete. If you replaced it with something like 16F628A, you'd gain a few I/Os, a lot of RAM, a couple timers, a USART and the internal oscillator you mention - and save a couple bucks too. It's the same package. Mind that it's been a while since I've used PIC (I'm an AVR convert now) and I recall some issues with the programming pinouts changing and old programmers not working on the newer ICs.

I would also suggest you add some power reservoir caps. Those little 0.1uF ones aren't going to handle 140mA switching currents very well. I can't tell if you're driving them common cathode or common anode, but I would recommend adding space for a larger bulk cap at the power input (at least a hundred uF). At least give yourself the option. I recall working on a digital clock project in college driving 6 7-segment displays and beating my head on the desk for 2 days before I realized I needed more decoupling. The bouncing ground was messing up the logic. You don't want to try and debug your code around issues like that :X That was on 4 breadboards connected with 24awg wire though, so maybe it's not that big a deal here

And I'll echo tangent's comment on the trace width.

Edit: And one more thing - it looks like you've put the current limit resistors at the end of each column. Since the PIC can potentially drive more than one LED in a column at once, the current (and thus brightness) will be shared (probably not very evenly due to Vf tolerances) between all the LEDs that are lit in that column. Probably not desirable. Since each PIC output will only ever drive one LED at once, it would be better to put the resistors there instead.

 

[tangent]

Quote:

Originally Posted by kipman725 /img/forum/go_quote.gif I want to display 24FPS animations which require at least 1536Bits/s of animation (hence the rather large eeprom). Also using cheap EEPROM's allows changing of the animation by just plugging a different EEPROM in

Well that changes everything. If you're going to be swapping chips to swap displays, you can put the display data in code space. You'd only have to use an external EEPROM if you needed to update it without changing chips. Instead of swapping the EEPROM, you'll swap the PIC.

Also on the PIC: if you can gain 5 I/O pins, you can also do without the decoder chip. You'll definitely need the external drive transistors then, but they were a good idea to begin with.

 

[kipman725]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Well that changes everything. If you're going to be swapping chips to swap displays, you can put the display data in code space. You'd only have to use an external EEPROM if you needed to update it without changing chips. Instead of swapping the EEPROM, you'll swap the PIC. Also on the PIC: if you can gain 5 I/O pins, you can also do without the decoder chip. You'll definitely need the external drive transistors then, but they were a good idea to begin with.

Tangent the EEPROM memory of the PIC is too small to store more than a second of animation. Each frame is 8x8bits (the last bit is used to signify if this is the end of the animation or not) and there are 24 of these 8x8 grids a second which means that the PIC's internal EEPROM would last less than a second of animation. Wheras the External eeprom is 64Kbits and therfore can store aprox 40S of animation. I have done a new design with the new pic but forgot the multiplexer has inverting outputs, looking on the net there dosn't apear to be a version of the multiplexer with non inverting outputs and I don't feel like scaling down the design so I will do anther design using a PIC with enough I/O to avoid the need for the multiplexer. All this drawing has made me better at using the scematic software though!