pcb layout question - Page 1
Forum section: electronics / beginners

author: whitevamp
author_rank: Regular Contributor
timestamp: November 02, 2017, 03:04:09 am
content: sorry if this is the wrong thread.ok so i have made a led chaser circuit and have decided to get get it manufactured, and this is the first time of doing a board layout and getting one manufactured.so i thought i would post in and have everyone createk it.now this is just the power supply and logic board section.the circuit is going to drive the 7 transistors to drive a total of 87 LED'S.the traces are 25thou and the power trace is 30thou.and i have not added in the ground plane yet.so are the trace width's ok or should they be bigger, from my calcs they should be ok, but im not sure.and how is the lay out.and a question about screw holes to make them you just make a through hole for the size of screw you are going to use.in the pic i placed a pic of a screw next to where the mounting holes are going to be at.is this the right way of doing it?

author: mariush
author_rank: Super Contributor
timestamp: November 02, 2017, 03:48:00 am
content: You have so much room on the circuit, it would be a shame not to make the traces thicker.why are you bringing the J3 header so far down? Looks like it could easily be placed somewhere near the fuse, above the 7 resistors?Also, what's the deal with all those 1n4001 diodes? Why go with diodes that can only handle up to 50v, when it costs the same to use diodes rated for higher voltage?Or better yet, looks like your circuit could use components with two diodes in them, with common cathode or common anode... for exampleBAV23AorBAV23C(200v 400mA) or BAV70 / BAW56 if lower currents are ok.Wow the board is so confusing... at a brief look it would seem to me like it would make more sense to move U2 and the other IC below the 7 resistors (which by the way could so easily be replaced with a resistor network)Could you post the schematic and / or explain what you're actually trying to do with 87 leds and so many diodes?

author: whitevamp
author_rank: Regular Contributor
timestamp: November 02, 2017, 06:41:02 am
content: Quote from: mariush on November 02, 2017, 03:48:00 amwill increase the trace width.I placed it there at the time made the most since because it was in the middle of the two section of where the power trace is going.and yes now that you mention of putting it near the fuse would also make since.Quote from: mariush on November 02, 2017, 03:48:00 ami was thinking that 50v diodes would be adequate. and its what i had on hand.Quote from: mariush on November 02, 2017, 03:48:00 amnever thought about using those type of diodes.Quote from: mariush on November 02, 2017, 03:48:00 amwill be moving those around.and never thought about a resistor network. will be changing to that, instead.and its what i had on hand, instead of thinking about a resistor network.Quote from: mariush on November 02, 2017, 03:48:00 amwhat im doing is writing out a name in leds, 3mm leds and for each letter and that section lights up then it moves to next letter and lights it up. and its a 7 letter name.and each letter has 12-13 leds in it with 2 leds in series. and a 220R resistor for the pair of leds. and a 440R for the single led.so it would goA ( gets light.)then as it goes offBcomes on.then as it goes offCcomes on.then as it goes offDcomes on.then as it goes offEETC: till all 7 letters are done then it starts all back over.and the speed of the the LED'S turning on and off is controlled by R8just came across this on youtube and its what im doing.

author: MarkF
author_rank: Super Contributor
timestamp: November 02, 2017, 07:59:37 am
content: Quote from: mariush on November 02, 2017, 03:48:00 amI would suggest cleaning up the part layout and keeping the traces as direct and clean as you can.Something like....

author: MarkF
author_rank: Super Contributor
timestamp: November 03, 2017, 08:51:44 am
content: Possible simplification using a grid with vertical diodes placed at the corners to do the logic???

author: whitevamp
author_rank: Regular Contributor
timestamp: November 06, 2017, 08:45:00 pm
content: sorry about the delayed reply back.so I have taken and made 2 different versions of the board lay out and the one i think is #2 its the shortest trace runs and the neatest, #1 has longer runs, and more straighter runs.and i would take and use the suggestions that was given of using a resistor network, and diode pair packages, but i all ready have these parts on hand, and $$ is tight.now while doing this i also came up with a question and that is.shorter and straighter traces are better, but is it better to have a bit longer trace verses a via, or is the via a better way to go. in turns of resistance, and impedance.and by a bit longer i mean going around a trace verses going a placing a via and cross a trace.i hope i explained that ok.

author: Kilo Tango
author_rank: Regular Contributor
timestamp: November 06, 2017, 09:58:59 pm
content: A couple of ideas :You have put in a lot of work into this, have you used a PCB design prog ?. 30 thou is a bit small, I use tracks as big as I can. Trouble is if the power feeds all the LED’s first, then goes to the logic chip, any volt drop due to the LED’s switching will affect the chip's power rails, and in a bad case can cause it to randomly reset. In fact I always use a ground fill, this fills all unused areas with copper at 0V so keeps everything close to ground which keeps it stable. Also less copper to etch away so less pollution.Looking at the bottom layout : You are going to have to use a thin track between the central IC pins 10,11 (?), nice to get it outside the IC, means it can be thicker and less prone to damage. I like to keep traces as thick as possible to avoid chance of a pad or track being lifted due to heat when soldering. You have two red tracks next to the RHS of D19, they are thin and quite close together. If you move the top one up and pull the bend on the lower one down you can double track widths. Layout, as a first step why not move the output connector to the edge of the board it will be easier to get to.Looking at the circuit, all the diodes seem to be purely signal routing, feeding the signals from the counter to the transistors, so 1N4001's is definate overkill. You could use basically any diode here, 1N4148 or similar would work fine. In fact........Has anyone figured out what diodes D12, D13, D14, D15, D16, D17, D18 and D19 are doing ?. If its an attempt to reset the counter when it fills up, shouldn't a wire go to the reset pin ?.Also on the circuit diagram D4 and D8 cathodes connect together but don't go anywhere. Should they go to R4 instead of R4 going to R3/D3/D7 ?I think getting good at PCB layout comes with practise, and having to build what you design. I had several goes using a Daylo pen and baths of ferric chloride and over etching boards and having to patch them up with jumper leads !.Ken

author: MarkF
author_rank: Super Contributor
timestamp: November 07, 2017, 07:46:08 am
content: Layout guidelinesLaying out the circuit into functional groups will minimize trace crossings and crosstalk. You have four main functions: logic, output drive, oscillator, and power.I try to keep power and ground on their own layer. Allows for copper pours. Usually power on top and ground on bottom.You will probably want to add decoupling capacitors.Try to minimize layer changes for traces.I laid out a second example to illustrate the four groups. This should minimize a lot of your trace overlap. I leave the logic area for you to do. Also, think of each transistor and it's base resistor as a pair and keep them together as a unit.Out of your last two layouts, I think #1 is better because you don't have the oscillator and counter mixed right in the middle of your logic. It may be better to move the counter into the logic area if it reduces trace routing. You may want to divide the logic into: the counter, rest logic and drive logic areas.Edit:On layout #1 some of the traces are far too close together. Since the DIP pins are on 100 mil centers and you have the space, I would maintain a 50 mil minimum spacing for your traces. This will allow one trace to run between the pins with a 15 mil trace width.

author: whitevamp
author_rank: Regular Contributor
timestamp: November 08, 2017, 12:35:25 am
content: Quote from: Kilo Tango on November 06, 2017, 09:58:59 pmyes I have used an IDE MultiSIM BLUE from Mouser.thanks for the suggestions.and yes since you mentioned it i went back and started looking and yes i had a couple nets missing from where they should have been. thanks for spotting it.

author: whitevamp
author_rank: Regular Contributor
timestamp: November 08, 2017, 12:40:23 am
content: ok with all the great suggestions this is what I have came up with.and i have not added in the ground plane yet on the bottom layer.

author: MarkF
author_rank: Super Contributor
timestamp: November 08, 2017, 01:28:38 am
content: You should re-examine the configuration for the LM555 Timer. I don't recognize it as the Astable Operation configuration. It 's not going to provide a clock for the counter.UPDATE:Looking over the logic, there are several diodes that either don't go anywhere or are in parallel with another. Is there an error in the logic or are there unused diodes and diodes duplicating functions here? See figure with crossed out diodes in question.

author: whitevamp
author_rank: Regular Contributor
timestamp: November 16, 2017, 04:19:10 pm
content: sorry about the late reply back.I am going to have to put this project on the back burner, as my main hard drive, and back up drive to a dump,and other issues have arose.and i would like to say thanks to every one who gave me suggestions on this project.