General PCB Check

Hi, before posting any schematics, I just wanted to test the water. I have been working on a design for a set of PCBs to build into a 3d printed housing I've designed, which hopefully with 3 teensy arduinos installed will work as a functioning dashboard for a project car that I'm building (though it is still in bits).

I would like to have the PCBs prototyped in China, so I can then power them up on the bench and start looking at making a working model. I've tested most of the bits on a breadboard individually, but this is the first time I'll put it all together.

Since the prototyping is a reasonable investment, I really don't want to screw it up. I've spent a lot of time checking and rechecking and redesigning where necessary and am pretty much at the point where I'm ready to send off for a quote. Since I'm relatively new to all this though, I'd really like someone with some experience to check over the schematic and maybe PCBs and make sure I haven't made any silly errors but I don't know anyone who could help me personally.

So my question is, would anyone on this forum be willing to help? It's a bit more than a question on the forum so I would be prepared to compensate for time spent within reason. I'm a mechanical engineer by trade so I'd expect the same if someone was asking me a similar question!

Thanks!

My experience with China was great. I used JCLPCB, everything is on line:
PCB Prototype & PCB Fabrication Manufacturer - JLCPCB I had very good results. I did my schematic capture with KiCad operating on Linux Mint 18.3, I think it is available for windows and Mac. This software is available free but it is worth donating something it is that good. I have done all my routing manually with this, it works very well. You will find there pricing very competitive. When you upload your Gerber files they will do a check with system and then again manually. For me from uploading until I was assembling boards was in the range of 7 days. If this is your first experience with a PCB see if you can find a buddy to help you with the CAD. It is like a new language. It will run a DRC (Design Rules Check) for you. They do not have a limit on board size or layers. The board matched my drawing exactly, including the mistooks. When they checked they found some and notified me about them.

Remember when you get your board back FAIL is an acronym meaning Forward Action In Life.
Good Luck & Have Fun!
Gil

I use DirtyPCBs for most of my PCB fab work. Dirt cheap, and they do good quality work. They do not DRC check it for you. If you submit something that doesn't meet the DRC, you generally just get back bad boards.

I have also used PCBWay - they are full-featured unlike DirtyPCBs, but they cost more.

@450Nick,
What program did you design the boards in?
I use Eagle and could help with the Design Rule Checks to make sure that someone like iteadstudio could make successful boards for you.
You don't really send off for a quote, all the sites have online tools where you enter the board size and they tell you how much bare PCBs will be. Such as $19.90 for ten boards up to 100mm x100mm. Then shipping is whatever you select.

Hi thanks for the replies guys... Bob I've just dropped you a PM. I'm using Autodesk Eagle, and then the board overall assembly integration in Autodesk Inventor.

As my soldering skills are more on the automotive wiring level rather than small PCBs, I think I would like to have the board delivered fully assembled. For this reason I am trying to be very careful to specify the BOM clearly (the board includes 4 servos that aren't too common) and have had to make a couple of components myself in Eagle. There's a lot of stuff across 4 separate PCBs (two of them double sided) with board to board connectors between.

I've run the ERC and DRCs in Eagle and everything seems to check out, but little things like ensuring I have specified reasonable capacities for my components given an automotive environment, making sure the trace widths are sufficient for the loads etc. I have checked everything, but working in engineering I am well aware that things get missed even when you're not a novice - especially when you've been looking at it for a lot of time, so a sense check look over would be greatly appreciated, and I don't think that the PCB manufactures would necessarily do this? I know they'll check the BOM for availability, and will test the board for continuity and validate it against the schematic, but I don't think they'll sense check the schematic.

The last thing I want is to order them, plug them in and generate some kind of small explosion :slight_smile:

At the moment I'm talking with RayPCB (https://www.raypcb.com/) and they seem decent, but their English is not perfect (though it is pretty good) so I am a little worried about misinterpretations... Any good suggestions for some more quotes?

If you post your layouts and schematics (as images) we can critique them.

This sounds like quite a project, so I definitely recommend trying to get people to sense check your design.

Fully-assembled is an exceptionally expensive way to buy prototype PCBs. Something that takes an hour or two for an amateur to assemble will take a professional longer because they really have to check it and test it before it goes out the door.

Even in China, those professionals don’t work for $5 per hour.

If this is your first PCB then you will certainly make a simple mistake. Do you want to spend hours on the phone with the assembler or do you want the $4 PCB in your hand so you can look at it and see you forgot one power wire?

I got into PCB design because I had a project with one particular component that was only available in surface-mount. (The ADXL345 accelerometer.) With a “laser” infrared thermometer and a frypan on the stove I was able to build that project on a PCB with very low cost in dollars or hours. The only delay was waiting 2 weeks for the version 2 PCB with the power trace I forgot in version 1.

DrAzzy:
If you post your layouts and schematics (as images) we can critique them.

This sounds like quite a project, so I definitely recommend trying to get people to sense check your design.

I tried to attach photos but maybe as there's too many it kept crashing the browser so I just put them on my OneDrive. There's also a zip file with the Eagle files and images in there too if anyone is interested...

OneDrive Link

MorganS:
Fully-assembled is an exceptionally expensive way to buy prototype PCBs. Something that takes an hour or two for an amateur to assemble will take a professional longer because they really have to check it and test it before it goes out the door.

Even in China, those professionals don't work for $5 per hour.

If this is your first PCB then you will certainly make a simple mistake. Do you want to spend hours on the phone with the assembler or do you want the $4 PCB in your hand so you can look at it and see you forgot one power wire?

I got into PCB design because I had a project with one particular component that was only available in surface-mount. (The ADXL345 accelerometer.) With a "laser" infrared thermometer and a frypan on the stove I was able to build that project on a PCB with very low cost in dollars or hours. The only delay was waiting 2 weeks for the version 2 PCB with the power trace I forgot in version 1.

You are probably right, but I'd be much more comfortable someone putting this together who has some experience. The trick is to not screw up the schematic as you say, hence hopefully checking it many times before pulling the pin...!

That's a good point - I would really want to get a batch of bare boards and assemble them the first go around, because you will do something wrong. It is rare for the first rev ("Rev. -" in the industry, though some start from Rev. A) of a board to work 100%.

Ok I tried a few more times and figured out that the forum doesn't accept .png images so here are the .jpeg versions. I would very much appreciate a bit of a peer review of the schematic if anyone has a few spare moments. The Eagle files are available on this link: OneDrive Eagle Files

And rear PCB jpeg

Nobody? :frowning:

There isn't much we can say about the schematic without diving deep into the datasheets of each component. The only advice we could give quickly is artistic and it looks reasonably neat, given the number of nets you have.

The real question for a schematic is "Does it do what you want it to do?" Does it include all the correct control lines? Does it use Arduino PWM pins for analog outputs (I have forgotten that one a few times.) Do you have Tx and Rx on Serial the right way around? We can't answer those questions for you.

When you get to the actual PCB, then you will get more advice from the forum. Things along the lines of "Is that power trace wide enough for the current it's carrying? Does that high current trace go under any sensitive analog components? Where is the return current from that component flowing in the ground plane and does that go under any sensitive components?"

Hi Morgan,

Thanks for your response! The PCBs and all Eagle files are on the link attached. 4 layer boards with ground planes and I think trace widths that work for the service I'm expecting.

For the schematic review, I'm really just hoping for someone with a trained eye to look over the boards and say, "this looks sensible". I have checked all nets, routes and connections and I am happy that everything is plugged in where it should be - the circuit isn't hugely complicated, there are just a lot of connections to route. I'm mostly worried things like resistor and capacitor values, particularly the ones interfacing with the I/O signals from the main car harness connector as I know auto environments are harsh electrically. I'm hoping someone can look at it and verify there are no silly errors to give me a little confidence to go ahead and order the boards Rev A.

As said above, I'm very happy to pay someone for their time to make a proper check if it will take more than a few minutes so that I can get a decent peer review on both schematic and board before proceeding.

450nick:
Nobody? :frowning:

Quick glance - seems to be a complete lack of decoupling caps?

Correct, this was one of the things I wasn't sure of. There are several types of inputs I have here:

  • 12v analogue; these I've run through quite high division voltage dividers with the assumption being that the division is enough to counter voltage spikes

  • 5v analogue; these will take a 5v reference from the self contained 5v power supply on the PCB and return a 5v voltage - since this is a closed loop, my thought was that this would only need a resistor.

  • 12v digital; these are all run through opto-couplers to decouple them fro the arduinos

  • 5v digital, I think there is only 1 and it just switches the arduino pin to ground

  • power; the 12v feed comes from a motorsport PDU system (this one) which provides reverse voltage protection for all circuits, and you can set clamps on each channel to limit current (not sure about voltage, need to check) so there is an element of protection upstream of this unit. The 5 volt feed uses one of these, and I think 2A should be plenty - it has caps built into it so the datasheet claims no external ones are needed.

So presently this is how I've set it up, but I'm definitely not an expert so if you think I should have some caps in there, please do say and I'd love to hear your comments.

Always consider wires entering your box as untrustworthy. Even if the upstream device has current limits, that does not prevent that wire from being dragged across the battery terminals during maintenance.

Usually the voltage divider on 12V inputs is sufficient for most automotive wiring screwups. But static discharges are thousands of volts. ESD protection via TVS diodes is pretty easy these days.

Look up "load dump". That is the most violent electrical event that can occur in a vehicle.

IMO, the 10k series led resistors for the PC817’s are marginal. Given the current transfer ratio of those parts, I’d consider lower value series resistors for better noise immunity. They may work on the bench but add some noise and a higher ambient and you may have issues.

I’d also be cautious of relying on the internal pull up resistors. They are relatively high values and not the best for noise immunity. In addition, there is the question of minimum load requirements when you’re replacing filament lamps with solid state inputs. Lots of unknowns there, depending upon the age of the vehicle.

MorganS:
Always consider wires entering your box as untrustworthy. Even if the upstream device has current limits, that does not prevent that wire from being dragged across the battery terminals during maintenance.

Usually the voltage divider on 12V inputs is sufficient for most automotive wiring screwups. But static discharges are thousands of volts. ESD protection via TVS diodes is pretty easy these days.

Look up "load dump". That is the most violent electrical event that can occur in a vehicle.

Thanks for the input, would you recommend a TVS diode on all inputs then to be sure? I will read up but is there a particularly good PN you'd recommend for this application?

WattsThat:
IMO, the 10k series led resistors for the PC817’s are marginal. Given the current transfer ratio of those parts, I’d consider lower value series resistors for better noise immunity. They may work on the bench but add some noise and a higher ambient and you may have issues.

I’d also be cautious of relying on the internal pull up resistors. They are relatively high values and not the best for noise immunity. In addition, there is the question of minimum load requirements when you’re replacing filament lamps with solid state inputs. Lots of unknowns there, depending upon the age of the vehicle.

Thanks for your input too! Having read the datasheet, I've now changed the values of the input resistor to 470 ohms - the PC817 can handle max forward current of 1A and min of 50mA so 470 should do the job I think...

When you mention not relying on the internal pullup resistors, what other method would you recommend?

With regard to the minimum load piece, the wiring loom is going to be completely replaced so nothing will be existing. The power distribution will come from the units mentioned above which will replace all relays and fuses, and the engine/alternator will be a 2019 Chevrolet LT4 motor so very new design and hopefully modern levels of electrical noise if such a thing exists...