Laying out my own PCB - what 'options' are useful?

[JellyBeanDriver]

I'm going to lay out my own MPGuino board but was wondering if anyone had any improvements that they've needed to make that I should consider in my board?

- conditioning of the VSS or injector signal (selectable pullup/pulldowns, feed through a comparator, etc)
- input power conditioning
- USB port like what FL used to include
anything else?


Anyways I thought I'd solicit input from you all on any things I should consider adding/improving on the design.

TIA,
Dave

[JellyBeanDriver]

Schematic is done for just the 'plain ol vanilla' version. Will try for a single sided board this weekend so I can etch it myself.

[mpgx2]

Hi,

A really handy additional option would be an incline meter
to know when you 'should' coast down a hill. For manual
or stick shift of course.

btw, how does one make their own PCB? I really want to
know. I found freepcb and tinycad. to make gerber files.

But what is the best way?

[dcb]

I like using a sharpie or a laser printer for individual pcbs, but double sided has to be lined up just right and all need lots of drilling/etc. So I usually use a prototype construction method, a-la deadbug or veloboard If you are just making a couple, that is the quickest/most cost effective route, or start with an arduino.

the veloboard version actually had a connector added for a bootloader controlled reflash/comm using rs232 or ftdi cable.

[JellyBeanDriver]

I do the layout on the PC, laserprint onto vellum and use that as a mask to expose onto a pre-sensitized PCB. Then develop, etch and drill. I've done 7mil line/clearances. Pretty handy to be able to whip out a prototype that way. For my 2 sided boards I either use 4PCB.com $33 special in the US and for large quantities an offshore board house.

I don't think an inclinometer would work well as roads themselves undulate and there would be so much measurement noise from the josling car. The instantaneous MPG meter and your speedometer together should tell you when you're coasting, no?

Anyways, think I'll include a comparator/buffer on the VSS and injector input so if needed I can add some hysteresis.

[dcb]

re metro vss, thecheese429 JUST got a gear sensor working for a vss, you might look in that direction if the reed switch proves challenging. there are existing buffers/tweaks for noisy reed switches, so not sure you want to "fix" that till you see if it is broken.

here is the solid state vss thread:
http://ecomodder.com/forum/showthrea...tml#post178770

[JellyBeanDriver]

Thanks but I'll be doing a PCB and might as well have it in there though I may not stuff the comparator. Thought about what I'd do if I find my VSS signal is flakey - might put a hall effect in the speedo if the reed output is totally unuseable.

[JellyBeanDriver]

Decided to just do SOMETHING. Can always make another board down the road.
This version is just plain vanilla. The only difference is I brought out the RS232 lines.
Hope to have this wired in next weekend - work gets in the way of having fun.

Here's the etched board.



Here's the partially stuffed board installed on the LCD. Some SMT stuff on the back for convenience. Regulator is at the bottom if I decide to bolt it to a case to heat dissipation. Should be enough room for a small heatsink.



Front side view - the rear board is the same outline as the LCD.



I was going to use a slimmer LCD (has 2x7 terminals on the side instead) but I'm pretty sure it's an older one that takes 150mA just for the backlighting. This one is < 20mA.

[mpgx2]

Yeah, its really cool.

Exactly what I'm after. How do you do pcb's like that?

I'm just a computer guy and have no idea. Do tell...

[JellyBeanDriver]

- PCB software to do the schematic which then drives the layout of the board. There's some free ones out there.
- Laserprint artwork onto vellum to be used as a photopositive
- cut pre-sensitized PCB blank to size, remove the plastic film, put the artwork on top, sandwich it in a frame and expose the board through the artwork using a fluorescent lamp. Boards are pretty cheap. I bought 6" x 10" for I think $6 a board years ago that I'm still using.
- develop the board
- etch the unwanted copper off
- drill, then stuff.

Exposure has to be experimentally developed. My time is 14.5 mins.
Develop is ~ 5 seconds, or until emulsion clears fully
Etch is ~ 8 mins

Drilling is the biggest problem if you don't have the carbide bits on hand already.

Note that laserprinters are not exact in X and Y so long dimensions can walk off on you. My HP4100 is pretty good, my HP4+ I had before was worse.