Open source PCB design

[dcb]

Need is a strong word, and it is a little specious to suggest the failure mode of andrews LCD AND to go from "person" to "people", but nonetheless...

On the one hand, there are plenty of examples of 7805's in automotive service in the homebrew/diy niche, where it is deemed "good enough" with a couple capacitors on it. With the standard LCD we are barely at 1/5 the capacity of the regulator.

On the other hand, there are a couple examples where they go to great lengths to protect the voltage regulator, the megasquirt comes to mind, but its schematic seems to be something of a paradox in itself.

Tell you what, Sourcing a 30 volt zener isn't terribly bad, it adds a component, but I cannot believe that there isn't a common 5 volt voltage regulator in existence designed for automotive use. Can somebody mouse around and see if the 7805 should be obsolete for automotive use and replaced with something better?

[dcb]

One power conditioning design I did see that made a lot of sense to me was a large capacitor on the input and positive was connected to automotive power "in case the starter tries to drain the capacitor" For some reason I have a real hard time putting a voltage regulator (zener) on a voltage regulator, but a large capacitor should smooth things out and reduce/eliminate brownouts, which the atmega definately doesn't like, especially with a diode there. Thoughts? Is that a reasonable compromise between DIY "good enough" and "clean room" mentality and complexity? Which would also add a fair degree of reliability to the power supply at the same time?

edit: updated schematic/board in post 9. Also note futurelec sems to have very reasonable prices for no-frills prototype boards, especially single sided. Any others? I have zero experience here.

Also what about buttons? I wind up custom fabbing them to keep the front profile to a minimum, but what are the options there? Also I like pressing down on them since it doen't test the strength of my velcro so much.

[MilesPerTank]

Originally Posted by dcb

With the standard LCD we are barely at 1/5 the capacity of the regulator.

At ambient room temperatures maybe. Our end goal has this device powered by a 13-14V alternator inside a hot car. 14V * 200mA is 2.8W (LDO). We only use 5V * 200mA = 1W . That's 1.8W burned in the regulator.

The way I have mine mounted is non heat sinked with no airflow. I admit I've got it in a project box pretty sealed up which isn't the best idea. Its 100 degrees ambient this week (~36C , North Carolina) not counting any solar loading.

http://www.fairchildsemi.com/ds/LM/LM7805.pdf

Max junction is 125C with a RthetaJA of 65 C/W. Using the above 1.8W*65C/W = 117 C. You can see if our 200mA is correct we are already in violation of the max temp even under a 25C air conditioned lab when using the cars voltage range. Anyone got some good temperature probes at home to mount to one of these? I'll take the board into work tomorrow and try to get some load current measurements.

I plan to mount a nice sized power resistor in front of the 7805. It will help current limit and burn some of the initial 14V .

[dcb]

I'd suggest a metal project box that would double as a heatsink if you are going to box it. You shouldn't have any problems with 200ma then. FWIW, my open air installations haven't complained yet about the heat, I haven't even noticed the regulator getting warm.

We do need to wait on the autopsy report on Andrews before deciding on a course of action there. I really want to keep the final product as simple and inexpensive as possible.

Edit: I should also mention that it isn't *really* necessary to run the backlight at 200ma, I run it on its lowest setting day and night without a problem. I turned down the brightness values a bit in the sourcecode, that is far easier than changing the circuit.

[dcb]

Here is a proto-prototype for the pcb version (I needed another guino for testing anyway). It was a PITA, took some planning, but it is loosely based on the pcb planform above and uses the same schematic. Playing with the layout in eagle helped out a bit.



For some reason this lcd isn't turning off all the way, no biggie. But need to figure out why.

The onesided preprogrammed version currently only has one airwire, and needs some tweaking yet. But I just checked futurelec and their price per board just took a big jump?!? 10 single sided boards would be $39 shipped. And that is with doing your own custom buttons. Where are we with the buttons anyway? I've gotten pretty good at fabbing them and tucking them into the LCD border, but that doesn't make it the best solution.

Also don't know about programming headers or onboard usb/rs232 chips. I'm comfortable using my existing freeduino to program a "headless" guino chip and pop it in to the socket, especially if the software gets pretty stable. I could crank out quite a few of them for friends and family who would never have need for either the usb chip or a programming header. Is assuming the builder has an arduino at their disposal to program the chip setting the bar to high?

[ConnClark]

Originally Posted by MilesPerTank

http://www.fairchildsemi.com/ds/LM/LM7805.pdf

Max junction is 125C with a RthetaJA of 65 C/W. Using the above 1.8W*65C/W = 117 C. You can see if our 200mA is correct we are already in violation of the max temp even under a 25C air conditioned lab when using the cars voltage range. Anyone got some good temperature probes at home to mount to one of these? I'll take the board into work tomorrow and try to get some load current measurements.

I plan to mount a nice sized power resistor in front of the 7805. It will help current limit and burn some of the initial 14V .

We are supposed to be saving energy here ;-)

Here is a Switching 7805 drop in replacement project
http://www.narwani.net/neil/tech/elec/78SWxx/index.html

Also for an OpenSource layout and schematic program look at kicad. It supports Linux/Unix and windows.

[jmonroe]

When a PCB design gets finalized, is anyone planning on putting together a kit with the board, components, and LCD un-assembled in a bag and selling that? I would be interested in at least one of those.

I am sure there would be room in that to make profit.

[dcb]

I can look into it, but need to understand the intended audience better, so let me get some feedback here:

The first run as it stands would be:
The above schematic, zeners, big old capacitor power supply, atmega with a bootloader and the then current version of the code, header pin interface with the car with a small female plug you solder your wires too, LCD, IC socket so you can update the software, single sided PCB, etc.

Not included:
Exact instructions for your vehicle
fuses for power lines
fancy wiretaps, you strip and solder to your own wires or come up with something.
A chip programmer or header, use an existing DIP duino to update the software and swap in the updated chip, or come up with something.
Warranty, or any liability on my part.
Solder, tools, skill, nerve.

Rough guess would be $25ish +- 20% depending on quantity and using the current parts set. Might be a month before the software is really ready for pre-loading.

If you are interested let me know, I've not done this before so be forewarned

[SVOboy]

I'm DIYing it, but if you're interested, dcb, I'm sure the site would be willing to help make it a reality so that the project is more accesibly for those who need it.

[dcb]

FYI, added eagle files to the repository. There's no onboard programmer or programming adapter due to lack of standards. Use an existing duino to program the chip.

Revision 51: /trunk/mpguino