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The pulldown would ensure it doesn't start on while the outputs are high impedance, once they are set as outputs and either high or low it doesn't matter, you could probably get away without them but thereon almost always there
how much draw on the 5v line are you using? Dropping ffrom 12v to 5v is basically wasting more than half the power as heat with a linear regulator

the 5v line draw is minimal - if i remove the status LED's (or put them on the emitter side of the transistors) - the draw would be the Arduino, DS1307 & MCP23008 + the current draw on the transistor gates =  i am guessing it is not using very much power but its not sipping power either... also there are a ton of pulldown/pullup resistors on the pins.   The draw on the opamp is < 10ma. I was going to limit the LED draw to 140ma.

I have pretty much -0- experience with power supplies - i have always used a 9 or 12 v wallwart with LM7805 .... having said that, my prototype circuit is using the L4931 regulator in a TO-92 (!) package - it is running a bit hot.      I was planning on switching out the L4931 in the RBBB i am using for the prototype with a LM7805 but have not gotten around to doing it (desoldering is my least favorite activity)

If you have any power supply recommendations to improve the circuit, i would appreciate!

regards,
david
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How about one of these little assemblies for 12V to 5v conversion?

http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/2259781
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Those seem perfect for your situation,  simple replacement for twice the efficiency atleast and minimal heat,
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I ordered the pcb from seeeed on Sunday! Will update with pics in due time.

How about one of these little assemblies for 12V to 5v conversion?

http://www.digikey.com/product-detail/en/OKI-78SR-5%2F1.5-W36-C/811-2196-5-ND/225978


This will be going into my next mouser order as well smiley

Thanks everyone for the feedback.
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Something I noticed was the traces coming off the row of transistors up the top. If the through hole pads are on both sides, the trace from the middle pad gets VERY close to the right pad on some of them. Some of the other clearances look a bit tight too.

You can set up EAGLE to run DRC to check for minimum clearances etc, might be worth checking the specs of the fab house you're going to use and check your layout with those values.


Good luck!
« Last Edit: June 14, 2012, 07:30:47 am by squarebear » Logged

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Something I noticed was the traces coming off the row of transistors up the top. If the through hole pads are on both sides, the trace from the middle pad gets VERY close to the right pad on some of them. Some of the other clearances look a bit tight too.

You can set up EAGLE to run DRC to check for minimum clearances etc, might be worth checking the specs of the fab house you're going to use and check your layout with those values.


Good luck!

Hi Squarebear - i see exactly what you are talking about.   Firstly, it passed DRC with 8mil clearance and the fabhouse DRC file has 6mil clearance.   It passes DRC until i change the clearance to > 10mil.     It does look VERY close though...  If I hide the bottom layer, you can more clearly see the space between the top trace on the middle pad and the pad to the right of it. As it turns out it looks like i fixed 2 of the traces on the file I submitted to the fabhouse, but one is still there like in the last picture I posted....   We will see what happens... this is my first fabricated PCB so i budgeted for one round of mistakes  smiley-eek-blue

regards,

david
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Hey Folks - thought i would update - PCB arrived from Seeed and i finished populating and wrote some test programs over the weekend.  Here are some pics.

I ended up with 3 stuipid mistakes but they were all fixable - 1) a missing capacitor i had on my hand drawn schematic but not on eagle, 2) a pin was connected to GND that s/b connected to VCC (same cause as item 1) and 3) one of the power trace widths was incorrect.

Thanks everyone for your help it made my first PCB order a successful one.



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Nice!
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Very nice, love seeing the finished project
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