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I have been working on an Arduino based project for a while now and it's getting the point I'd like to make it more permanent/professional looking by building a custom PCB for it.  I'm not looking to make a shield, but rather I'd like to convert my project to a standalone board.   I know the next step is to breadboard everything out (no more arduino boards, but use a ATmega328 with Arduino Bootloader).

The project currently consists of two Uno's talking over a simple 315Mhz RF link.  One side collects temperature readings via some thermistors and the other has an LCD display, SD card & DS1307 for data logging.  I need to add some buttons for navigation and a buzzer/alarm on the receiver as well.

I'm learning Eagle and have even started creating custom parts that I'll need, but I could use some advice:

1. The thermistors, DS1307 & RF links run at 5V while the SD/LCD are 3.3V.  I'm trying to figure out the most cost effective/simplest way to generate both voltages off a pair of AA batteries.   

I know I can use an NCP1400 to step up to 5V... should I split that output to a 3.3V regulator?  Seems like I'd be wasting a fair bit of power stepping up, only to step back down.

Perhaps the best way is just use 4xAA's and independent 3.3V & 5V regulators?  Prolly would work as long as I don't use rechargeable batteries right?

2. How can I calculate the power draw of a thermistor?  Yeah, I'd read the spec sheet, but there isn't one.

3. Any suggestions for references for de-Arduino'ing a project?  I know I'll need to build a regulated power circuit and provide a crystal for timing.  Makes sense to add a FTDI programming header too.  Any other gotchas or things to think about?

Thanks!
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Put your atmega on a socket, or socket strip, then plug it back into your Uno for programming.
http://www.dipmicro.com/store/ICS328
http://www.dipmicro.com/store/HDR40X1FM
This place also has 16MHz crystal, 22pF caps, 100nFcaps, 10K pullup resistor for the reset pin.
All the pieces on the lower left are the 'arduino' bits on a little board I made up.
I have a 5V walwart for power, so no regulator is included, just a 100uF cap on the 2 pins where power comes in.

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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Thanks for the tip.  I was aware I could always swap Atmega chips, but frankly that seems like more work then it's worth each time I want to update the code- shouldn't be that often, but I guess I'm at the point in this project that if it's worth doing, then it's worth over doing!  I'm actually hoping to design my own PCB's (one for the receiver & transmitter) and either etch it myself or have them done via something like BatchPCB.  That's a little bit farther down the road I guess though... I still need to figure out a good way to power both ends since I don't want to use a wall wart.
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If you don't want to swap, then put in a 6-pin header like the promini uses to connect something like an FTDI-Basic to, and be sure to keep Rx/Tx free, or add a switch to disconnect the lines, so you can program freely.
http://www.gravitech.us/arliandpropr.html  best price around, mine has been used many, many times since August.

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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Anyone with suggestions on cost effective ways to power a project like this?  One idea I had was using a MAX756 setup like MintyBoost to generate 5V off 2-3AA's and then a 5V->3.3V step down regulator like the LM3901 for the SD/LCD, but I'm looking at over $10 in parts which seems a bit much (and that's not even using tentalum caps).

Sparkfun has a $6 NCP1400 5V step up breakout board which would shave off a about $3, but I'm a bit hesitant to use a breakout board for limited gain (I'm hoping to open source the project hardware & software so other people can build it and you never know when Sparkfun will discontinue a part).  Trying to solder such a small SMD part like the NCP1400 seems way over my abilities (or the average hobbyist).

Still trying to figure out how to determine the power draw of a thermistor so I can run some calculations on my power requirements.  I suppose I could just "wing it", but I figured that it wouldn't be too hard to do it right.
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