Arduino+Breadboard to custom PCB project

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?

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

  2. 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!

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.

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.

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.

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.