Thanks for the suggestions! I’m thinking of making my software a bit more modular to allow for a HD44780 display in 4-bit mode connected to the 6 AUX pins on my schematic. I’m also thinking of having a jumper so the user can choose to between a thermocouple with the MAX6675 or a thermistor, which I’ll probably put on ADC6. These changes could really lower the overall cost, but still allow for flexibility.

The internal oscillator idea is interesting… I use a bus pirate with stk500 emulation for programming, so I could always set the fuses to use the internal osc. I’m not doing much in the way of critical timing in my code, so the accuracy shouldn’t make a difference. Looks like I would have to make some changes to my code and use a different bootloader, as the default Arduino environment and bootloader assume a 8mhz frequency.

I’m also working on getting a PCB routed for this project, I’ll post some updated info once I have the design finalized. If all goes well, I’ll probably post the board on BatchPCB marketplace and/or post the Eagle project here once I’ve gotten a couple of test boards fabbed.

1. HD44780-compatible parallel LCDs are available just about everywhere, starting at about $7/1 for a 16×2 character module and possibly cheaper if you can find them surplus. These require 6 or 7 GPIOs for control in 4-bit mode. See the LCD4BitLibrary for Processing code.

2. As long as you have a free analog input, you could save some money and hassle by using a $1-$2 thermistor as a temperature sensor. Some are rated up to 150°C, which ought to be plenty for a boiler. You can use the Thermistor4 component library to convert from raw ADC values to temperature with a tunable tradeoff for accuracy vs. calculation cycles. Pluses: the voltage swing is as wide as you like, you don’t have to worry about acquiring or dealing with an external SOIC if you don’t want to. Minuses: uncalibrated thermistors aren’t terribly accurate, can be slow to arrive at their final reading (some have a thermal time constant of 7s or more) which could require multiple derivative terms and/or feed-forward in your controller.

3. Consider using a $0.25 ceramic resonator instead of a $.50 quartz crystal and caps. Frequency accuracy is not terribly important in an application like this so a few percent either way won’t hurt as long as it’s not jittery. Also, most or all ATmega and ATtiny devices currently in production have 1% calibrated internal oscillators, usable at the minor cost of adjusting your workflow a bit and using a separate programmer such as the Bus Pirate. But if you’re using the Bus Pirate, you don’t need the bootloader and therefore aren’t restricted so much in your chip choices.