It would only take one more resistor to add a switch (the safety switch electromagnetic relay), correct?
What functions were you thinking of, more firing modes or more refined logic?
Originally, I envisioned an I²C library to have it connect to other devices such as a display and scroll wheel. Crazy, I know. I shelved the idea when I realized it would probably severely impact battery life and reliability.
The safety switch relay does not connect directly to the ATtiny44. Instead, the relay controls the power to the chip Vcc (Pin 1). To simulate this on the breadboard, I just plug and unplug the power source.
A mixture of both. The stock board has the ability to detect low voltage from the battery and indicate that the battery is low to the user. I have not finished researching this, but I don't really have the space for it on the chip either. It was that or presets. For firing modes, I don't have any ramping. I would like to include some version of it. I'm trying to optimize the code a bit (without going to assembly) and might be able to make room for one more firing mode (fingers crossed).
Crazy? Absolutely! But we're all a little crazy. We've had similar ideas about adding peripherals (kill counter, ammo counter, ...). For this, I think we would include a separate enclosure with a picatinny mount so it could store a separate battery for itself, and link up to the stock board with a cable tied to the pinouts (drawing as little current as possible to get the job done). I'm not a hardware guy, but I think this is possible.