That processor will do just fine on a single LiPo (3.2-4.2V depending on charge level). You may have to limit the speed to 8 MHz or so.
This requires more explanation. A Google search told me that nitinol is a shape memory alloy, and there’s nothing about it that normally needs to be powered. It’s a metal spring. So what are you actually trying to do here?
Trouble is, I need more voltage to get the proper current for the nitinol actuators (about 320 mA), which essentially act like resistors.
To explain further, the circuit has several (at least 6) nitinol springs surrounded by regular springs that work as reversible actuators. When powered, the nitinol (hopefully, since I have trouble calculating the spring constant) overpowers the steel spring, expanding outward as current flows through it and heats it up due to resistive/joule heating. When not powered, the nitinol cools down, exerting less force and allowing itself to be pulled back to the original position by the steel springs. The amount of current needed by the nitinol is determined by the actuation time wanted (in this case, 1 second assuming no heat lost to the environment) and the radius of the wire (smaller requiring less current).
While I could use a single LiPo to power the ATtiny itself, the max available current from the highest-density single cell I could find that meets space limitations is around 200 mA (part# RJD2450 on DigiKey), below what I need. I could, in theory, decrease the radius of the nitinol wires to lower the current required, but past a certain point they wouldn’t have enough force on expansion to override the steel springs.
The output voltage is set by the ratio of R2 and R3. Increase R2 for a higher output voltage. From the data sheet the voltage on pin 6 should be 800mV, calculate the new value of R2 to give 800mV with the output of 6.55V.
You might not be able to get 6.55V from 7.4V input as the input to output difference might not be enough. No harm trying. Also, that assumed the batteries are fully charged.
Hmm. I guess a voltage regulator would be the best option, then, if it meant it could take any input voltage (within reason) and turn it into the target voltage - but how do voltage regulators affect current? If I’m stepping voltage down, I’d like to step current up as a safety factor in case I need to adjust for heat loss to ambient air.
How about one of the Pololu regulators like this one?
0.5" x 0.4" should fit your requirements.
That does look like what I want, thanks! I’ll give this one a try and see if I can make it work.
I did also make a spreadsheet to keep track of required parameters for this project (it’s mostly calculations for the nitinol), so maybe that will help clear some confusion up. Unfortunately, I can only upload it as PDF, so it looks a bit ugly (second page should be on the right of the first one).
nitinoljouleheating.pdf (139 KB)