hambleturtle wrote:I'd like to connect a Raspberry Pi to a roomba 530, powering the Pi (5V 700mA) from the roomba ...
Sure, Roomba's have their own +5VREG node that is derived from its battery power; but, I'd bet money (not much, of course) that the SMPS (buck converter ckt) that down-converts battery voltage has no where near "700mA" of margin to share with any add-on peripheral. In fact, it is my guess that Roomba's buck converter's maximum current draw is way less than 0.7A. AFAIK, none of us have determined the power output capability of that buck converter.
Are there any off the shelf solutions (or partial solutions)?
There might be on this board, somewhere, but I don't recall what or where.
I'm thinking I could use a switching ... dc->dc converter ... to get the 5V power from the Roomba. ...
You may be able to do that by directly tapping into raw battery voltage and returning current to battery negative, and completely side stepping Roomba's elex.
Don't plan on tapping into the battery voltage appearing on sockets 1 & 2 of the SCI Mini-DIN receptacle -- that power has a 0.25A (fused) limitation.
When making direct connections to battery terminals, keep in mind a couple thoughts: 1) Charge depletion: 0.7A represents about 35% of Roomba's Clean-Mode current, so, if Roomba has all its motors running when your Pi is controlling it the robot's run-time will be reduced. And 2) EMI coupling: Your add-on SMPS down-converter may require added components between it and the battery terminals to attenuate the converter's switching noise enough to not confuse Roomba's controller.