I'm in the same situation. I wish I could just be honest about my Mental Illness but that's too big a risk to take. You could tell the interviewer that you wanted a break from the duties of your past employment and decided to get a volunteer job to keep your personal skills sharp and see what else is out there that might bring you more satisfaction. I don't think any interviewer would fault you for wanting to widen your horizons.
Have you ever read "What Color is My Parachute?" There are a couple of chapters dealing with the question you are asking. One of the chapters even deals with being released from prison and how to present that diplomatically.
Personally, I am looking for a book that specifically deals with Mental Illness and work related repercussions and doing research on the Internet on the same subject maybe from a blog or other resource. With all the individuals that are challenged with Mental Illness I would think someone has addressed the issue in writing. I'll make sure I share the information on this forum if I come up with something helpful.