<p>I'm starting up a personal project to develop some open source software.  I want to use Scrum as the PM process on this (as I like the Product Backlog, prioritisation, and if I can get them, the burndowns) but it seems to me that I won't get the full value because  I can't at the outset guarantee the amount of time myself and my collaborators will be able to commit to work during a given sprint.</p>

<p>I know there are other benefits that I will still get from using Scrum but are there variations or tricks and techniques I am unaware of which will enable me to get the value of things like burndown charts and timeboxed iterations?  Or am I just being too hopeful?</p>

<p>TIA.</p>

<p>Regs, Andrew</p>

In a by the book setting you won't use real time for calculation of the burndown chart but rather story points. After a few sprint you will see an average velocity and thus be able to generate a burndown chart and use this velocity for commiting to the sprint items.
And I strongly disagree with
warrens post
on the scale-down point. The main problem I see is a strongly varying velocity between two sprints, since it is only a hobby.