cheap diy shutter speed tester

P.S.
Computer logic, mice, keyboards, USB ports use 5VDC; CD, DVD, hard drives use 12VDC for their drive motors and 5VDC for their logic. Industry standard.

OK, I've got my little tester working. Tuns out I needed 6 volts. So now we have here a screen shot of an actual shutter test--I forgot which speed, but it's not important for now. My problem in interpretation. I'm dealing with a 100 year old Betax shutter that I worked on, and I knew all along that I had a rather week shutter-closure spring. If you will note the attached shot, you will see that after the initial exposure, there is an equally long time before the graph returns to zero, which I am supposing to be caused by the slower action of the weak spring. Would you agree, or is this to be expected? And if so, should I include some portion of this slower upward sweep in my computation? Thoughts? Thank you.

If I understand what I'm seeing, that thin line below the axis and rising to meet it is just an AC coupled circuit recovering from a change in DC input and could be ignored. That is, it's a manifestation of the tester/audio circuit and not the shutter.

(One of these days I want to build one of these, I have the parts -- but I think part of me doesn't really want to know! )

definitely different pattern from what I'm accustomed to seeing.