Measuring rotational speed with a phone and an LED

Over the weekend, I was having fun with using an LED as a photodiode, and hooking it up to my oscilloscope. This can be used to measure the speed of a drill (just stick a reflective spot on a matte chuck and use a flashlight). I was going to make an Instructable on measuring rotational speeds of various objects, but my son told me that most people don't have an oscilloscope. But then I found you can just connect the LED to the microphone input on a phone and use a free oscilloscope app, and use that to measure rotational speed. And so I made an Instructable that doesn't need an oscilloscope.

My unkillable Treo 700P

I bought my Treo 700P phone second-hand on ebay in 2008, and it's served me faithfully.  I've written enough apps for it so it works very much like I want it to work.  I only really wish it had a better web browser, but it's good enough to check my email on.

At one point, maybe a year or two into its service to me, I had to use an app to turn on the microphone and speakers to make phone calls, but this was fixed when I blew out the headphone jack--I think it was stuck in thinking that there were headphones attached. Two or three times, I've had keys become less reliable, but that's an easy fix--I just disassemble the phone, peel back the keyboard, and clean the contacts (acetone works well).

Yesterday, I thought it had finally kicked the bucket.  We had an on-campus Fiesta event, and there was a small pool of bubble solution, and my son and I were making bubbles, and the Treo slid from my shirt pocket into the bubble solution.  I checked that it didn't work, removed the battery, disassembled and dried it at home, and it still didn't work.

I then spent several hours looking at what Android options Sprint had for me.  I wanted a large screen (4.3" is really the smallest I'd want, at least in wide-screen format) and a hardware keyboard (on-screen keyboards aren't very good for typing serious technical emails, especially if you need to use braces and the like--apparently a lot of people don't use them much).  Alas, nothing met my desiderata.  The Galaxy S II had an OK sized screen (4.5") but no keyboard, and the Galaxy S had a keyboard but the screen is little too small (4").  Granted, my Treo's screen is much smaller, and its keyboard symbol support isn't great (but I wrote an app that helps with that), but if I am going to upgrade, I'd like to upgrade to something that will satisfy me, rather than make me wish for something else.

I was planning to drive to the Sprint store and get a Galaxy S this morning, when I did the last check of my Treo and found that after drying out more fully overnight, it's now back to good working order.  I wonder how many more months or years it'll last me.

Update: It's finally dead--see comments.