If you choose to alter your air popper you are assume the risk that it will no longer work as it was designed to, that it might not work at all, that you might injure yourself in performing these tasks or in using the altered device. This information is here for you to peruse, but Sweet Maria's accepts NO responsibility in what the results may be.
When I started roasting with an air popper, I found that timing the roasts was difficult. If I watched the clock on the wall I would lose track of the minutes, and it wasn't very accurate anyway. If I used a timer on my watch, I would forget to stop it, or even look at it, during the critical moment when the roast is done and you need to be dumping beans and agitating them. Kitchen timers are available in both analog and digital, but I would forget to stop them too.
Then I remembered all the photo darkroom equipment I had in the basement, and my old Gra-Lab darkroom timer. The popper plugs into the timer and is turned on and off by it. I turn the popper on and off with one switch on the timer, so the actual roast time is recorded nomatter. I actually own a fancier digital timer for photo purposes, but the old Gra-Lab is a wonderful workhorse.
Another advantage to using the timer is that you can dial in a particular roast time and, all other things being equal, you can reproduce your favorite air popper roasts without paying attention too much! Remember, roasting is a potential fire hazard, so use some sense and never leave the room when you're roasting.
Gra-Labs are readily available at used darkroom equipment shops at very reasonable prices. Like the old Bell telephones, these are built to last forever!