This metronome i use.
You can set it into any time interval with vibration.
Then have it at pocket and when you glance at wheel to estimate for a short moment, then touch your pocket and feel the vibration.
This way works when you take rotor speed and when you estimate ball using metronome (special method).
http://www.garret.ru/En_Timer.htm#Metronome (http://www.garret.ru/En_Timer.htm#Metronome)
You need a cable to connect your phone with our computer to install the software.
This is a metronome watch, just to show you other options.
But the mobile phone option is excellent and free.
And a mobile phone is not a device in the same sense like a metronome watch.
(http://i40.tinypic.com/2hz2vwm.png)