Interval Clock guides you through interval training with a countdown timer that alerts you when you have to go faster or slower. This is ideal for use on a stationary bike, treadmill, or other piece of exercise equipment that allows you to look at your phone (although you can use the app for just its audio alerts without machinery if you prefer).
The app originally worked for a 20 minute workout with short intervals, as this was found to be an ideal way to train (see this Lifehacker post for details). Now, in version 1.2, the app supports interval lengths and timers of your choosing. It will also remember the last interval you chose so you don’t have to change it frequently.
To use the app for the 20 minute biking exercise with 12 second slow intervals and 8 second fast intervals, all you have to do is launch it and press start. The screen will turn yellow during the slow interval and a pixel dog will walk casually to indicate the slower movement. When it’s time to speed up, the screen will turn green and the dog will run. If you prefer this specific workout you don’t have to do anything. If you want intervals of your choosing, however, just press the + and – buttons next to the slow and fast interval length to set them to however many seconds you like. You can also change the minutes and seconds in the same way with the + and – buttons above. You already know the start button starts the timer. Unsurprisingly, the stop button stops it and the reset button brings it back to the last time you chose (20 minutes by default).
This is a free app, currently without ads. If you find issues with it, please let me know and I’ll do my best to fix the bugs.