This Android app vibrates the current time when the display is locked and the power button is pressed twice in a row with a delay between 50 and 1350 milliseconds. If the double click is performed accidentally while the display is still active, the app warns with a long, persistent vibration.
You may also use Tactile clock to keep informed about the current time. For example let the app vibrate the current time every 5 minutes or every hour.
The background process automatically initiates once the system has finished booting.
The app supports all devices with Android version >= 4.1 (API 16).
Basically there exist two different vibration patterns: A short vibration stands for the digit 1 and a long one for the digit 5. The 2 is represented by two consecutive short vibrations, the 6 by a long and a short one and so on. The 0 constitutes an exception with two long vibrations.
Examples
01:16 = .. s ... s .. l . s
02:51 = .. s . s ... l .. s
10:11 = s .. l . l ... s .. s
Explanation
The time is processed digit by digit. s = short, l = long. A leading zero at the hour field is omitted. To simplify the recognition of the vibration pattern, there exist three kind of gabs with different durations, marked by the number of dots in the examples above. A single dot stands for the pause between two vibrations, two dots symbolize the separation of two digits within the hour and minute field and three dots split hours and minutes.
This app is also available in the Play Store.