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