One of my hobbies is building and flying small quadcopter radio control (RC) aircraft. Most RC aircraft use a one way transmitter – the remote control is the transmitter and there is a receiver on the aircraft to process received commands for flying the aircraft. But the aircraft does not transmit anything back to the control unit – you have no way of knowing when the battery is about to reach empty!
Some RC aircraft use Wi-Fi or proprietary links sending data in both directions – those quads send an alert when the battery is low.
But for simple systems, there is no alert!
A crashed my small quad recently due to draining the battery. I was flying it back to land when its battery died. While only 2 meters off the ground, it came down hard and damaged a prop and motor. (I was flying at an AMA approved RC model airfield so this was all done safely over a grassy area.)
To solve this, I decided I needed a count down timer app that gives me an alert after 10-15 minutes of flying.
That’s where App Inventor comes in.
User Interface View
To set the countdown timer, move the slider control to select a number of minutes between 0 and 59. Or, select one of the “preset” buttons which set the time limit to 10, 12 or 15 minutes.
Press the Start Countdown button to activate. The count down timer shows the remaining time in minutes and seconds.
Once the timer reaches 3 minutes, the remaining time color changes to red and an audio alert “3 minutes” is played. At 2 minutes, there is a “2 minutes” alert and then a “1 minute” alert when there is 1 minute remaining.
The Stop button stops the timer and resets everything to zero. The Exit button closes the application.