Elderly help app: Do you know an app with this specs?

Hey people, maybe you can help me with this...

I need an app that works with a phone's accelerometer and detects movement while sending a signal to a second phone connected through WiFi, the second phone then rings an alarm.

It appears to be quite easy, but I couldn't find anything similar, so I'm planing to do it myself, although I would prefer to get it already made :P

The idea is to attach the "sensor phone" to my grandpa's arm and give the "ring phone" to my grandma. My grandpa needs assistance to move, but he forgets about it due to his mental confusion and tries to get up, risking to fall. 

This app would allow my grandma to continue her activities without the need to be at his side all the time.

Any help with similar apps or tips on how to do it would be great!  Thanks a lot!

This may not be as easy as it sounds. Programming the accelerometer is not so difficult, but what is a movement that requires action from your grandma and what is just grandpa waving his arm? Have you thought of looking at fall detectors? The commercial ones are rather expensive, but maybe you can find an app in the Play store.
Then, App Inventor may not be the right platform, because the app must be active all the time. That is possible with some tricks, but it will drain the battery rather quickly. The app for grandma would be doable.

Thanks for the thoughts! Actually you already helped me, I didn't think about this key word "fall detector", how stupid, I see many options now. Thanks for the battery heads up too. 

Nevertheless, the ones I found all seem to be indeed "fall detectors", and I want some sort of "pre-fall detector".

I guess if 1)the sensor was connected to his chest/belly and 2) I could measure the positioning of the phone in a 3D set, it would be doable. 

A fall detector must work with the intensity of acceleration, I suppose, so going to bed or siting is not taken as fall, even though you move in a up-down axis. That makes it hard, because, as you said, "what is a fall?".

I my case, I need simply a series of unidirectional movement in a certain axis to indicate a change in position.  If up-down axis is positive for a certain period it indicated that my grandpa got up from his bed/chair. 

Does that make sense? I'm not familiar with using an accelerometer...

You could use tinywbdb so make it when moment is detected it set a value on a tinywbdb to one. then on the second get a clock and make the clock look for the change in vales then once it has changed make it play the alarm.