When Samsung launched the Galaxy Note 5 earlier this month, there were quite a few improvements to the device from last year's Galaxy Note 4 and other previous models. The design was an obvious big change as the Galaxy Note 5 took on the same look as the Galaxy S6 and the Galaxy S6 Edge, with a metal and glass construction and slim design. Another new feature which was introduced to the Note series for the first time was the Screen Off Memo function which lets users jump right to a low-power consumption screen to write out notes and memos after pulling out the s-pen.
Since the Galaxy Note 5 features a non-removable battery this time around whereas all prior Galaxy Note devices had a battery which could be easily swapped out by the user, any moves made to improve battery life on the Galaxy Note 5 are smart additions by Samsung, especially factoring the power consumption of a Super AMOLED display that is Quad HD. The good news for older Samsung Galaxy Note owners is that the Screen Off Memo feature has been made available for the Galaxy Note 4 and Galaxy Note Edge as well as the older Galaxy Note 3. It's worth mentioning this is an unofficial version of the app and it is not developed by Samsung, so there may be some differences in what the original feature on the Galaxy Note 5 offers and what's available for the other devices.
If you're still running an older Galaxy Note device out of the three that are listed here, you can head to the XDA Developers thread from the source below where the apk is hosted and grab the apk file from the links there. All you have to do is download it and install the app as you normally would, and root is not required to install or use the app but remember you'll need to head into the settings menu and check the box for unknown sources to install the apk file. The user responsible for getting this up and running also mentions that the app is still a work in progress, which means you may stumble upon a bug or two, and that there could be further development to improve the app and add new functions and such so it might be worth keeping an eye on.