I have a number of old phones. Mostly they just hang around in a box of all other computer hardware. I have two smartphones, one feature phone and one dumb phone.
First one is Intex Aquafish. Great phone, but since it can't connect to my office network (Crazy!!!), can't really use it.
Next one is Karbonn S1 Plus which is ancient now. Gifted this to sister, she used it for 2.5Yrs., and by then it was quite busted. Battery gone, frame cracked, power button barely working, and horizontal white lines on screen. I gifted another phone to sister. But this phone is not totally dead. I replaced the battery for 300Rs and reset the phone. Turns out it works fine as a standalone GPS navigator. So that's it. Its hanging around like that.
Next one is a Nokia C5. This was the first phone I gifted to sister, before the Karbonn. It works fine, but no it's quite obsolete. No wifi, Symbian etc etc. But the thing is it's still a fine phone. I use this as mp3 player these days. Works great.
I also have a Nokia 1280 dumbphone. But it's battery is dead and keyboard has all letters peeled off. So it's resting in the junk box. That's it.
Coming back to the Karbonn S1 Plus. It's power button is almost dead. So I thought to see if there's something to be done on software side. After going through some links I found a nice app called Volume button to Power button. This triggers power button press on volume button press. It's not instant, but quite okay. And that means my main use case of unlocking the phone via power button is satisfied.
There are a few other use cases like reboot and power on when phone is shut down. Few solution are provided for these too. Seem like it will work. Let's see.