...that doesn't require a lot of background resources? I understand that a non rooted standard S5 doesn't support lockscreen widgets (which is lame). I use the fingerprint scanner as my lock. I used this for my iPhone as well, and Apple has a nifty control panel that can be swiped up while the phone is locked to activate a few basic settings including a flashlight.
I found one that can run in the background and you shake to turn it on.. blah.