Simple battery threshold circuit for 9v powered projects. LED will flash once if available voltage is 7.5V or greater. If the LED doesn't flash, then it's probably time for a new battery soon.
This circuit is taken from the wonderful FDeck HPF-Pre Series 3 circuit and is used with permission. https://sites.google.com/site/hpftechllc/home/technical-diy-stuff/hpf-pre-series-3-technical-info?authuser=0
I was looking for a small, yet effective, battery indicator and this one works great. PCB is available at OSHPark and the board is on 21.5mm x 18.7mm so it fits in tight spaces. This means you can probably retrofit it to some of your favorite projects.
Link to PCB at OSHPark is here- https://oshpark.com/shared_projects/PLOuj0qq