Here’s the all-in-gadget from Fender, work as a guitar tuner, bottle opener and a flashlight. The LED flashes at the same frequency as your low E string it is in tune. So just put this Strobe-e near your Low E, adjust the string until the LED stay still to show that it is in tune. It is powered by CR2016 lithium battery and it measures 2 1/4″ long x 1 1/8″ x 3/4″thick. Available for $9.99, just make it $10.