The headstock tuner has taken off in a big way, certainly judging by the number of them out there. While most get the tuning job done well enough, many have poor battery life, and when the required batteries are invariably those expensive button types, things can start to get rather costly.
Seiko's answer is a USB-rechargeable tuner that also features a very handy dual-power level torch - ideal for those backstage/onstage low-light situations, or even to illuminate your pad. It's larger than many, but that means the clip will fit any acoustic and electric headstocks and the actual tuner can be rotated 360 degrees and tilted up or down.
Fitted to the back of your headstock, the note display is upside down, but the bright bar-tuning display is still easily used in this fashion. The STX-7 features the usual tuning calibration and either mic or clip (vibration) mode.
It may not be the most discreet headstock tuner we've encountered, but it's faultless in operation, with fast and accurate note recognition in both tuning modes. Charge time from zero is roughly three hours, and so long as you remember to press and hold the switch to turn the torch on, it proves plenty bright enough to illuminate amps, effects or your chord charts. In low-light mode, it offers 30 hours use per charge.
Not the cheapest headstock tuner on the market, but as far as we're aware, the only one that's USB-rechargeable. Plus that torch is extremely handy for live use.