A used Petzl Swift RL 900, this is the older version but still works great! The front sensor detects ambient light and adjusts the brightness. Charges with a micro-USB. 

*From Petzl*

Ultra-powerful, lightweight and rechargeable: the ultimate multi-sport headlamp.

With 900 lumen brightness at only 100 g, SWIFT RL is the most powerful of the Petzl compact headlamps. With REACTIVE LIGHTING® technology, a sensor analyzes the ambient light and automatically adjusts the brightness to user requirements.

The headband is reflective for night visibility. Its two-part construction ensures excellent stability in dynamic and intense activities such as mountaineering, trail running, and skiing. SWIFT RL is intuitive, with a single button for easy