How to Build a Gamma 40 Hz LED Dimmer

This tutorial serves a few audiences. (1) Engineers who need a really simple, current-source square wave generator and (2) those who think brain entrainment through the visual system is therapeutic. It's about the cheapest method you'll find to having a self-contained, fixed frequency, variable duty cycle 5VDC device, based on a simple 555 timer and comparator/op-amp.