Apple Laptop LED PWM Frequencies

Written 2014-12-07

Tags:PWM LED MacBook PowerBook 

Although the LED on the lid latch of an Apple Laptop is a simple device, there's a little more complexity than would be expected. Since the LED is lit while the laptop is suspended, it is important that it be efficient to avoid draining the battery unneccessarily while suspended.

One trick to driving LEDs more efficiently is to use pulse-width modulation(PWM) to temporarily overdrive the LED for a short period of time, then underdrive or not drive it at all for another period of time. This works due to something called the flicker fusion, where you perceive small enough pulses of light to be a single, continuous light.

But, it's possible to see behind the curtain. When you move a PWM'd light source, the human vision system tends to see each strobe individually, like a stroboscope, but only if the light is being moved fast enough for the strobes to occur at different locations - otherwise you won't see the strobes and the pulses will blur together.

It turns out, if you move a PowerBook(6,8) and a MacBook together, you'll see that the strobes from the PowerBook LED are further apart than the MacBook LED. This indicates that the PowerBook strobes its LED at a lower frequency than the MacBook's LED. If both laptops used the same led it would be possible to estimate the duty cycle by comparing the perceived length of each strobe, but since the MacBook has a much smaller LED, the results would be skewed.