How to Build a Gamma 40 Hz LED Dimmer

Recent work from the labs of MIT has revealed that flickering light emitting diodes (LEDs) 40 times per second (40 Hz) significantly decreases beta amyloid in mice for up to 24 hours. The precise mechanism underlying the beta amyloid clearance is unknown, although it has been suggested to reflect activation microglia that act to "clean up" beta amyloid clumps. For more information about beta amyloid and gamma light therapy please visit our homepage or FAQ.

Described here is how to leverage an inexpensive LED dimmer to produce a 40 Hz signal suitable to flicker many common LED lights. Dimming LEDs is fundamentally different than dimming incandescent lights because they have a very narrow and unstable threshold between their on and off state. Therefore, LEDs are typically dimmed using high frequency pulse width modulation (PWM) by adjusting the duty cycle which is expressed as a percentage of on time versus off time. 

The flicker itself is controlled by the PWM frequency whereas dimming is controlled by the duty cycle. LED dimmers usually use a PWM frequency greater than 500 Hz to ensure that the flicker is blurred by human vision, although some research has suggested using frequencies as high as 10,000 Hz for consumer applications (e.g. LED brake lights) to avoid eye strain.

The circuit design inside the dimmer itself is exactly how most electrical engineers would build a PWM controller on a minimal budget and with minimal components. In brief, the dimmer uses an astable 555 timer to produce a "shark fin" waveform which is sent to an op-amp comparator that uses a variable voltage set by the dimmer potentiometer to adjust duty cycle. The output of the comparator goes to a PWM optimized FET to supply the LEDs at the output. The circuit uses a common 7805 regulator on the front-end to convert the incoming DC voltage (9-24VDC) to 5VDC used by the rest of the circuit.

Some considerations: 

  • The incoming ground is not directly tied to the output ground
  • The FET requires a load to properly generate the square wave
  • The output square wave (pin 3) of the 555 is not used

The only difference between the off-the-shelf dimmer and one that flickers at 40 Hz is the R2 component shown in the schematic and circled below on the circuit board. Originally the R2 resistor has a value of 10kΩ resulting in a factory PWM frequency of 687 Hz (calculated using the 555 Astable Calculator Tool). By modifying this single component to 180kΩ the PWM frequency will change to 40 Hz. Find 180kΩ Surface Mount Resistors at Digi-Key.

The dimmer unit can be used with any 5VDC compatible LEDs. It is advised to use a low current (~1A) 12VDC power source because this circuit has no built-in current limiter and some LED strips may get hot if the current is left unrestricted. The flicker effect should be noticeable from low-to-midrange dimmer settings. The dimmer never influences the PWM frequency, but when set to the maximum level the duty cycle is nearly 100%, and therefore the LEDs will appear to be fully on.

By recording the LED lights with a high speed camera and using spectral analysis software it is evident that the LEDs are producing flickering light in a narrow 40 Hz gamma band (note: the small peak near 80 Hz is a harmonic).