How Does an LED Control Its Brightness?#

Take a close look at Christmas tree lights. The lights don’t just turn on and off — they gradually get brighter and then gradually dimmer. In other words, the brightness is being controlled.

Is the LED’s brightness really being controlled? No! In fact, it’s tricking our eyes into thinking the LED’s brightness is being adjusted. Let me explain the principle.

A Simple LED Circuit#

I’ve prepared a battery, an LED, and a switch. By connecting them in series, I built a simple electrical circuit. As shown in the figure on the right, when the switch is closed, current flows and the LED turns on. Conversely, as shown on the left, when the switch is opened and the current is cut off, the LED turns off.

Let’s Gradually Increase the LED Blinking Speed#

Let’s say we turn the LED on for 0.5 seconds, then turn it off for 0.5 seconds. If we express this as a waveform on the time axis, it looks like the figure below.

Next, let’s blink the LED 10 times per second. The waveform looks like the figure below.

Now let’s blink the LED 100 times per second. At 100 times per second, it will blink very quickly. The waveform looks like the figure below.

Now let’s blink the LED 1000 times per second. We have to turn the LED on and off within 0.001 seconds, so it blinks really, really fast. The waveform looks like the figure below.

Can our eyes see something blinking 1000 times per second? The human eye cannot see a 1000-times-per-second blink. So how does it look? It looks as if the brightness has been reduced.

This is because the human eye has the characteristic of leaving an afterimage (persistence of vision). Therefore, the eye perceives brightness as the integral of the actual amount of light that was on over a certain period of time. Since light was present for half of that period and absent for the other half, it feels dimmer than when the light is constantly on.

How to Decrease or Increase the Brightness Further?#

Let’s take this a step further. What if we want to reduce the brightness further? As shown in the figure below, if you shorten the LED’s on-time within one cycle, the area becomes smaller, so the light feels even dimmer.

Conversely, what if we want to make it brighter? As shown in the figure below, if you extend the LED’s on-time within one cycle, the area becomes larger, so the light feels even brighter.

By blinking the LED hundreds to thousands of times per second and adjusting how long the LED stays on, we can make the light feel stronger or weaker.