One fundamental use case for LEDs is to signal a state: best known and ubiquous are power indicator LEDs which are simple LEDs with a current limiting resistor, directly connected to a power source. They light up once a device is powered on.
Signal LEDs can do much more, though:
Bi-color or RGB LED indicate more information than just an on/off state.
- With two colors, standby and ready or charging and full can be indicated by different colors.
- RGB LED provide even more options and effects (such as breathing color (color transitions), i.e. while connecting to a network).
Multi-Color LEDs
There are three different popular packaging forms:
LEDs with Legs
Multi-color LEDs typically come with more than two legs, although bi-polar bi-color LED can have just two, like a regular LED (controlling emitted color by applied polarity):
- Two Legs: either a monochrome (regular) LED, or a bi-polar LED. Bi-polar LEDs are really two LEDs in opposite direction. Depending on applied polarity, either one lights up in a separate color.
- Three Legs: bi-polar LED with a common anode or cathode. Again, two separate LEDs are used inside, and depending on whether you supply power to an outer leg, it lights up. Since with this design, you can also power both LEDs at the same time, effectively mixing their colors, such LED can emit three colors.
- Four Legs: RGB LED with three internal LED and a common anode or cathode (longest leg). RGB LED can be used as a simple bi-color LED, indicating state in green and red (by ignoring the blue LED). By combining all three red, green, and blue LED and adjusting their brightness individually via current or PWM, this LED can emit any color.
- Four Legs (programmable): Indistinguishable from the former, they can contain a microchip that manages RGB colors and current limiting. Such LEDs (like the APA106) take 5V power with two legs, and a control signal with the other two legs. These LED can be daisy-chained, and a microcontroller can control as many of these LEDs as needed with just two GPIO.
LEDs on Star-Shaped Heat Sinks
The most flexible type of RGB LED provides separate connections to each internal LED and does not use a common anode or cathode.
Such LED are typically high power illumination LED (1W or even 3W per LED, with a RGBW LED bein rated for a maximum overall power of 12W). As such, they are typically mounted on a heat sink (star) and can have additional white LEDs (RGBW or RGBWW).
Nevertheless, even if this might be technical overkill and not economical, they, too, can be used to signal state.
Programmable SMD RGB LED
Programmable LEDs (such as the popular WS2812) are commonly used in individually addressable LED light strips. Since these LED are individually addressable, they can also be used as single signal LEDs. The only prerequisite is that your project uses a microcontroller that can produce the control signal required to program the LED.
Such LED are typically SMD (Surface Mount Devices), so they are directly soldered to a PCB (printed circuit board), and have no legs. Breakout boards exist with premounted SMD LED that are easier to use in DIY projects.
SMD LED are available in different sizes (i.e. 3528, 5050, and 2835). These numbers refer to the size of the LED. A 3528 SMD LED for example is 3.5mm x 2.8mm.
Light Patterns
Blinking and other light patterns (like breathing color changes) are another great way of visualizing information.
PWM (pulse width modulation) is just an extreme variant of blinking, with a frequency so high that the human eye cannot distinguish the on and off phases anymore, effectively dimming LEDs - again another option of signalling state.
GPIOs
Simple indicator LEDs are either directly connected to a power source (to indicate power on), or controlled by firmware using a dedicated GPIO.
Devices with more than just one or two indicator LEDs can quickly require a lot of GPIOs just for the signal LEDs, and since GPIOs are often scarce, there are better ways:
- clever wiring can signal GPIO state without requiring another dedicated GPIO
- programmable RGB LED need just two GPIOs for any number of indicator LEDs which is one of the best ways to scale.
- Dedicated LED driver ICs just require two GPIOs (typically using I2C) and can control a great number of LEDs or RGB LEDs. These specialized ICs often come with built-in support for current control and/or blinking and other patterns.
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(content created Feb 18, 2024 - last updated Aug 25, 2024)