Many custom LED lighting designs require the ability for the user to choose a specific output setting other than full power. Perhaps the application will operate during night and daytime hours, but requires a constant relative output. For example, LED truck lights operating a full power will obviously appear much brighter during the night. The solution may be to dim the LED truck lights in order to provide the illusion of a consistent output, when compared with daytime viewing. This feature can be an automatic function based on a clock or light sensor. It could also be a manual function allowing the user to select between various modes of operation. One might consider several possible configurations for a PWM LED dimming circuit. However, dimming an LED truck light is not always as simple and strait forward as one might consider.
The easiest way to dim an LED array is to decrease the forward current. There are several effective ways to accomplish this including variable resistors or even a voltage regulator with a variable programmable output. It is important to recognize the danger associated with dimming LED truck lights in such a manor. The problem ultimately begins with the light emitting diode manufacturing process. LED manufactures are unable to produce identical LED lights. Therefore, the light emitting diode manufacture will test and separate dissimilar LED lamps basic on numerous characteristics. This is the process know as binning. Numerous LEDs within a specific bin share similar, but not identical characteristics. Some may appear slightly brighter, while others are a slightly different color. The voltage threshold at which the LED light will begin to illuminate can also vary slightly. When driving an LED light at the recommended forward current rating, these variations should never become apparent. However, driving at unusually low forward currents will cause one or more dissimilarities to become obvious. As a result, dimming LED truck lights by simply varying drive current may cause some light emitting diodes to appear as a different color, while other may simply stop illuminating entirely.
Since varying the forward drive current to achieve LED dimming may prove problematic, an alternative method is required. Pulse width modulation, or PWM, can effectively control the pulse width and duty cycle causing the LED light to vary its intensity. This works by turning the light emitting diode off for a very short period. To produce an increased dimming effect, the LED will remain off even longer. The human eye does not interpret a flicker or strobe when utilizing the proper frequency. Instead, the eye essentially will detect the LED as a continuous light stream, but the light appears dimmer due to the short periods during discontinued operation. To produce the appropriate frequency and pulse width, the LED drive circuitry requires some form of programmable timer. Many older PWM LED dimming circuits utilized the 555 timer, and controlled the output pulse width using a simple variable resistor. This method works sufficiently. However, recent advanced in microcontrollers have created new opportunities for PWM LED dimming circuits. The microcontroller is custom programmable, and provides greater application flexibility when compared with traditional 555 circuits. In addition, many new microcontrollers actually require fewer external components than the 555 integrated circuit.