This article describes the implementation of a tunable white light design, which enables a user to change the color temperature of white light between 2000K to 25000K using RGBA LEDs. The design uses the DMX512 protocol for communication. DMX512 derived from Digital Multiplex, is a digital communication standard, which is widely used in stage lighting and entertainment lighting solutions. This approach can also be adapted to implement RGBA color mixing. The DMX512 protocol can be used for individually controlling the dimming level of each of the LEDs up to 512 channels.
Adoption of high brightness LEDs is increasing in the lighting market due to numerous advantages that LEDs offer when compared to the conventional lighting technologies. The various benefits include extended service life, better efficiency, energy reduction and low maintenance cost. Other than these benefits, intelligence can be added to the LED-based lighting drivers in order to add new functions that are impossible to achieve with the conventional lighting technologies. One of these new functions is Tunable White Light where the color of an LED-based white light fixture can be varied from warm white to cool white.
Different control interfaces are used for providing control information to the LED fixtures. Some of these are Digital Addressable Lighting Interface (DALI), PowerLine Communication (PLC), Digital MultipleX (DMX512) and Wireless protocols. DALI is the popular in the building lighting applications whereas DMX512 is popular in the entertainment industry most commonly stage lighting. This article will describe how DMX512 can be used to implement Tunable White Light.
Tunable White Light
Theoretically, heating of a black body generates white light, which glows first deep red at low temperatures and eventually blue-white at very high temperatures. This temperature is referred to as the color temperature (CT). Its unit of measurement is degrees Kelvin (K) and is used to identify the color of light a source produces. The white light at temperatures below 3000K is referred to as warm white and, as the color temperature increases to 3000K - 3500K, the color of the light appears less yellow, or more white. When the color temperature is greater than 5000K, the light produced appears bluish and is referred to cool white.
A light bulb that produces light perceived as yellowish white will have a color temperature of around 2700K. The only intelligence that can be added to this is the control of the brightness of the light. With the introduction of solid-state lighting, a single LED fixture can be tuned to white light varying between 1000K and 25000K. The key benefits of controlling the brightness include enhanced productivity in the workplace, beautification of architectural buildings, providing an attractive and welcoming setting in hospitality areas, attraction and flow of customers in retail industries, generating a soothing and relaxing atmosphere in homes, and so on. The white color can be changed based on mood and season. This concept is very useful for retail industries where the light can be selected based on the product on display. For example, gold looks best in warm white light whereas fruits and vegetables show their true color in neutral white light (i.e. light at around 4000K).
DMX512 is derived from Digital MultipleX with 512 pieces of information. It is maintained by an ESTA publication, and goes under the title of Entertainment Technology - E1.11-2004 USITT DMX512-A - Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories. The DMX512 standard covers electrical characteristics, data format, data protocol, and connector type.
DMX512 describes the digital data transmission between controllers and lighting equipment and other accessories. It is a serial, digital, packet-based protocol with a transmission speed of 250 Kbps. It deploys a multi-drop bus topology called 'daisy-chain' where the slaves are connected one after another to the DMX512 host. Each host can connect to a maximum of 512 slaves on one bus with slot addresses starting from 1 and ending at 512. Figure 2 shows an example of a DMX512 host.
The electrical specifications are governed by the EIA-485 standard (differential signaling) with 5-pin XLR as the interface connectors. A DMX512 connection point has five assigned end-points: data 1-, data1+, data2-, data2+ and a common reference. data2- and data2+ are used for creating the daisy-chain network, they carry the same signals of the first differential line (data1- and data1+).
The data link requires a terminator to eliminate ringing issues or signal reflection. This terminator must be a 120 Ohms +5%/-10% impedance placed between data+ and data- lines. DMX512 Controller and other transmitting device use the Female connectors. Slaves or Receiving device use male connectors.
The protocol uses 1 start bit and 2 stop bits with a little endian data format. The start of a packet is signified by a break of at least 88uS followed by a "Mark After Break" (MAB) of at least 8uS, extended in 1990 from the 4uS specified in the original 1986 standard. The break is a signal to receivers to start reception. After the break, up to 513 slots are sent. The first slot is interpreted as a "Start code" that tells receivers what kind of data will follow.
The remaining slots make up the actual level data. Up to 512 slots can be sent, and it is the job of the receiver to count the slots to keep track of the channels. As there is no error detection or correction in DMX, it is vitally important for receivers not to miss slots, and to discard packets if framing or buffer overflow errors are detected.