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Intelligent Control of LED Lighting Enables Even Greater Energy Efficiency



This article recognizes that the adoption of LED lighting is a major step towards a greener economy, with significant energy savings possible through the replacement of both conventional incandescent lights, and even the more energy-efficient fluorescent tube and compact fluorescent lamps. It expands this theme by arguing that more efficient lighting sources are only part of the solution. Responsible use of our precious energy resources demands that we apply intelligent controls to ensure that the lighting in our homes, offices, factories and streets is only switched on when required and at levels appropriate to these requirements. This article aims to show that for these controls to be effective, they need to balance convenience and ease-of-use with a level of automation that can respond to movement or room occupancy, adapt to ambient light levels, and ultimately prevent energy waste.

The article will highlight a number of LED Lighting, sensor and control products available from Hotenda including standard LED replacement lamps like ROHM's R-B15L1 and R-FAC40BN1, PIR movement and automatic lighting sensors from Zilog (ZEPIR0AAS02MODG), OSRAM (SFH 5711-2/3-Z), and Intersil (ISL29029IROZ-T7), and driver and control devices such as Diodes’ ZXLD1374EST20TC and Atmel's AT90PWM316-16MUR.

What is the attraction of LED lighting?

The headline benefit of using LEDs to light our homes, offices, factories and streets is undoubtedly the energy savings achieved from replacing incandescent bulbs, fluorescent tubes, and most other kinds of lighting technology. Incandescent (tungsten filament) bulbs are notoriously inefficient, with as much as 90% of the input power converted to heat rather than light. Fluorescent lights, either the tubes predominantly used in commercial premises or the compact fluorescent lamps (CFLs) we have been encouraged to use in our homes over the past decade or so, are more efficient, but LEDs can save between 30 to 50% of the energy used by fluorescent lights.

The exact saving varies according to lamp design, their intended use, and how they are specified, e.g. aspects such as color temperature, more often expressed as warm (yellow) or daylight (cooler blue). We can see this with a couple of product examples from ROHM. Their R-B15L1 LED light bulb (shown in Figure 1) replaces a conventional E26-style light bulb and has the same color temperature as an incandescent lamp. It consumes 8 W compared to a 60 W incandescent lamp, a clear energy savings of more than 85%. Even if it only replaced a 12 W CFL with equivalent light output, that is still a 30% savings.

The ROHM R-FAC40BN1 linear LED light bulb is a direct replacement for a standard 4 ft. (1.2 m) G13 2-pin base cap fluorescent tube. It provides a cool white output, more often preferred in work environments, and delivers the equivalent light output of a 40 W fluorescent tube, yet consumes just 22 W, a close to 50% energy savings.

Figure 1 (left): ROHM’s 8 W LED light bulb replaces standard E26 60 W incandescent lamps.
Figure 2 (right): ROHM’s 22 W LED linear light bulb replaces G13 40 W fluorescent tube lamps.

While LED light bulbs are still much more expensive than incandescent or fluorescent bulbs, the actual bulb cost is only part of the cost of ownership. This is because, apart from the energy cost, the life expectancy of an LED bulb is 50,000 hours, compared to 10,000 for a CFL and just 1,200 for an incandescent lamp. Therefore, over 50,000 hours, the total cost for an LED lamp is marginally less than a CFL, but only one quarter of the cost of using incandescent bulbs. There are other factors that weigh in favor of LED versus CFL, including their instant turn-on response and the fact that they do not contain hazardous materials such as mercury, further adding to their eco-friendly credentials. For governments and businesses, the need to change light bulbs much less frequently provides a further cost savings in reduced maintenance.

There is more to be gained than just switching to lower-energy lamps

We are all encouraged to switch off appliances when they are not in use, even to the point of being told not to leave TVs and the like on standby, and to unplug or switch off phone chargers at the wall when the phone is no longer connected. This is an attempt to save the residual energy that consumed by these devices, which can be quite apparent if you have an energy monitor in your home. But for some, the inconvenience of having to get up to switch on the TV or plug in a charger outweighs the cost saving, which is why the industry still needs to address these types of issues more effectively.

The same arguments apply to lighting. There is only so much energy that saved by switching to more efficient sources. While LEDs will certainly enable substantial energy savings on a global scale, especially compared to incandescent lighting, there is a lot more we can do to ensure we do not waste energy by having lights on unnecessarily. However, this does not just come down to personal responsibility. Much can be done to automate the control of lighting in both an environmental- and human-friendly manner. Potential solutions, using an appropriate combination of sensors and control/driver circuitry, can be as simple or as sophisticated as required for a given application. The challenge for designers is often less about implementation but more about identifying the components that can get the job done quickly and effectively. Below we will consider a number of products available from Hotenda that may help achieve this.

Switching lights on and off in response to movement is something with which we are all familiar. Typically, this is done with PIR sensors, and the most common usage is for outdoor security or courtesy approach lighting. The same principles can be applied within buildings, so lighting can be switched on when someone enters a room and either turned off or dimmed when there is no longer anyone present, usually after a pre-set timeout period. However, as we have probably all experienced, simple PIR solutions do not always behave as expected, e.g. the after-hours office worker who is plunged into darkness simply because he sat too long at his desk without moving.

This is where products such as Zilog’s ZMotion Detection Module offer a more flexible solution, reducing design effort and eliminating development risk. The module is a small circuit board (just 25.5 mm x 16.7 mm and less than 10 mm thick) that combines a pyroelectric sensor and low profile lens with Zilog’s Z8FS040 Motion Detection Microcontroller. The device can either operate standalone, where it simply outputs a signal when motion is detected, or as part of a larger system, communicating with other processors via a serial interface. Apart from the communications aspect, the primary benefit of incorporating a microcontroller (MCU) is the ability to operate advanced software-based motion detection algorithms, e.g., automatically increasing sensitivity after motion is detected. The MCU also makes it easy to set the sensitivity level and output activation time, as well as support for ambient light sensor input. With a low-power sleep mode, we can be confident this device is not wasting any of the energy we are trying to save. More details of this product can be found in the module brief.

Figure 3: Zilog’s motion detection module combines PIR sensor, lens and microcontroller.

The requirement for an ambient light sensor (ALS), as mentioned above, is to ensure that lighting is not activated when ambient light levels are already sufficient. Apart from their use in conjunction with PIR sensors, they are commonplace in street lighting where, rather than timers, they can more effectively cope with the changing times of dawn and dusk throughout the seasons, and even allow for the differences due to fine or overcast weather conditions. Ambient light sensors also have a role to play in managing interior lighting. The office environment is a good example of where these sensors can be effectively deployed to balance natural and artificial lighting.

It is not unusual to find offices constantly illuminated with fluorescent strip lighting throughout the day, regardless of whether there is daylight illumination from windows. While ALS devices could be used to switch off some or all of the lights located near windows, lights further away from windows would probably still need to remain on. The ideal solution would be lighting that can provide fully variable illumination levels controlled by ALS devices. This has always been a problem with fluorescent lights, which do not readily lend themselves to being dimmed – even specifically designed dimmer circuits only allow a limited level of dimming before the lights go out. LED lighting on the other hand is fully dimmable and can be partnered with ambient light sensors, either built into the individual luminaire or used to control defined lighting zones as part of a more comprehensive lighting control system.

The high accuracy ambient light sensor from OSRAM (SFH 5711), shown in Figure 4, is the perfect solution for such applications as it has a logarithmic current output, matched to the sensitivity of the human eye. It also has a low temperature coefficient of spectral sensitivity, ensuring high accuracy over a wide illumination range. As well as use in general lighting applications, this device is also aimed at automotive applications to control headlights (automatically switching them on in low light or poor visibility conditions) and display backlighting levels (which need to be brighter under high ambient light levels).

Figure 4: OSRAM’s hybrid ambient light sensor is matched to human eye sensitivity.

Intersil offers an interesting device (ISL29029) that simultaneously measures both ambient (i.e. visible) and infrared (IR) light using two independent ADCs. It is billed as an “ambient light and proximity sensor” since it uses the IR channel to measure proximity determined by the level of light reflected from an external infrared LED. Unlike PIR sensors, which detect movement as the result of a change in temperature when a warm body enters the field of view, this form of proximity sensing can be used to control light levels according to the range of the detected object. Consequently, this device is more likely to find use in more specialized lighting applications, e.g., industrial or medical.

Figure 5: Intersil combines ambient light and IR proximity detection in a single sensor.

We have seen how various sensors can be used to automate lighting, but we also need to understand how to control these LED lights, i.e. to switch them on or off or provide a dimming function. There are many potential solutions, ranging from simple discrete circuits comprising as few components as two transistors and two resistors, through to quite complex integrated circuits. A device such as Diodes’ ZXLD1374 LED driver/converter is a good example of an IC solution that can accurately control the current through a string of up to sixteen series-connected LEDs. It employs a multi-topology converter that operates in buck, boost and buck-boost configurations to provide the most efficient operation from input voltages up to 60 V with LEDs running at up to 1.5 A. This LED driver supports dimming over a wide dynamic range, either 20:1 using just a DC input voltage to set the LED current, or 1000:1 using an input waveform to pulse width modulate the output current. Other features of this device include a temperature adjust input for LED thermal control, e.g. using a thermistor to reduce the output current when the lamp temperature exceeds a preset threshold.

Figure 6: Diodes driver provides PWM dimmable control for 16 series-connected LEDs.

All of the things we have looked at so far play a role to play in “smart lighting”, but, as the term implies, this requires some level of intelligence in our lighting solution, rather than the ad-hoc control of individual lights. This role typically falls to the ubiquitous microcontroller (MCU), although these days MCUs are no longer the general-purpose devices they used to be. Increasingly, the peripheral circuit functions integrated on the chip are tailored to specific applications. So it is with lighting control, as witnessed by the Atmel AT90PWM316 8-bit MCU that includes power stage controllers with PWM capability for LED driving and dimming. It also includes a fully programmable USART for serial communications that supports a specific mode for DALI (Digital Addressable Lighting Interface), a standard for the network control of lighting in buildings. As can be seen from Figure 7, this Atmel device is a small form-factor IC, but still includes all the regular peripheral blocks: ADC, DAC, comparators, timers, interrupt unit, among others. Together with 16 k bytes of Flash program memory, this is a truly versatile device for lighting automation projects.

Figure 7: Atmel’s 8-bit controller enables low-power, low-cost lighting automation.


Summary

This article investigates some of the technology that can be used to automate lighting systems. The intent is to realize further energy savings beyond the benefits obtained from simply replacing incandescent and fluorescent light bulbs and tubes with efficient LED lamps. Devices available to do this fall broadly into three categories: sensors that tell us when we need to switch lights on or off or vary their brightness, driver circuits that make this happen by controlling the current through the LEDs, and microcontrollers that provide overall intelligence, along with network communications, to enable a complete lighting solution. This has been illustrated with product examples that are both representative of these functions, but also aim to demonstrate ease of implementation.

References:
  1. ROHM “LED Fundamentals” (1) & (2)
    http://micro.ROHM.com/en/products/lighting/led/index.html
    http://micro.ROHM.com/en/products/lighting/led/index2.html
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