Sensors for Smart Lighting

At the heart of a smart lighting system is the sensor. Detecting changes in light and environment is a key part of developing a power-efficient lighting system for homes, offices and the outside environment.

Lighting is responsible for over one fifth of the energy produced around the world. With over 3.5 TWhours of energy needed every year, adding sensors to save even a few percent of consumption would eliminate the need for hundreds of power stations and millions of metric tons of carbon emissions.

Smart lighting aims to do just that, adding sensors to the not-so-humble light bulb to help reduce the global power bill. As the world moves from the power hungry incandescent bulb and inflexible fluorescent tube to more efficient LED lighting, there are more opportunities to control lighting depending on changes in the environment.

There are many ways to do this with a wide range of different sensors. Some of these approaches can be as simple as a photodiode or as complex as ultrasonics and there are many different ways to implement such systems. There are also moves to bring such systems together and develop open standards that drive both the lighting and smart control industry forward.

Different areas of lighting

The power used for lighting is split between residential, commercial, industrial, and outdoor lighting, with each sector having its own particular needs and requirements with different sensors for each sector. Residential lighting, with an energy requirement of just over 1 TWh, needs a low-cost simple solution that can be easily retrofitted such as using the ambient light sensors of simple proximity sensors. Manufacturers are moving to more efficient high-power LED lighting to help save power, but there are also opportunities to reduce the power consumption further by using these sensors. If the light can detect when there is no one in the room, it can switch itself off, saving money and power. This can be done in a number of different ways.

Commercial lighting in shops and offices consumes 1.5 TWh while industrial units use 630 TWh. These can make use of more complex smart lighting systems such as passive infrared or ultrasonics that can cover larger areas. These also include wireless networks with radio receivers to link lights together and even connect them to the Internet. This allows centralized processing to switch the lights on and off depending on how many people are around and the commercial requirements. This can also lead to the more futuristic approaches where the lights follow you as you walk around a building – the ones behind switch off as you pass and the ones in front switch on, so you can see where you are going. These lights can all be linked via low-cost LED emitters and sensors or even by modulating the output of the main light with low-bit-rate data and using a photodiode sensor to pick up the signal.

Outdoor lighting, in streets, car parks and even traffic lights, takes around 300 TWh around the world but has more safety requirements. The impact of such power consumption is clear when local government in the UK, for example, is switching off the traffic lights at night to save money.

Sensor technologies

Some of the simplest ways to implement smart lighting come from the innovative use of sensors. Ambient lighting sensors, proximity sensors and even photodiodes can be used for smart lighting.

Ambient light sensors can be used to switch main lights off when there is sufficient illumination available, but it is important to have a sensor that is aligned with the sensitivity of the human eye. These devices also benefit from being used in other high-volume applications that bring down the cost of the sensor, particularly controlling display backlighting. With millions of sensors being used in the backlights of cellphones, PDAs notebooks and tablets and even TVs, video cameras and Digital Still Cameras, the cost comes down to a few cents.

The APDS-9007 from Avago is an analog current output Ambient Light Photo Sensor, packaged in a miniature lead-free surface mount package that can be used for both residential and commercial lighting control. It provides a logarithmic response over a wide dynamic range of 3 lux to 70 K lux and has a low sensitivity variation across various light sources. This means it is well suited for applications that operate under high ambient brightness.

Similarly the PDV-P9008 from Advanced Photonix is a CdS photocell designed to sense light from 400 to 700 nm and switch at a value of 10 lux. It comes in a range of different resistance values to suit different system designs. These are simpler devices often used for night-light detectors but can be used for larger systems; and while they do not switch particularly fast (60 ms) this is not an issue with such smart lighting systems.

The Everlight ALS-PT19-315C/L177/TR8 provides a phototransistor in a miniature SMD package that has a high rejection ratio of infrared radiation and so the spectral response of the ambient light sensor is close to that of human eyes. It comes from cellphone backlighting applications but can also be used for automatic management of residential and commercial lighting and ambient-light monitoring devices for street lighting.

There are additional levels of integration that help reduce the complexity of the system. An example of this is the OPT101 from Texas Instruments, which is a monolithic photodiode with an on-chip transimpedance amplifier where the output voltage increases linearly with light intensity. The 8-pin amplifier is designed for single or dual power-supply operation, allowing a small, simple implementation.

The integration of the transimpedance amplifier and photodiode on a single chip eliminates the problems commonly encountered in discrete designs, such as leakage current errors, noise pick-up, and gain peaking due to stray capacitance.

Proximity sensors

Proximity sensors are another good way to implement a simple smart lighting system, detecting when people are present and providing the appropriate areas of light (see Table 1). The PING ultrasonic distance sensor 28015 from Parallax provides precise, non-contact distance measurements from about 2 cm to 3 m. It is very easy to connect to microcontrollers, requiring only one I/O pin. It works by transmitting a short 40 kHz ultrasonic burst, well above human hearing range, and providing an output pulse that corresponds to the time required for the burst echo to return to the sensor. By measuring the echo pulse width, the distance to target can easily be calculated.

Passive infrared sensors are another way of providing the signal for smart lighting. Zilog provides a module, the ZEPIR0AAS02MODG, which is a complete, fully functional motion detection solution including a low-profile Fresnel lens. This includes the microcontroller and advanced software based motion-detection algorithms, which provide superior sensitivity and stability within a wide 5 m x 6 m, 60-degree detection area. It can also use a photocell input for ambient-light detection in lighting applications and requires no temperature compensation. The surface-mount pyroelectric sensor and Fresnel lens combine to provide the lowest possible profile, allowing a module that is only 25.5 mm x 16.7 mm and so can easily fit into many size-constrained applications, particularly important for lighting.

The signal from the pyroelectric sensor is sent directly to the ZMOTION microcontroller, providing it with the true, unaltered signal. This allows the software to identify and react to false trigger sources such as drift, EMI and ESD, therefore providing a more stable motion detector. This direct interface also eliminates the need for external components, including large electrolytic capacitors, therefore improving reliability and reducing the cost.

Control over sensitivity and output timing is provided through a simple hardware interface. More advanced settings and status are available through an asynchronous serial interface mode.

	Hybrid or Combination Sensors	Integrated Daylight Sensors	Wall Switch Sensors	Wall or Ceiling-mounted Sensors	Specialized Sensors
Combo of	Two or more technologies to minimize false detection, usually PIR and ultrasonic, or PIR and audio	PIR or ultrasonic sensors with a light-level sensor.	PIR, ultrasonic, or combination/hybrid sensor and control circuitry packaged into one unit, sized to fit in a standard wall box.	PIR, ultrasonic, or hybrid sensors designed to be mounted separately from the control unit(s), usually in high locations.	PIR or ultrasonic sensors designed specifically for bathrooms, hallways, and stairwells.
Advantages	Can be very foolproof, allowing wide coverage and applications.	Can be wired to a dimming circuit to control room lighting based on available light and occupancy.	Small, inexpensive, and easy to install.	Can cover wide areas effectively; switching units can control a variety of equipment.	Specifically designed for these spaces.
Disadvantages	They can be more expensive (for small area applications), and may require more adjustments since sensors contain more than one sensing unit.	They can be difficult to adjust and require a dimming ballast or special wiring.	Their range can be limited, and depending on the location of the switch, they can easily be obscured.	They tend to be more expensive and often necessitate rewiring.	Rewiring may be necessary if certain lights need to stay on.
Recommended for	Large, open areas and areas with unusual occupancy patterns or work requirements.	Areas that receive large amounts of daylight.	Smaller meeting rooms, individual offices, and storerooms.	Large areas.	Specialized areas.

Table 1: Available types of occupancy sensors. Source: US Department of the Interior.

Of course you need other components to support the sensors in smart lighting, from low-cost low-power microcontrollers to DC-DC and AC-DC converters with more controllability.

NXP supports a total 'smart lighting' solution with its GreenChip range of devices. These range from high efficiency, low standby power converters, CFL, solid state LEDs (SSL) and TL drivers to high-performance low-power IEEE802.15.4 compatible single-chip transceivers and microcontrollers and the main lighting-relevant software stacks like Zigbee and JenNet.

Standards

This highlights the range of different architectures that are available for smart lighting. However, there are also moves to standardize interfaces to the sensors to create more standard architectures where a wide range of devices can all work together.

The Semantic Interoperability Architecture for Power Management of Smart Lighting is being developed at the University of Eindhoven to tackle this as part of a European project called WASP.

This divides a building into high-priority rooms that are allowed to use power according to whatever demand there is at that time and low-priority rooms that are obliged to use the power that is left over. This means the high-priority rooms use power-measuring lamps, light sensors, and motion sensors so that the light output changes depending on the activity in the room.

The low-priority rooms have to rely on smart lighting using a proposed standard called the Open Service Architecture for Sensors, or OSAS, which is an architecture for programming a network for sensors and actuators.

The researchers designed a platform that allows sensors to be reprogrammed over the network by translating a single program (for the entire network) into a set of configuration messages and by the code that is run by a small interpreter on a local microcontroller.

Conclusion

There is a tremendous drive towards finding innovative smart lighting solutions; and sensor technology is at the heart of it. Combining the appropriate sensors with microcontrollers and power management can produce cost-effective low-power control systems that save both power and money. Discrete ambient light sensors and photodiodes integrated with other parts of the system such as amplifiers and microcontrollers offer designers smaller systems and lower costs, while other technologies such as ultrasonic sensors and passive infrared can be used for longer range sensing and larger rooms.

By reducing the energy requirements of one of the world’s largest consumers, system developers, building operators, consumers and ultimately the environment all benefit through less power stations and lower carbon emissions. Small, simple changes that bring about smart lighting can have significant impact around the world.

References:

1. http://www.doi.gov/whatwedo/energy/index.cfm
2. http://www.win.tue.nl/san/wsp/activities.html