Where are those Wireless Sensors?

-October 31, 2012

Recently we talked about who controls the thermostat in our home. This topic pushed me to investigate a solution that would help me position the controlling temperature sensor where the temperature needed to be controlled. Many wireless thermostats are available where the sensor could be located just about anywhere you like.

These systems have one thing in common: batteries, usually two AA. They also recommend high capacity, 3000 mAh, and lithium cells. These batteries are supposed to provide about one year of service before they need to be replaced. I thought I could just add it to my list when changing out all the other once-a-year batteries – like the smoke alarm, CO detectors, glass breakage, and remote controls.

When a device uses this size of batteries, it seems that the manufacturers take the opportunity to add other features like displays, buttons, and such. Although they may add some level of convenience, they also limit where you might place the device. I really was looking for something about the size of a thumb tack; even the size a quarter would do. I hoped to find a replacement for the wall thermostat, then use small remote wireless temperature sensors small enough to be discrete and out of sight. No such luck!

When you can’t find something on the market, usually there is a reason why. It could be that today’s technology just isn’t there yet, or the more unlikely reason is that no one has thought of it. There have been considerable publications on miniature smart sensors all the way down to smart dust, not that anyone in my house would want dust to be any smarter than it apparently is already. So why can’t I find a simple small wireless temperature sensor to connect to a thermostat?

What does it take to connect a wireless sensor to a thermostat? Some devices today can send a single Bluetooth low-energy (BTLE) packet every second, requiring an average current of about 25 µA during this period. If you increase the period to 15 seconds, the average current drops to less than 4 µA. Updating the temperature to once per 15 seconds for a room temperature control is within reason.

Powering up the sensor and computing the packet of information does not add appreciably to the 15 second average current. The average current for this function is less than 2 µA. This brings us to a total of about 6 uA. At 3.0 V input, we need about 18 µW average input power.

Figure 1 shows the maximum power delivered from various types of photovoltaic cells under typical office fluorescent lighting. Lighting illuminance is measured in Lux and measures luminance per unit area: one Lux is one lumen per square meter. For example, a typical desk surface may have 500 Lux, which provides 12 to 14 uW per cm2. However, ceilings and walls may have considerably less illuminance, typically 200 Lux. So in these situations the maximum power may be 5 to 7uW per cm2. These indoor harvesting values are expected to improve over the coming years and may approach 20 µW per cm2 by 2015.

Taking into account that lights are not always on, the total area for our 18 µW need may require as much as 10 cm2 of photo cells. In this case I assume the lights are on about 25 percent of the time. This is considerably bigger than our quarter-sized wireless sensor. The area of a quarter is about 4.6 cm2. We can always reduce the update time to 30 seconds. When the lighting drops down to very low levels, < 50 Lux, the power from the photovoltaic cells is disproportionately lower. So we can’t depend on very low light to help us gather more energy.

When using harvested energy to power our wireless temperature sensor, we need some storage. When combining power conversion and storage management, an integrated solution is best. Several IC solutions are available. Important parameters are its startup voltage, quiescent current, and efficiency. These devices also have other functions such as indications of low-energy storage and even autonomous switching between energy storage devices.

What if we use CR2032 instead of any harvesting? The capacity at 20°C and discharging at 18 µW is about 170 mAh when discharged down to 2.8V. To use this complete capacity, a boost converter is needed to provide a 3V output. The mWh capacity is about 490 mWh. At 18 µW average power, the coin cell could power our wireless temperature sensor for about three years. In this case, using a harvester might not be the better idea. Changing a battery every three years is not that inconvenient.

So why could I not find a wireless temperature sensor that is about the size of a thick quarter? I really don’t know. Maybe some marketing guy said I would be the only customer – ever. Maybe these systems always have to have a local display, so the coin cell could not power the total system long enough. Even accounting for conversion loss of 10 percent and quiescent current of 300 nA, I would only have to swap the battery every two-and-a-half years.

Let me know what you think about a peal-and-stick wireless temperature sensor that talks to your wall-mounted thermostat that is about the size of two quarters, one stacked on the other. It just may be the ideal solution for our family room!


Figure 1: Maximum power from various photovoltaic cells from fluorescent lighting.

For more information about this and other power topics, visit TI’s Power House blog: www.ti.com/powerhouse-ca.

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