Accurate, self-sufficient wireless temperature sensor design

Rather than facing the challenges and high costs of running cables throughout a factory, it is now possible to install reliable, industrial-strength wireless sensors that can operate for years on small batteries or by harvesting energy from available sources such as light, vibrations or temperature changes.

Linear Technology provides all the components needed to design high performance, reliable, low power wireless sensor networks. The example described in this article is a real-world design that integrates a high-resolution temperature sensor, a power management circuit powered by solar energy (when available) and a backup battery (when needed), and a low-power radio module that automatically forms a reliable mesh network that wirelessly connects all sensors to a central access point.

Design Overview

Figure 1 shows the block diagram of the design. The temperature sensor is based on a thermistor that is biased by the low noise LT6654 voltage reference. The 24-bit ΔΣ ADC LTC2484 reads the voltage of the thermistor and reports the reading through the SPI interface. The LTP5901 is the radio module that contains not only the radio unit but also the networking firmware required to automatically form an IP mesh network. In addition, the LTP5901 has a built-in microprocessor that reads the LTC2484 ADC SPI port and manages the power sequencing for the signal chain components. The LTC3330 is a low power, switch mode dual output power supply that is powered by a solar panel when sufficient sunlight is available and by a battery when insufficient sunlight is required to keep the output voltage regulated. The LTC3330 also contains an LDO to set the duty cycle of the temperature sensor power supply.

Figure 1: A wireless temperature sensor is constructed by connecting a radio module to an ADC, reference, and thermistor. The circuit is powered by an energy harvester that can draw power from a battery or solar panel. (BATTERY: battery; SOLAR PANEL: solar panel; DUTY CYCLED: set duty cycle; WIRELESS NETWORK: wireless network; THERMISTOR BRIDGE: thermistor bridge)

Signal Chain

This design uses a thermistor to measure temperature. Thermistors are well suited for reading temperature values ​​in temperatures far beyond the typical ambient temperature range of interest. A thermistor is a resistor with a large negative temperature coefficient. For example, a thermistor with part number KS502J2 (as specified by US Sensor) has a resistance of 5kΩ at 25°C and changes its resistance from 88kΩ to 875Ω over the temperature range of -30°C to +70°C.

The thermistor is connected in series with two accurate 49.9kΩ resistors and biased by the LT6654 precision voltage reference (Figure 2). The LTC2484 ΔΣ ADC measures the voltage divider ratio of the resistor divider with 24-bit resolution. The total unadjusted error of the ADC is 15ppm, which corresponds to a temperature uncertainty of less than 0.05°C for the thermistor slope used in this application. The specified temperature accuracy of this thermistor is 0.1°C, so no calibration is required to measure the temperature to this accuracy.

Figure 2: Using the LTC2484 24-bit ADC to read thermistor voltage. Because the input common-mode voltage is centered, the Easy Drive ADC does not draw input current, making it easy to obtain accurate ratiometric readings. (3-WIRE SPI INTERFACE: 3-wire SPI interface)

The ADC has a noise of less than 4μVp-p, which corresponds to a temperature change of less than 0.005°C. Therefore, with calibration, this system can be used to measure temperature with extremely fine resolution. Since the ADC measures the ratio of the thermistor voltage to the reference voltage value, strictly speaking, the reference voltage does not need to be accurate. However, it must be low noise because variations in the reference voltage can cause errors when the ADC is converting.

The LTC2484 ADC uses an Easy Drive input structure. This means that the net differential sampled current is close to zero during conversion. Therefore, the input sampled current flowing through the resistive thermistor network does not cause any measurement error, which means that a separate op amp buffer is not required. The bypass capacitor provides a low impedance path at high frequencies. In many cases, the temperature does not need to be measured constantly, but only once per second or even once per minute. It makes sense to save power when the system is not measuring temperature. This application circuit does just that, as described below.

The resistor network draws a maximum of 25μA from the 2.5V reference. To avoid power loss between measurements, the reference supply is duty cycled to be on only during measurements. The RC time constant of the ADC input is approximately 5ms. By turning on the power supply 80ms before taking a measurement, the ADC input is ensured to be fully settled. In practice, since both input nodes turn on at the same slope, it takes far less than the theoretical settling time for the reading to be accurate. The LT6654 is powered by the 3V LDO output of the LTC3330. The LTP5901 microprocessor drives the enable pin of the LDO in the LTC3330 high and low at the appropriate times before and after taking a temperature reading. The LTC2484 automatically enters sleep mode when not converting. The 1μA sleep current is low compared to the already low power of the radio. Therefore, it is not necessary to set the duty cycle to the ADC supply. By keeping the ADC's supply voltage the same as the LTP5901, the logic levels on the SPI interface are ensured to remain constant, which helps to achieve a simple design.

After providing the conversion result through the SPI port, the LTC2484 automatically starts a new conversion and stores the conversion result in its internal register until the user requests to read the conversion result again. This mode of operation is very convenient in systems that need to read temperature values ​​very frequently. However, some ultra-low power applications may wait a long time between two readings. In order to ensure that the temperature data provided to the user is always a "fresh" reading, such applications first toggle the CSb and SCK pins to remove the "old" temperature reading from the ADC register, and then automatically start a new temperature conversion. The microprocessor waits until the conversion is completed and then reads the result through the SPI port. Even though the new temperature reading process will start automatically again, the system will then shut down the thermistor network (by turning off the LDO) because these additional temperature readings will then be ignored.

The total power consumption of this temperature sensor circuit can be estimated as follows. First, sum the currents of the reference (350uA), the thermistor network (25μA), and the ADC (160μA during conversion), which gives a total current of 535μA (see Table 1). Then, consider how long this current is sustained. The ADC takes about 140ms per conversion, and we wait 80ms before each conversion to allow the reference and thermistor to stabilize. Add in some SPI reading time, and the turn-on time is about 300ms. 535μA consumed in 300ms corresponds to 160μC of charge. To this charge we should add the charge required to charge the 4.7μF power supply bypass capacitor to the voltage reference, since this node is charged from 0V to 3V during each reading. Add this 14μC charge, and the total charge required for each temperature reading is 174μC. If the temperature data is read every 10 seconds, the average current consumption can be calculated to be 17μA. Other examples of average supply currents are given in Table 2.

Table 1: Signal chain current consumption (operating)

Table 2: Average current consumption of the signal chain for power management based on temperature reading frequency

The LTC3330 manages all power for this application. The chip contains two switch-mode power supplies and a linear regulator in a small monolithic package. A buck-boost converter draws power from the battery to maintain a regulated output voltage (set to 3.6V for this application). A separate buck converter draws power from the solar panel and also regulates the output voltage to the same value. An internal prioritizer ensures that solar power is used whenever possible, drawing power from the battery only when needed (Figure 3). For other applications, the LTC3330 also supports AC energy harvesting sources, such as piezoelectric crystals that generate an AC voltage proportional to vibration energy (see Figure 4).

Figure 3: The LTC3330 takes power from either a solar panel or a battery, automatically prioritizing the two power sources to maintain a stable output voltage. An additional LDO output is controlled by a logic input pin, which is used to set the duty cycle of the temperature sensor power supply. The LTC3330 generates an output flag to indicate whether solar power or battery power is being used. (SOLAR PANEL: solar panel; BATTERY: battery)

Figure 4: The LTC3330 Energy Harvesting DC/DC Battery Life Extender harvests energy from piezoelectric, solar or magnetic sources. Drawing less than 1μA of quiescent current, the LTC3330 is well suited for this low power wireless application. The power supply consumes only a small fraction of the total power, so most of the power is available for the “load” (i.e., the temperature sensor and wireless network).

In addition to the two switch mode supplies, the LTC3330 also includes an LDO with a separate enable pin. This feature is very useful for this type of duty cycle application. The voltage reference and thermistor network are powered from this LDO. This not only reduces switching noise, but also allows the application to switch the signal chain power on and off while keeping the radio module power always on. Even though the radio module does not consume much power between transmissions, it must always remain biased to keep the timer running correctly so that the entire network can stay in time synchronization. The microprocessor inside the radio module sequences the LDO enable pin at the appropriate time to prepare the signal chain for reading the temperature data.

The LTC3330 provides an output flag (EH_ON) that indicates whether the system is powered by the battery or the solar panel. Having real-time access to this information can be important to the end user. Therefore, we have the microprocessor in the radio module read this output flag and transmit this information over the network along with the temperature data. The logic level of the EH_ON output is an internal bias voltage for the LTC3330 that varies depending on the operating mode and can be higher than 4V. Rather than connecting this output pin directly to the lower voltage radio module logic input, we divide it down and feed it to a built-in 10-bit ADC that is part of the microprocessor. In this case, we simply use this ADC as a comparator to indicate which power supply the LTC3330 is using.

Wireless network

The LTP5901 is a complete radio module that includes a radio transceiver, an embedded microprocessor, and networking software. Its physical design consists of a small printed circuit board that can be easily soldered to the main circuit board containing the rest of the application (signal chain and power management).

In this application, the LTP5901 performs two functions: wireless networking and housekeeping microprocessor (Figure 5). When multiple LTP5901 nodes are powered up near a network manager, they automatically recognize each other and form a wireless mesh network. The entire network is automatically time-synchronized, meaning that each radio module is powered only for very short, specific time intervals. As a result, each node can function as both a source of sensor information and a routing node to forward data from other nodes to the manager. This allows a highly reliable, low-power mesh network to be established, even if all nodes (including routing nodes) operate at very low power, with multiple paths available from each node to the manager. This radio technology has a typical node-to-node transmission range of 100 meters, and can reach even longer distances in favorable outdoor conditions.

Figure 5: The LTP5901-IPM requires very few connections to run the entire application. All wireless networking functions, including firmware and RF circuitry, are already built into the module. A 3-wire SPI master communicates with the LTC2484’s SPI port. A GPIO pin (DP2) controls sensor power sequencing. The built-in ADC acts as a convenient level translator to read the energy harvesting status flag, EH_ON, from the LTC3330.

The LTP5901 contains an ARM Cortex-M3 microprocessor core that runs the networking software. In addition, this core can be programmed by user-supplied firmware to perform tasks specific to the user's application. Therefore, many applications can be implemented without any third-party microprocessor. In this example, the microprocessor inside the LTP5901 manages the power sequencing of the temperature sensor by turning on and off the LTC3330's LDO at the appropriate time to save power between two temperature readings. The LTP5901 communicates directly with the SPI port of the 24-bit ADC, which reads the temperature value provided by the temperature sensor. Finally, the LTP5901 reads the power status output flag (EH_ON) from the LTC3330, which indicates whether solar energy or battery is used to power the circuit.

The power consumption of the radio modules can be estimated using the SmartMesh Power and Performance Estimator tool available online at Linear Technology. For a typical network of 20 nodes, 10 of which are wirelessly connected directly to the manager (1 hop) and the other 10 are indirectly connected to the manager (2 hops), the average power consumption for the 2-hop nodes is about 20μA and for the 1-hop nodes is 40μA. These figures are based on the assumption that each node reports temperature data every 10 seconds. The reason that the 1-hop nodes consume about twice the power is that they not only send their own sensor data, but also act as routing nodes, forwarding some of the sensor data of the 2-hop nodes. This power consumption can be further reduced by a factor of two if a feature called "Advertising" is turned off. Once the "advertising" feature is turned off, the network no longer recognizes new nodes that want to join the network. Other than this difference, turning off the advertising feature has no effect on the operation of the network.

Overall power consumption

The total power consumption of the complete application circuit varies depending on various factors, including how often each sensor measures temperature and how all nodes are configured in the network. For a sensor node reporting temperature data every 10 seconds, typical power consumption is less than 20μA for the sensor portion and perhaps 20μA for the radio module portion, for a total average load current of about 40μA.

A small 2" x 2" solar panel (such as the Amorton series) can produce 40μA even in relatively moderate indoor lighting conditions (200 lumens), and can produce much higher currents in bright light conditions. This means that in many conditions, this application can run entirely on solar panel power. If the circuit is in the dark and needs to run entirely on battery power, a 2.4Ah AA battery (such as the Tadiran XOL series) can power the application for almost 7 years. In low or variable light conditions, the circuit automatically switches back and forth between solar power and battery power to maximize solar power and extend battery life.

in conclusion

Linear Technology's signal chain, power management and wireless networking products can be used to implement the design of complete, true wireless sensor network products. This time-synchronized wireless mesh network ensures that data is reliably transmitted between nodes using minimal power. The built-in microprocessor can set the duty cycle of the sensor circuit power supply. Highly efficient, highly integrated power management ICs can completely power the application with a small solar panel, or a small battery can power the application for many years.