Applications such as home automation, activity monitors, remote sensor nodes, and tire pressure monitors operate on small batteries and require long runtimes. The discussion of a wireless magnetic window alarm sensor system illustrates the challenge of achieving extended operation on small coin cells. This design solution demonstrates that a high-performance quad-band multichannel transceiver integrating a microcontroller and flash memory helps maximize the battery life of remote wireless sensors.
introduce
Battery life is a critical function for remote wireless sensors. Wireless sensors are becoming ubiquitous, penetrating the most diverse applications, such as home automation, activity monitoring, remote sensor nodes, and tire pressure monitoring, to name a few. Operating time requirements range from one to twenty years, posing significant challenges to the design of these mobile devices. This design solution will first review some of these runtime challenges. Secondly, it will discuss typical implementations of wireless sensor systems and their drawbacks. Finally, it will introduce a novel ultra-low-power RF ISM transceiver that helps maximize the battery life of remote wireless sensors.
Runtime requirements
For example, wireless magnetic window alarms and wireless locks are becoming increasingly popular in homes and hotels. They must operate on a single battery for at least a year. In other examples, remote meter readers monitoring water and leaks, earthquakes, gas, humidity, and temperature need to operate autonomously for up to 20 years. In each case, the remote system needs to be small and inexpensive, operating on very small batteries with limited capacity, typically using only a fraction of an ampere-hour (Ah). To meet such demanding operating time requirements, designers must use every coulomb in the sensor battery wisely.
Typical sensor system
A typical wireless magnetic window alarm sensor system (remote sensor subsystem and central control console) is shown in Figure 2. The 1.5V button battery powering the remote sensor subsystem is regulated by a boost converter. The remote sensor subsystem is in deep sleep mode most of the time, with the boost converter and transceiver off. Whenever the magnetic sensor is activated, the subsystem is awakened (by pull-up resistor R and transistor T), sends an alarm signal to the intelligent transceiver for processing, and then transmits the alarm to the central control console.
Figure 2. Wireless magnetic window alarm sensor system.
During the remote sensor's deep sleep mode, it needs to consume as little power as possible. During regular wake-ups and infrequent transmission periods, it can absorb short pulse currents up to 100mA.
The transceiver of the wall-mounted central control console communicates wirelessly with each subsystem in the building or on-site. Its transceiver communicates with the main controller CPU, which is responsible for controlling the entire system, via UART.
Battery life
Let's assume the coin cell battery has a capacity of 150mAh and a required lifespan of 2 years. The math is very simple: the sensor operation must be managed so that its average current consumption does not exceed 8.5μA (8.5μA x 365d/y x 24h/d x 2y = 150mAh)! There's no way to pass that.
Typical solutions
The typical solution currently available is to consume 1μA of current during deep sleep, which still consumes 12% of the total battery runtime! Clearly, every component in the sensor needs to be improved to reduce runtime losses to a more acceptable level. Ideally, if the remote sensor were kept an order of magnitude below this level (100nA), runtime losses would be reduced to a more acceptable 1.2%.
Ultra-low power transceiver
The MAX7037 sub-1GHz, ultra-low power RF ISM transceiver meets such stringent requirements. Figure 3 shows its functional diagram.
Figure 3. Functional block diagram of MAX7037
The MAX7037 is a high-performance, quad-band, multi-channel transceiver integrating an 8051 microcontroller, flash memory, and sensor interfaces. It supports the standard ISM bands from 300MHz to 930MHz and delivers up to 10dBm of output power. Its supply voltage range of 2.1V to 5.5V allows it to handle various energy sources, such as solar cells, electromechanical, or thermoelectric power. In Figure 2, the MAX7037 replaces each "transceiver + μC" module. Hardware-implemented transmit and receive routines enable the high-efficiency transceiver system for wireless fail-safe multi-band/multi-channel communication, featuring advanced FSK (Frequency Shift Keying) and ASK (Amplitude Shift Keying) protocol capabilities. A sleep mode allows for easy implementation of low-power applications with fast response times.
Ultra-low power solution for wireless units
In event-driven wireless sensor applications, the MAX7037 spends most of its time in deep sleep mode. In this mode, current consumption is 100nA (maximum). When an event is detected, the sensor typically signals the microcontroller via an interrupt. Short data packets are generated by the microcontroller and transmitted at a specified frequency. The data packet needs to contain some identifier of the sensor, such as flag bytes indicating its status. It also needs error detection or some kind of checksum or other error detection code applicable to the packet content. Furthermore, the first byte of the data packet indicates the number of bytes in the packet payload. Aside from requiring the first byte to indicate the length of the payload, there are no restrictions on the content of the packet payload.
The radio frequency used can be selected from standard sub-GHz ISM frequencies; for example, 315MHz or 868MHz. Depending on where the system is deployed in the world, the ISM band for that region should be used. If a frequency is available, the lowest frequency used in the building is ideal, as lower frequency signals propagate better through walls. However, interference factors may dictate the frequency choice.
Using the remote wireless magnetic window alarm sensor example we mentioned earlier, the MAX7037 reduced runtime degradation from 12% (3 months) to 1.2% (9 days)!
nanopower boost converter
The MAX17222 nanopower boost converter is also ideal for this application. With a minimum input operation of 400mV, it can draw the last drop of power from a 1.5V battery. It also offers a 0.5A peak inductor current limit and an optional output voltage using a single standard 1% resistor. Its novel True Shutdown™ mode produces nanoamp-level leakage current (0.5nA typical), making it a truly nanopower device.
Central console
The central control console is always on. The main controller CPU periodically polls the MAX7037 located in the same position to check for data packets. Upon receiving a data packet, the CPU confirms that the content is error-free; if so, it broadcasts an ACK (acknowledgment) data packet. When the MAX7037 in the remote subsystem receives an ACK with an ID, it returns to deep sleep mode. If a data packet from the subsystem contains an error, the main controller CPU will instead send a negative acknowledgment (NAK). If no ACK is received or a NAK is received within a certain timeout period, the subsystem controller will retransmit the original data packet. The described handshake protocol is a possible alternative method. When using FSK modulation, the MAX7037 can transmit data up to 125kbps.
in conclusion
We discussed the extremely demanding battery runtime requirements in many wireless sensor systems. We reviewed a typical wireless application system that wasted 12% of its battery life due to excessive leakage in deep sleep mode. We demonstrated how using the MAX7037 can reduce runtime loss from 12% (3 months) to 1.2% (9 days)! The MAX7037 features an ultra-low 100nA deep sleep current, significantly reducing leakage current and contributing to the long battery runtime required for such applications.
Conversely, the MAX17222 nanopower boost converter features a true shutdown mode, providing ideal power boost for the MAX7037 transceiver.
