Transmit humidity and temperature data with a fully functional, off-the-shelf, battery-less wireless sensor node.

Bluetooth low energy (BLE) beacons have been around for a while and can be seen everywhere. Normally, they run off of a battery or a line-connected power supply like a USB port, but not always. The prospect of a solar-powered BLE beacon device is intriguing. In this article, we explore such a device from Cypress Semiconductor.

A BLE beacon is a device that only advertises. That is, the transmission is in only one direction. Unlike other BLE peripherals, they are not connectable by design. Instead, they simply advertise a very brief message. Generally, they are not continually transmitting the message, they transmit the message at intervals. The shorter the interval between transmissions, the greater the power used by the beacon.

BLE beacons are always short-range devices and their range is largely a function of the transmission power. Therefore, power requirements and, consequently, battery life, can be estimated by the power of the transmission and the interval between transmissions. Oftentimes, battery life for a beacon is quite lengthy and can be measured in months.

A received signal strength indicator (RSSI), from the receiver side, can be an important part of the transmitter-receiver relationship. The RSSI value can be used to provide a measure of the distance between the beacon and the receiver. For example, the beacon can transmit the RSSI at one meter. The receiver can then use its own RSSI in conjunction with this transmitted RSSI to get a rough measure of distance (see here for more on calculating distance from a beacon).

A typical example of using proximity is when an establishment is able to deliver targeted messages through a smartphone application depending on where you are in the store. See this link to view ways the NBA champion Cleveland Cavaliers use beacons (can you guess which NBA team I root for?).

In addition to commercial advertising, beacon technology has also been used in healthcare. It seems that, in certain instances, “people tracking” can be implemented with BLE beacons more cheaply than GPS and with less privacy invasion than cameras.

 

A Solar Beacon

The Solar Beacon from Cypress Semiconductor is about the size of a US quarter.

 

The solar beacon from Cypress Semiconductor uses beacon technology to transmit temperature and humidity measurements embedded in the advertising transmission. So, in addition to the aforementioned uses, these devices function as short-range, wireless sensor nodes.

These beacons are available in two kits:

  • CYALKIT-E02 Contains one solar beacon and one BLE-USB Bridge (Debug Board) that can be used to program beacon parameters and more.

  • CYALKIT-E03 Contains five solar beacons.

 

What’s Inside?

A look under the hood of a solar beacon (click to enlarge). Image courtesy of Cypress Semiconductor.

 

  • Humidity and Temperature Sensor - Si7020-A20. The low power sensor features an I2C interface for precision digital temperature and relative humidity measurement. While the chip provides 16-bit sensor values (with 14-bit resolution), for both temperature and humidity, the values are truncated to 8 bits and embedded into the advertisement transmission.
  • EZ-BLE PRoC Module - CYBLE-022001-00. The brains of the beacon are contained in the BLE module. This low-power device includes a 2.4 GHz BLE radio, an ARM Cortex-M0 CPU core, and a royalty-free BLE (4.1) stack. The beacon comes pre-programmed with software to transmit temperature and humidity. 
  • Energy Harvester - S6AE103A. If the EZ-BLE PRoC Module is the brains of the beacon, then the S6AE103A is the heart. This is a power management IC (PMIC) specifically designed for use with solar cells. The PMIC stores power generated by solar cells in a capacitor, and it also controls how power is delivered to the load. If the solar cell is not generating sufficient power, the chip can deliver load current from the storage capacitance or from a battery. The S6AE103A also contains an LDO regulator that provides a stable voltage for sensors or other peripherals.
  • Solar Cell- AM-1606C-MEL. The beacon uses a Panasonic 15 mm × 15 mm amorphous silicon solar cell. The Maximum Power Point (MPP) for the device is 3.58 μW/cm2 @ 200 lux (VMPP = 2.6 V).
  • Capacitors -
    • The “system load” capacitor is made up of a bank of 4 × 100 μF chip capacitors.
    • The surplus energy storage capacitor is a 0.2 F DCK-3R3E204T614-E supercapacitor.
  • Slide switch - The beacon includes a slide switch to set operation to “demo” mode or “timer” mode. In demo mode, the supercapacitor cannot be fully charged, and the interval between transmissions is 3 to 60 seconds, depending upon ambient light levels of 1000 lux (or greater) to 100 lux, respectively. In timer mode, the interval between transmissions is fixed at 5.17 minutes and the supercapacitor is charged with surplus power from the solar cell, when available. With the supercapacitor completely charged, transmissions can occur for 30 hours without ambient light.

 

Beacon Format

The figure below illustrates the beacon transmission format. A general scheme can use the UUID to identify devices of the beacon family and the MAJOR ID to identify specific beacons. The temperature and humidity sensor values are contained in what is normally the iBeacon MINOR ID field.

 

Beacon advertising format (click to enlarge). Image courtesy of Cypress Semiconductor.

 

Debug Board

Debug board (click to enlarge). Image courtesy of Cypress Semiconductor.

 

Using the BLE-USB Bridge (Debug Board), the user can easily program elements of the Beacon’s advertisement (see table below). After connection of the board and a beacon, programming is accomplished using a USB terminal program such as the open-source software Tera Term; a version of Tera Term is included in the Solar Beacon software download package.

 

Debug board with a Solar Beacon attached.

 

Programming command list (click to enlarge). Image courtesy of Cypress Semiconductor (PDF).

 

Although I have not yet attempted to do so, the documentation indicates that the debug board along with PSoC Creator software can be used to directly program the EZ-BLE module using hex files.

When a beacon is connected to the debug board, it will charge the onboard supercapacitor; a half charge requires two minutes and a full charge requires “ten minutes or more.”

 

Making a Hybrid Sensor

With some "steady hand" soldering, you can easily make the solar beacon a hybrid by adding a 3-volt coin cell battery as shown in the figure below.

 

Wiring connections for attaching a coin cell battery. Image courtesy of Cypress Semiconductor.

 

Documentation is not as in depth on hybrid usage. Information from the Reference Design Kit Guide (PDF) shows a sample waveform for energy drive mode only (which I believe includes demo mode on the beacon); the documentation does not clearly state if battery backup operates in timer mode.

 

Software Suite

Several programs for use with the Solar Beacons can be downloaded for free from Cypress, including a PC-based utility, Cypress BLE-Beacon.

A screenshot of the program at work appears below. The contour map resulted from placing several beacons in various areas, including a cold window sill. While I made no attempt to adjust the underlying “dwelling location” map, you can get a good idea of the visual capability of the software.

 

Screenshot from BLE Beacon Software (click to enlarge).

 

In addition to the PC-based version, there are Android and iOS mobile applications. I tested out the mobile application (see below) on my Android device using three beacons, each at a site with different light levels. I checked the light levels using a simple BH1750FVI ambient light sensor with an Arduino UNO. Far from the rigorous level of a testing laboratory, I simply wanted to see if I could visually confirm different advertising intervals under the different light levels using indoor lamps.

Sensor 1 was at 702 lux, 3 was at 8150 lux and 4 was at 970 lux. All three beacons were set to demo mode. Although I am not sure that the precise transmission frequencies indicated in the Reference Design Guide were achieved (there are, admittedly, many variables), you can clearly differentiate the advertising intervals of the three sensors and they are positively correlated with light level.

 

Screenshot from the Android mobile application (click to enlarge).

 

Closing Thoughts

These solar beacons are well-documented and include a great deal of functionality within a very small footprint. They have off-the-shelf utility, and they also serve as a showcase for the included technology.

In the corresponding project, we will test drive the devices by building an Arduino-UNO-based multi-node temperature and humidity monitor.

 

Comments

1 Comment


  • ricsilvs 2017-04-28

    We live in interesting times. Nice solution for energy harvesting indeed.