First investigation into wireless weather sensor Station
A quick summary of parts and functionality involved in developing your own wireless mesh network based sensor systems.
Wirless Mesh Network Based Remote Sensing
So it seems the equipment being used currently for remote sensor logging is a little bit limited. The interfaces are not open so the systems cannot be modified for customization, and there's no ability to use mesh networking. Also Cell coverage is spotty and expensive to have for each station. Here's what I would do.
Zigbee (802.15.4)
Zigbee is a great protocol that has been around for a while now. People have know that mesh networking was the future for a while now and have been trying hard to develop protocols and devices that allow an end user device to exploit the benefits of mesh networking technology without having to deal with all of the underlying complexity involved. A good example is this simple little $19 Zigbee Module. Here's the datasheet
, and I will list some of the key parameters below:
- 100m Outdoor Range (This can be extended to 1000s of m for a higher power cost).
- RF Data Rate of 250Kbps. Seems plenty for sending around some charaters of Sensor Data.
- 3.3V Supply. This means it can be run off a typical Lion battery, which should also be easy to re-charge with Solar.
- Transmit current: 45 mA (@ 3.3 V)
- Receive current: 50 mA (@ 3.3 V)
- Power-down sleep current: <10 µA
Analyzing the power draw numbers above, IF we wanted to have a device Tx/Rx at all times we will need about (45+50) * 24 mA hours of power for a power source. I'm not sure what you get out of a typical Solar Cell, but I'm sure we can find something to fit the bill easily. This will be investigated further later. as you can see <10uA of standby current means if we send data in bursts, it shouldn't be too much of a problem.
While 100m seems pretty good for a mesh network, the reality is that either large clumps of tightly packed sensors, or individual sensors themselves may have to be spaced over MUCH greater distances. The simple answer is to use a more fancy $179 longer distance mesh network adapter.
Zigbee allows you to configure stations to be either low power simple nodes, or administrators which can forward messages etc. Here are some of the key parameters for this particular extender:
- Power output: 1 mW - 1 Watt (0 - 30 dBm), software selectable
- Indoor/Urban range: Up to 3000 ft (900 m)
- Outdoor/RF line-of-sight range: up to 40 miles (64 km)
- RF data rate: 9.6 or 115.2 Kbps
- Supply voltage: 2.8 - 5.5 VDC regulated
- Transmit current: 730 mA (@ 5V, 1W TX Power Output)
- Receive current: 80 mA (@ 5V, 1W TX Power Output)
- Shutdown power current: 5 µA typical
It is easy to see that this device will use MUCH more power for transmitting as you turn the power level up to support the longer distance. Basically we can make stripped down nodes with NO sensors to function as relay devices, or still populate them with sensors as individual long distance sensor stations.
Next up I will review the Solar Power and Battery needs for the system, and then jump on the sensors and uController to drive the system.
8/31/08 Update
I found a company that appears to make cool mesh networking boards and gateways that might be worth further investigation. Also a wicked case study power point document.
-Tom
