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building_a_ukhasnet_node

An example sensor node, reporting Temperature and Battery Voltage To build a UKHASnet node, the key components are the Radio Transceiver to communicate with the rest of the network, and the Microcontroller that handles the packet generation and repeating. You may also want to attach as sensor, such as temperature, humidity, voltage, or GPS position.

This page is still in development, if you have any questions please don't hesitate to jump on the irc channel!

Radio Transceiver

Currently the favoured module is the HopeRF RFM69HW 868 MHz FSK Transceiver. This is a SPI-controlled module capable of upto 100mW RF output with a power supply of 1.8 - 3.6 V. This is not compatible with 5V Arduino boards

There are other radio transceivers under consideration, however these are not guaranteed to be compatible. See Radio Transceivers for more information.

Antenna

For the antenna a commercial SMA antenna is ideal, however for prototyping most of us will use a carefully cut 1/4 wave whip straight off the RF pin on the module.

To calculate the wire length required:

  • (300 * 10^6) / (869.5 * 10^6) = 0.345 = 34.5cm wavelength
  • 34.5 * (1/4) = 8.63cm quarter wavelength
  • 8.63 * 0.95 = 8.2 cm for a wire in free space

Microcontroller

Choice of microcontroller is far less critical, as long as it talks to the Radio Transceiver over SPI and can implement the basic UKHASnet protocol logic, it'll work!

As with nearly all modern projects, the favourite is the ATMega328, the chip underneath the popular Arduino platforms. We use the Arduino IDE to program these for convenience.

However there are members of the community looking into implementation on Freescale KL25Z ARM Cortex-M0+ boards, and more!

Sensors

The most commonly used temperature sensor is the DS18B20, however the combined temperature/humidity sensor of the DHT22 is also in use.

Batteries

For 100mW output, the RFM69HW transceiver requires 3.3V supply. A 1.8V supply could be used with a maximum RF output of 50mW.

Switch-mode step-up/step-down regulators may seem the most power-efficient, however the extra quiescent current (current the regulator itself draws) can outweigh the regulation advantage at the low current that UKHASnet modules use, sometimes making low-quiescent linear regulators more efficient, particularly when the radio is not always active.

Rechargeable

Example Node with small LiPo battery and charger A single-cell LiPo battery will supply a range of approximately 4.2V - 3.5V across its capacity and can then be linearly regulated down to 3.3V with a low-dropout regulator such as the MCP1700.

Non-rechargeable

3x cheap AA batteries will give a peak voltage of 4.5 V, and can then be regulated down to 3.3V with a low-dropout regulator such as the MCP1700.

building_a_ukhasnet_node.txt · Last modified: 2015/11/03 09:22 by craag