Personal Beacons, Arduinos and Raspberry Pi’s

-An On-going Project

Brian Flynn GM8BJF

Acknowledgements

• Project has been a cooperative exercise

– Chris Tran GM3WOJ houses the transmitter on one of his towers.

– Briain Wilson GM8PKL who houses the receiver set-up and controls my lack of internet security awareness. More on that later! He also keeps the Rx and audio stream “tweaked” on a day to day basis.

Background

• Beacon building has been a long term interest.

– GB3EDN, 70 cm GB3ANG, 3cm beacons GM8BJF, GM8FFX and GM3WOJ.

• Recently completed a rebuild of the GM3WOJ 3cm beacon.

• Simplified the hardware to make it more reliable.

• Upon completion of the rebuild Chris ‘WOJ suggested that it would be good to try an develop some “experiment” around the beacon.

Location of GM3WOJ

Path Profile

Path Profile

• Path is far from L0S

• There are interesting propagation effects

• Rainscatter

• Aircraft reflections

• Refraction and diffraction effects

• Signal is always audible (as long as Tx is on!)

• Sometimes only with Spectran

Signal Monitoring

• For the about past 3 years we have monitored the signal and streamed the audio onto the internet with Icecast and Darkice.

• Streaming audio of GM3WOJ/B

• Spurred on by Chris’s suggestion of a “more scientific” experiment it was decided to attempt to log the signal strength over a period of time and make the data available.

The Receiver

• Antenna is a re-purposed Sky dish with a modified LNB whose LO is frequency locked to a good 10 MHz reference.

• The receiver is a Yaesu FT790RII on the 70cm IF.

• At present the audio is streamed via a USB sound dongle connected to a Raspberry Pi running as an “internet radio station”.

The Gorse Green Sky Dish

Discreetly Installed Dish!

Inside Briain’s Cellar

Back to the Experiment

• The goal is to record the signal level by measuring the AGC voltage in the receiver.

• Digitise the AGC level and record (say) every 30 seconds.

• Make the information available as data files on a small web site.

The Hardware Implememtation

• Use Raspberry Pi as webserver, (already one there)

• Use Arduino as ADC and USB interface to Raspi.

• Also first line data storage to SD card. 32 G SD card holds a lot of numerical data!

• Why not use Raspi to do data conversion? No ADC!

The Hardware Implememtation

• The Raspi is a small computer running Linux with an operating system – Linux - Great !

• The Arduino is a programmable microcontroller with no operating system. Only does one thing.

• Both easy to interface over USB

• No need to solder up an ADC and get it working and Arduino Nano board is about the same cost as an ADC chip.

Raspberry Pi 3 Brief Specification

• SoC: Broadcom BCM2837 CPU: 4× ARM Cortex-A53, 1.2GHz GPU: Broadcom VideoCore IV RAM: 1GB LPDDR2 (900 MHz) Networking: 10/100 Ethernet, 2.4GHz 802.11n wireless Bluetooth: Bluetooth 4.1 Classic, Bluetooth Low Energy Storage: microSD GPIO: 40-pin header, populated Ports: HDMI, 3.5mm analogue audio-video jack, 4× USB 2.0, Ethernet, Camera Serial Interface (CSI), Display Serial Interface (DSI)

Raspberry Pi R3

Arduino Nano Specifications

• Microcontroller ATmega328 • Operating Voltage (logic level): 5 V • Input Voltage (Recommended): 7-12 V • Input Voltage (limits): 6-20 V • Digital I/O Pins : 14 (of which 6 provide PWM Output) • Analog Input Pins: 8 10 bit ADC 1024 levels • DC Current per I/O Pin: 40 mA • Flash Memory 32 KB (ATmega328) of which 2 KB used by

bootloader • SRAM: 2 KB (ATmega328) • EEPROM: 1 KB (ATmega328) • Clock Speed: 16 MHz • Measurements: 0.73" x 1.70"

Classic Arduino Uno

Arduino Nano

Arduino Data Logger

Data Logger Hardware

Complete Set-up

Raspberry Pi Software

• Install Apache2

– apt-get install apache2

• Save the USB data stream on /dev/ttyUSB0

– sudo cat /dev/ttyUSB0 >> data.txt &

• Very straightforward. (If it all works!)

Calibration of the Arduino ADC

0

200

400

600

800

1000

1200

-160 -140 -120 -100 -80 -60 -40 -20 0

AD

C V

alu

e

Input signal power (dBm

ADC output

Linear ()

Calibration of the Arduino ADC

Extracting a Relative dB Value

Excel regression formula (straight line)

y = 7.6x +1404

Arduino C code to convert adc value to dB

int sensorValue = analogRead(0); // Read analogue pin A0

float dB = (sensorValue - 503) / 7.6; //Convert to dB scale

logFile.println(dB);

(503 ADC value = 0 dB)

Data Format

• Data output once per 30 seconds • Stored on SD Card and to file on the Rpi. (32 G

card stores a lot of numerical data!) • Date • Time (Hrs:Mins:Secs) • UnixTime - Good for graph plotting! • Raw ADC value • Relative signal level in dB. • Use comma separated variable (*.csv) format. • Good for spreadsheets

Sample of data

Web Page from Raspi

Where next?

• Hardware side is mostly complete.

• Plan to run both the audio stream and the data server on the Pi3 along with monitoring a GPSDO frequency reference for the Rx.

• Get it installed and up and running

• Not to be under-estimated!

• No doubt there will be teething issues.

• I am a pessimistic old Engineer!

Questions ??