Internals of Ubiquiti ETH-SP ethernet surge protector

 A couple of photos showing the internals of Ubiquiti's Ethernet surge protector ETH-SP. 

There are eight 2RL090 90 Volt GDTs.  One between each Ethernet line and the metal grounding point.  

HTTP Time protocol

The HTTP Time Protocol (HTP) is used to synchronize a computer's time with web servers as reference time source. Very handy in situations where NTP UDP/123 is being blocked. http://www.vervest.org/htp/

Electrical Cable sizing calculator for AS/NZS3008

https://www.jcalc.net/cable-sizing-calculator-as3008

A nice write up on using and crimping wire ferrules.

Nagios plugin for monitoring last modified time stamps of website content

Inspired by this plugin and this discussion,  I have written a nagios plugin for monitoring the "Last-Modified" time of content on a website. The script uses curl to retrieve the header information and then warns if the content has not been updated within a specified period of time.   

I developed this plugin for checking that data & satellite plots on our aurora alert site were being refreshed at the expected intervals.

[james@orchid tmp]$ curl -s -I  http://auroraalert.otago.ac.nz/aurora/Suomi_NPP_DNBNCC_night_highresimage.png
HTTP/1.1 200 OK
Date: Tue, 14 Aug 2018 23:36:08 GMT
Server: Apache/2.4.6 (CentOS)
Last-Modified: Tue, 14 Aug 2018 05:56:06 GMT
ETag: "15cfb4-5735edda8345f"
Accept-Ranges: bytes
Content-Length: 1429428
Content-Type: image/png

 Nagios plugin shell script:

#!/usr/bin/env bash

DEFAULT_WARN_SECONDS=600
DEFAULT_CRIT_SECONDS=1200

while getopts  'hp:c:w:' option; do
    case $option in
        h) help=1;;
        p) path=$OPTARG;;
        c) crit=$OPTARG;;
        w) warn=$OPTARG;;
    esac
done
if [ -n "$help" ] || [ -z "$path" ]; then
    echo "usage: $0 -p [path (required)] -w [warning threshhold seconds] -c [critical threshold seconds]" 1>&2
    exit 3

fi
if [ -z "$warn" ]; then
    warn=$DEFAULT_WARN_SECONDS
fi
if [ -z "$crit" ]; then
    crit=$DEFAULT_CRIT_SECONDS
fi
modificationTimeStamp() {
    curl -s -I HEAD $path |
        awk '/Last-Modified/{ date=""; for(i=2;i<=NF;++i) date=(date " " $i); print date;}' |
        xargs -I{} date -d {} +"%s"
}
#seconds=$(($(date +'%s') - $(stat --format='%Y' $path)))
seconds=$(($(date +'%s') - $(modificationTimeStamp) ))
if [ $seconds -gt $crit ]; then
    echo "CRITICAL: $path was last modified $seconds seconds ago"
    exit 2
elif [ $seconds -gt $warn ]; then
    echo "WARNING: $path was last modified $seconds seconds ago"
    exit 1
else
    echo "OK: $path was last modified $seconds seconds ago"
    exit 0
fi

Add it to the command.cfg

#check last modified time on remote URL
define command{
        command_name    check_urlmtime
        command_line    $USER1$/check_urlmtime -p $ARG1$ -w $ARG2$ -c $ARG3$
        }

Example service entry

define service{
        use             generic-service         ; Inherit default values from a template
        host_name               auroraalert
        service_description     Soumi NPP satellite image
        check_command   check_urlmtime!"http://auroraalert.otago.ac.nz/aurora/Suomi_NPP_DNBNCC_night_highresimage.png"!86400!172800
        }

rsync over a non standard SSH port

Use the -e option:

rsync -avz -e "ssh -p $portNumber" user@remoteip:/path/to/files/ /local/path/

HMC5883L magnetometer to Raspberry Pi connection notes

Some quick notes for connecting Honeywell's 3-Axis Digital Compass IC
HMC5883L to the I2C bus on a Raspbrry Pi board.

HMC5883L breakout board from SparkFun:
https://www.sparkfun.com/products/10530

Connections

GND -> GND (pin 6 on RPi I/O header)
VCC -> 3.3 V (pin 1 on RPi )
SDA -> SDA1 I2C (pin 3 on RPi)
SCL -> SCL1 I2C (pin 5 on RPi)

Configuration

Enable I2C using "raspi-config". Add user to i2c group.

pi@raspberrypi ~ $ i2cdetect 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 1e --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Run

See below for example C code.

pi@raspberrypi ~ $ gcc magnetometer.c   -o magnetometer -lm

pi@raspberrypi ~ $ ./magnetometer
Identification: 'H43' HMC5883L sensor detected
1434246270.000000, -548, -649, 349, 67030.4 nT
1434246270.100000, -547, -650, 349, 67038.5 nT
1434246270.200000, -548, -648, 348, 66951.1 nT
1434246270.300000, -548, -650, 350, 67109.8 nT
1434246270.400000, -548, -650, 350, 67109.8 nT
1434246270.500000, -547, -650, 350, 67066.3 nT
1434246270.600000, -547, -650, 349, 67038.5 nT
1434246270.700000, -548, -650, 350, 67109.8 nT
1434246270.800000, -547, -649, 348, 66959.2 nT
1434246270.900000, -548, -650, 351, 67137.6 nT

Code

C code to read and display the magnetometer data

Red Hat and CentOS Linux: Use PEERNTP=no to ignore any NTP server info provided by DHCP

Any NTP server information provided by DHCP can be ignored by setting PEERNTP=no anywhere in the sourced network scripts (e.g. /etc/sysconfig/network). This is useful in situations where a specific NTP setup is required rather than a default configuration supplied via DHCP. 

https://bugzilla.redhat.com/show_bug.cgi?id=809367  

systemd service script for soundmodem

james@arm:/lib/systemd/system$ more soundmodem.service
[Unit]
Description=Soundcard Packet Radio Modem
After=sound.target network.target multi-user.target

[Service]
Type=forking
ExecStart=/usr/sbin/soundmodem --daemonize -s

[Install]
WantedBy=multi-user.target



james@arm:/lib/systemd/system$ more /etc/debian_version
7.8
james@arm:/lib/systemd/system$ uname -a
Linux arm 3.19.0-armv7-x3 #1 SMP Wed Feb 11 21:58:07 UTC 2015 armv7l GNU/Linux
james@arm:/lib/systemd/system$

Baofeng BF-F8+ battery voltage display

The Baofeng BF-F8+ has a built in battery voltage display.

Baofeng BF-F8+

Press and hold the "0 SQL" key

The display will show the DC voltage of the battery. 8.1 V in this example:

Accuracy seems to be OK

MIcroAPRS

http://unsigned.io/category/microaprs/

CrowdMag

Real-time crowd sourced magnetic data http://www.ngdc.noaa.gov/geomag/crowdmag.shtml

VLF E-field whip antenna design

Suitable for use as a VLF E-field probe

Storm warning: Silent threat behind the thunder

From the Australian newspaper The Sun-Herald dated February 9, 1958

Estimating the power of a signal at different frequencies: aka spectral density estimation

This report has a good write up on using FFTs to make signal amplitude measurements: http://www.schmid-werren.ch/hanspeter/publications/2012fftnoise.pdf

Some example MATLAB code. Three signals are generated at levels of -20 dB, -40 dB & -60 dB:

clear
clf

%sampling frequency
Fs=48000;

%test frequencies
f1=11300;
f2=1510;
f3=17750;

%time
t=(0:1/Fs:1);

%generate signals
% 0.1 amplitude = -20 dB
% 0.01 = -40 dB
% 0.001 = -60 dB
y=0.1*cos(2*pi*f1*t)+0.01*cos(2*pi*f2*t)+0.001*cos(2*pi*f3*t);

%add some random background noise
noise = randn(size(y));
y=y+1e-4*noise;

%wavwrite(y,Fs,'test.wav');

N=1024;
w=blackman(N);

Y=fft(y(1:N).*w');

%scaling factors
%e.g. see
%http://www.schmid-werren.ch/hanspeter/publications/2012fftnoise.pdf
scaling = 2/(mean(w)*N);

fx=Fs/N*(0:N/2-1);
Y=Y(1:N/2);

figure(1)
plot(fx,20*log10(abs(Y) * scaling))

xlabel('Frequency/Hz')
ylabel('Amplitude/dB')
grid

Boot old 3.2.x kernel on beaglebone white A6b

Need to add something like this to uEnv.txt

#To boot old v3.2.x based kernel enable: (BeagleBone only)

uenvcmd=run loaduimage; run mmc_classic_boot

Power down the BeagleBone Black using the on board power button

The beaglebone black board has a power button next to the RJ45 ethernet connector. Pressing that button can get the board to cleanly shutdown.

First install acpid:

root@beagle1:/home/james# apt-get install acpid

Now test the shutdown. Press the Power button:

root@beagle1:/home/james#
Broadcast message from root@beagle1 (Fri Sep 27 01:13:49 2013):

Power button pressed
The system is going down for system halt NOW!
Connection to 192.168.1.116 closed by remote host.
Connection to 192.168.1.116 closed.

Installed system was Debian 7.1

james@beagle1:~$ more /etc/debian_version 
7.1
james@beagle1:~$ uname -a
Linux beagle1 3.8.13-bone26 #1 SMP Fri Aug 16 20:56:24 UTC 2013 armv7l GNU/Linux

Enable UART5 on beagle bone running debian

First mount debugfs:
mount -t debugfs none /sys/kernel/debug

Enable UART5:
echo 24 > /sys/kernel/debug/omap_mux/lcd_data9
echo 4 > /sys/kernel/debug/omap_mux/lcd_data8

Hardware Pins:
P8: 37 UART5_TXD
P8: 38 UART5_RXD

Device to access:
/dev/ttyO5

References:
http://www.jerome-bernard.com/blog/2012/06/04/beaglebone-serial-ports-and-xbees/

Debian Linux on beaglebone notes

Purchased beaglebone from digikey
http://www.digikey.co.nz/product-detail/en/BB-BONE-000/BB-BONE-000-ND/2765688

Added DVI audio cape
http://www.digikey.co.nz/product-detail/en/BB-BONE-DVID-02/BB-BONE-DVID-02-ND/3503684

Installed Debian Linux "Wheezy":
http://elinux.org/BeagleBoardDebian
http://elinux.org/BeagleBoardDebian#Debian_Wheezy_snapshot

Update system:
apt-get update; apt-get upgrade; etc

To load the modules for the audio cape:

sudo vi /etc/modules

at the end, add the lines:

snd_soc_davinci_mcasp
snd_soc_davinci
snd_soc_evm
snd_soc_tlv320aic3x

Power

With DVI audio cape attached, the device would often shutdown when powered over USB.  A separate +5 V DC 2 Amp power supply solves this problem.

GeographicLib

http://geographiclib.sourceforge.net/html/index.html