SEXTA Construction Notes

SEXTA Construction Notes
SEXTA Construction Notes
Tony Barry, Dave Gault
Preamble:SEXTA is a system (hardware device, firmware, and application software) to create and
analyse optical timestamps as observed by a camera and recorder.
The paper provides information
about the method of use and properties (accuracy, resolution, etc).
This document is intended to provide information to allow the researcher to build their own
Bill Of Materials - BoM:- See Appendix - A
The BoM lists the parts required to make a SEXTA and lists suggested suppliers for the
Major components. Other suppliers may well have equivalent products. Minor components
can be sourced from any hobby electronics supplier. Information on major components;
a) Arduino Compatibles: Many firms offer Arduino compatible microcontroller boards and
most of these will be suitable as long as they use a crystal oscillator rather than a
ceralock resonator.
b) GPS Receivers: SEXTA will operate with almost any GPS receiver as long is outputs
the correct NMEA sentences (GGA at 57600 baud)) as well as a 1 pulse per second
(1pps) signal. The prototypes have been built using the GPS receivers listed in the
BoM. However to use a receiver other than that listed will require some tinkering with
the MEGA code. Use of an internal or external GPS or an internal GPS with an
external antenna depends on reception available at the site where SEXTA will be
c) Dot-Matrix-Display: An array of 32x16 LEDs - available in blue and red.
d) 7-Segment Arrays: Available in many colours. The use of blue or green will limit
overexposure to some extent.
e) Plastic Housing Case: Two prototypes were built using a case available at a local
electronics hobby shop, however almost any case would be suitable to house the
SEXTA electronics.
Layout:a) The DMD panel, the 7-segment displays, the lock and almanac LEDs and the 10 unit
seconds LEDs need to be coplanar, so that the camera can take a picture of the whole
panel and have them all in reasonable focus.
SEXTA-ConstructionNotesV6 – page 1
b) The reader application can take care of some mild key-stoning of the panel due to tilt,
but ideally the panel will be face-on to the camera system and the camera system will
have minimal optical distortions.
c) The two Arduino units are mounted in the case of on standoffs of some kind.
d) The 10 unit seconds LEDs need to be co-linear and equally spaced, because the
reader application interpolates ten equidistant "sample zones" between the 0 and the
9 LED click points. If the spacing is not regular, the reader will not reliably read the
unit seconds.
e) The GPS unit is quite sensitive to RF, and so if an internal GPS is used (without an
external antenna), it should be mounted a reasonable distance (say 150mm or 6")
away and at right angles to the face of the micro-controllers. The GPS unit can be
fixed in place with double sided tape or velcro.
Power Supply:Power may be supplied by either;
a) The USB ports on either of the Arduino boards
b) The DC input port on either of the Arduino boards. This will utilize one of the 5v
regulators on the Arduino
c) An external regulator using a LM7805 and two capacitors. A large heatsink can be
implemented with this option.
Wiring:It is not necessary to etch a board for the task of connecting components. The wiring and
connecting can be done in a few hours using standard hookup wire (rainbow cable) and 0.1"
single row
SEXTA-ConstructionNotesV6 –page 2
Loading the Software:a) The two Arduino microcontrollers are programmed using the Arduino environment.
This is available for PC, Mac and Linux and is available at
b) Arduino 0022 in particular is the correct version to utilize for both boards. This is
available at
c) When loading the code for the Duemilanove (which drives the DMD), the DMD
libraries must be installed into the libraries folder.
d) The DMD library will be included in the file
e) Do not use the DMD library which is freely available from the supplier. The library we
supply has a modification which sets the SPI frequency for optimal use.
Start Up:- When SEXTA is powered on, a Splash Screen is displayed for about ten seconds.
During this time comms with the GPS established. If this fails, an error 12 is reported.
This has to be fixed before you can proceed.
- After comms with the GPS is established, the 500 LEDs of the DMD panel are lit in
turn, and the last LED (Row 32 LED4) is left on until the GPS to begin outputting 1PPS
signals. This may take some time if you are not in a good signal area. Typical times to
first fix are under five minutes.
- Once the GPS begins to emit 1PPS, about 10 seconds is allowed for signals to
stabilise. In this mode, the text "STAB" is shown on the 7-Segment array, together with
a count down. During this time the GPS may stop emitting 1PPS as satellite signals
vary. It may take more than 10 seconds to get to a point where the satellite signals are
- The DMD lock-in procedure is begun. This takes less than a minute in most cases and
when complete, the Lock LED is lit.
- Your first power-on will require you to wait up to 15 minutes to download a current
almanac from the GPS constellation. When that is complete, the A-OK LED is lit.
Setting the Sweep Rate:a) The Sweep Rate is the time the DMD takes to illuminate all the LEDS, left to right.
b) The user sets the rate to suit the camera test exposure setting under examination.
The device provides 4 sweep time settings;
1 second
2 seconds
5 seconds
10 seconds.
Typically for test exposure durations briefer than a half-second, a sweep time of 1
second should suffice.
c) The Sweep Rate is set using Switch1 and Switch2 on the front panel in the following
Sw1 - off and Sw2 - off, then the rate is 1 sec
Sw1 - on and Sw2 - off, then the rate is 2 sec
Sw1 - off and Sw2 - on, then the rate is 5 sec
Sw1 - on and Sw2 - on, then the rate is 10 sec
d) SEXTA must be restarted when the sweep rate is changed.
Error Codes:Any errors detected are displayed on the 7 Segment LEDs, e.g. "E7 45 6" where the E7 is the
error number, and the 45 is the UTinteger seconds from the last correctly parsed GGA
sentence, and the 6 is the number of satellites registered in the GPS constellation.
- Switch3 is to reset after an error.
- See appendix C for a list of error codes
SEXTA-ConstructionNotesV6 – page 3
Piezo Buzzer:The piezo buzzer and switch attached to pin 10 of the MEGA is optional, to provide an
audible time reference in the following schema (provided the switch is turned on );
- On obtaining a 5 satellite fix, a melody is sounded.
- At the instant of each second a coded beep is sounded. The format is:
o beginning of the minute
= long beep
o 10 , 20 , 30 , 40 and 50 seconds
= short beep
o 55th, 56th. 57th and 58th second
= brief beep
o 59th second
= silent (no beep)
- If the A-OK LED is unlit, all beeps are sounded as Double.
o (other than the 0, 10s and the 55th to 59th)
DMD Array:The DMD Array consists of 32 rows of 16 LEDs, giving a total of 512 LEDs. The bottom 12
LEDs of row 32 are not used for SEXTA.
Leap Seconds:Leap seconds are implemented from time-to-time. SEXTA must be operated for at least 15
minutes after any leap-second is inserted to ensure the GPS has downloaded correct
almanac data.
GPS Battery:The PA6H GPS has a battery to maintain the RTClock and almanac memory. This must be
replaced periodically.
SEXTA-ConstructionNotesV6 –page 4
a) Bill of Materials
The following is Bill of Materials for SEXTA : Version: v3
Major Components
Arduino MEGA-1280 or 2560
Arduino Dumilanove or Equivalent
GPS - Global Top PA6H
Dot-Matrix-Display (DMD)
7-Segment LED Array
Minor Components
LED Green
Resistor 330 ohm
Resistor 1k ohm
Momentary switch
Toggle switch SPST
Header Pins (40)
Rainbow Cable (1 metre is more than enough)
Plastic Case 225x165x40
Miscellaneous bits and pieces
Available from many sources.
must have crystal, NOT a ceralock resonator
known as the Ultimate GPS
Various colours available.
Jaycar HB5972 Or Similar
Note, if the On Arduino regulators are not used (for whatever reason) then a voltage regulator circuit is
5volt Regulator LM7805
Capacitor 0.1uF
Capacitor 0.33uF
Ø2.1mm DC Socket
Vendor References
SEXTA-ConstructionNotesV6 – page 5
b) Circuit Diagrams
SEXTA-ConstructionNotesV6 –page 6
c) Error Codes
SEXTA-ConstructionNotesV6 – page 7
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