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Once the PPS signal has begun, the GPS unit will continue sending one pulse per
second until power is turned off, regardless of the satellite tracking state. The clock
will synchronize its second counter to this PPS signal.
In addition to setting the time on the first PPS signal, every morning at 2:45am the
clock will query the GPS unit over the serial line. It will adjust the clock to the nearest
half-hour offset of the UTC time provided from the GPS.
The GPS provides the Universal Coordinated Time (same as Greenwich Mean Time).
The clock assumes the user has set the time to within half an hour of the correct local
time (i.e., plus or minus 15 minutes) and uses the the UTC time from the GPS
calculate the exact time.
Parts
The first rule to tinkering is to save all the parts. -- Paul Erlich
Part Name Description
PIC16F688 Microcontroller Updated microcontroller with support for GPS
1 Watt, 100Ohm Resistor Replaces ½ Watt, 1k Ohm Resistor R1. This will allow more 5V power supply
current to power the GPS-18
Wire Optional, to connect nixie decimal point display to AUX_A or AUX_B driver
output. This will cause the nixie decimal point to light up a Pulse Per Second
(PPS) signal is being received
Garmin GPS 18 LVC The GPS antenna and receiver on a 5 meter cable. Make sure the GPS-18 has
a view of the sky, as unobstructed as possible. It will work indoors, but at
least placing the unit on a window sill is a good idea.
12V DC Power Adapter The 12V power adapter included with the GPS upgrade kit is a switch-mode
power supply. WARNING: Please make sure to use the GPS modified
clock ONLY with a switch-mode 12V power adapter, a 12V battery, or
other true 12V source. The 12V power adapter which shipped with the non-
GPS kit is transformer-based, and provides too high a Voltage for the GPS
modified clock. The transformer-based power adapter can actually output up
to about 18V, which will burn out the 100Ohm dropping resistor.
If you would like mounting hardware for the GPS 18 LVC, search on amazon.com or
the web for part numbers 010-10541-00 (suction cup mount) or 010-10453-00 (flange
mount).