[time-nuts] Oncore 1pps driver & LED
martelno at yahoo.com
Thu May 4 22:14:52 EDT 2006
Personally, i take great care about the 1PPS timemark
output from my GPS receivers. First thing of all: My
timemark line drives a single 74ACT04 or 40H69
inverter with short leads to get sure the line is as
little loaded as possible without ringing. What
happens after is far less stressing: If the line is
overloaded, the only component that will "suffer" is
the hex inverter, NOT the GPS board!
For the LED, my two GPS boards are a Canadian Marconi
(CMC) Superstar and a Trimble Lassen SK8 which deliver
VERY narrow pulses. (1ms for the Superstar and
something around 10µs for the Trimble (very difficult
to see on an analog scope)) These pulses are way too
short (especially the 10µs!!) to comfortably drive an
LED and give a visible signal. So, i simply woked up
an ooooold timer: The famous 555! In a monostable
design, i could easily stretch a 10µs pulse up to a
much more comfortable 20ms or 50ms and directly drive
the LED via a current limiting resistor. (470 ohms for
a 5V supply)
The PIC solution is not bad either, but personally i
prefer the old analog way!
Have a good day and 73 de Normand VE2UM.
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