If you've ever wondered (like I have!) how to make full use of the TELEMTRY
string which
the KPC-3+ can be set to beacon, keep reading...
The Kantronics KPC3+ TNC can be set to beacon telemetry, with five analog
to digital converters
and a binary string which I haven't figured out yet. Unfortunately,
from the DB-25 connector or
from the top of the board, you can only reach the first two (AN0 and
AN1) of the A/D inputs.
The others (AN2, AN3, AN5) are only accessible by soldering directly
to the chip. Even with
my experience soldering surface mount diodes the size of a comma on
your screen, I wouldn't
try THAT! The alternative is to disassemble the TNC and solder
to the bottom of the board,
but just try to find the pin numbers! That's why I decided to
find them myself.
Note: Use proper ESD grounding procedures
- wear a wrist strap!!!
Here's how to get there: First, remove the cover. Next,
remove the two front screws
and the faceplate. Then, remove the four stand-offs holding the
DB-9 and DB-25
connectors. Lift the front of the board up, and when it clears
the housing, move it
forward and out of the case. Turn it over and lay it down with
the LEDs facing you.
You should see this pattern near the middle of the board.
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Until an input is used, it's a good idea to terminate it with a 10k
resistor, or maybe even
ground it, so that your unused telemetry will read close to zero.
Now you can start using your inputs. Just remember - a reading
of 255 is equal to 5 volts
on the input, so if you're measuring input voltage, for example, start
with a voltage divider
with 4.7k across the input and 10k in series with the voltage source,
and use a trimpot in
addition to the 10k series resistor to reduce the reading to match
the voltage times ten
(126 is 12.6 volts, 84 is 8.4 volts, etc.)
Another example: I used a thermistor from an old Motorola battery pack
(a good source
of surplus AA cells too) in the following circuit on input AN1:
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8/19/01 - Update: I've got a KPC-3 working with 5 telemetry parameters:
internal
(battery) voltage, internal temperature, external (line) voltage, external
temperature,
and light intensity. I'll be adding this info some time this
month. In the meantime, here's
a short description: The external temperature sensor is the same
as the internal one.
The line voltage sensor is a SEPARATE power supply (wall wart) and
a voltage divider
to set the DC reading to match the AC line voltage. Use a separate
supply and NOT
your radio's supply to avoid any load changing the reading. The light
sensor is a Rat
Shack cadmium sulfide photocell (pack of 5, the small white ones) and
I'll post the
resistor values soon. It's set up like the temperature sensor, and
light decreases
resistance and gives high readings. This is so that I can see relative
cloud cover up at
the site, and also because I haven't had time to build a wind sensor.
More info at
http://home.teleport.com/~n3eg/signalpeak.htm
Things to seriously consider:
- Use slow changing parameters, and don't add to frequency congestion
by beaconing
too often.
- Don't use redundant parameters, such as speed, direction, altitude,
etc.
- Temperature inside the TNC doesn't change as quickly as the air temperature
outside.
- Consider installing 5.1 volt zeners across inputs with long lead
lengths, inputs used
with other equipment, or inputs which will come in contact
with foot-shuffling static
charge transporters.