[TangerineSDR] FW: details on dual rm3100 test

Mon Nov 30 21:34:56 EST 2020

From: Julius Madey <hillfox at fairpoint.net>
Sent: Tuesday, November 24, 2020 12:35 PM
To: Kim, Hyomin <hmkim at njit.edu>; David Witten <wittend at wwrinc.com>; Dr. Nathaniel A. Frissell Ph.D. <nathaniel.frissell at scranton.edu>; Dave Larsen <kv0s.dave at gmail.com>
Subject: details on dual rm3100 test

All,
I'm not sure whether these photos got through last time, but for the dual rm3100 test I felt it was necessary to try for the best relative and absolute alignment I could of the two units.  The installation used two of Dave Witten's old adapter boards with one of them just wired as a 'slave' unit with only the RM3100 and 9808 temperature sensor and connected to the full remote board only by ground, +3.3V, i2c clock and i2c data wires.

The 'slave' board was also modified to change the address of the RM3100 so that both RM3100's and both 9808 temp sensors could be independently accessed over the single i2c circuit from the local board on the Raspberry Pi.

The assembly required some lathe and drill press work with the PVC pipe and fittings which may not be easily duplicated unless one has some more extensive shop facilities but duplication by others was not the object since it's also possible to compare readings from two reasonably co-located magnetometers with separate board sets.  This object of this installation was as near as possible, an identical temperature and location environment for the two RM3100s to eliminate as much as possible, other variables in the comparison.

As others are currently doing with the new board sets Scotty sent out last week, I'm doing a single RM3100 mount.  The discussion on Zoom last night indicated that the current form of the remote board with a 40 pin female extended pin header installed and the side entry RJ45 jack might not be optimum for installation in the PVC pipe formats now being tried.

I'm leaning toward a two board set using a single PC board layout with the RJ45 connector rotated 90 degrees to the present orientation, like the earlier layouts which were passed around for comment and with one of the two boards in the set without the 40 pin female extended pin header installed .... maybe included as a loose part in the kit.   That would keep parts counts minimized while simplifying embedded pipe or more conventional enclosure mounting.

Without the 40 pin header and with the RJ45 rotated 90 degrees, the new board easily fits into a 1 1/2 in PVC pipe which provides for some compensation in possible vertical misalignment of the 3 inch augur bored hole for earth embedding.  Of course, if you use a post hole digger then even a three inch pipe is acceptable ...

Except .... the greater air volume inside the mounting pipe might increase the possibility of moisture condensation if the pipe isn't sealed in a low humidity environment before embedding.  My basement is quite dry during this season and I've also included some packets of silica gel desiccant to minimize any condensed moisture.  The first embedded trial earlier this year of a mag in a 24 inch 1 1/4 inch pipe mount showed no sign of moisture after 2 months in the ground at ~8 degrees C measured temperature.

My plan for the new board test is to glue a plastic block of the same size as the 40 pin header on the opposite edge of the board which provides a symmetric profile about the long axis of the board providing auto-alignment with the bore of the pipe if it sits against the pipe wall.  The RJ45 connector is a problem.

Dave Witten demonstrated a clever way on the Zoom to use the existing RJ45 with board positioned in a PVC T fitting with the length of the plug and its folded back wire accommodated by the side port of the T with the board centered along the pipe axis. But better position accuracy still requires something of the same size as the 40 pin header glued to the opposite side of the board.    I tried Dave's idea with both 1 1/2 and 2 inch Ts and both work, although will no longer fit in a 3 inch augur hole.

For my test of the new board, I'll desolder the RJ45 socket and directly solder the correct length of CAT5E or 6E jumper cable to reach the waterproof connector at the other end of the pipe and will provide photos of that.  Looks like it will work with either 1 1/2 inch or 2 inch PVC.

Regards,
Jules - K2KGJ
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