Re: Not-too-conductive material
- From: dplatt@xxxxxxxxxxxx (Dave Platt)
- Date: Fri, 25 Jul 2008 14:20:36 -0700
In article <4889b9a1.959500@xxxxxxxxxxxxxxxxxxxxxx>,
Robert Scott <---@xxxxxxx> wrote:
..you could fabricate the probes out of
a thin rectangle of PCB material (something water-resistant) with a
series of pads etched into it, and solder down to the pads a whole row
of SMT resistors (in effect, connected head-to-tail). You'd get a
discrete step in resistance each time the water rose high enough to
bridge a resistor. Choose the resistance per step to suit your needs.
Yes, that would give me enough resolution. But this probe may need to be
cleaned occasionally with a damp rag. Chip resistors would catch on the rag.
--unless I put the resistors on the back side of the PCB and put vias to the
pads that touch the water on the other side. But a PCB is still not the ideal
form factor. Something shaped like a dowel would be easiest to clean.
As for the range, it is 6 inches from full to empty. The resolution only needs
to be . 5 inches.
Oof. At that resolution I'm not sure I'd bother using a resistance
type of sensor at all. I'd probably just run a bunch of individual
sense wires down the inside of a rod to little contact on the rod
surface, and use CMOS inverters or something like that to detect a
discrete open/closed state for each.
Or, if you still want to use resistance sensing methods, here's a
possible way to fabricate it which will give you an easily-cleaned,
waterproof sensing rod.
Get yourself a block of Plasticene. Reshape it into a rectangle 8"
long or so, a couple of inches thick, and a couple of inches wide.
Press an acrylic dowel/rod around 3/4" in diameter halfway down into
it and then pull out the rod, leaving yourself with a trough as long
as you need.
Buy a box of "escutcheon pins" at your local hardware store... they
look like short round-headed brads made out of brass. Even better
would be round-headed copper pins, if you can find them, or silver-
plated PC-board turrets.
Push a series of escutcheon pins, head-down, gently into the bottom
of the trough in the plasticene... just deeply enough for the head to
stick in the plasticene and not fall over. Place one every half-inch
or so (at your desired resolution).
Mix up, and pour into the trough, just enough epoxy (or polyester)
resin to cover the heads and first parts of the shafts of the
escutcheon pins. Allow to harden... this will hold the pins in place
during the next part of the procedure.
Trim and shape the leads of a bunch of 1/8-watt resistors (leaded, not
surface-mount), and then go down the length of the rod, placing each
resistor between adjacent escutcheon pins and bendings its leads
around the pin to hold it. Crimp the leads gently into place with
needle-nose pliers.
When all of the resistors are in place, either solder them quickly to
the pins (use an active rosin solder, as brass can be hard to solder),
or apply a dab of silver-bearing conductive epoxy and allow it to
harden. Also, solder a wire lead to the pin at one end of the trough
and run it up out of the trough.
When you're done, measure the resistance from the wire lead to each
pin in sequence (making sure that the contacts are OK). When you're
satisfied, mix more epoxy or polyester resin, and fill the trough up
to the top (covering the ends of the pins, and encapsulating the
resistors). Allow to harden. Remove from the plasticene. Clean off
the slightly-projecting round heads of the escutcheon pins.
--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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