Date: Wed, 9 Jul 2014 08:17:46 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: First cleanup Re: Westy water tank level indicator problem
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>1) You can test the input with a potentiometer and voltmeter. Input
>lead is biased to B+ with one megohm, electrodes ground out
>successive resistors in a voltage divider inside the little black
>module by the tank. Result compared against similar network at the
>left-side LM324 chip.
First unplug the green wire at the tank. You can plug and unplug
freely with panel power on. None of the tank LEDs should be
lit. Then ground the green wire. The green LED only should
light. If either of these tests fail, skip immediately to the second
section, called LM324s die a lot. Return to this section for info
later. Otherwise continue from here:
To test the thresholds connect a digital voltmeter from the green
wire to ground and either:
a) operate the sender float on newer ones (be *very* cautious; if you
bend the rod the tiniest bit in the wrong direction it will break one
or both reed switches inside because no slack was left when soldering
them). The meter will pull the voltages down somewhat, see NOTE 1
below and method/formula at c).
b) ground all the sender terminals on the older tanks to the bottom
terminal and then remove them beginning from the top, same comments
about meter as above. The tank should be empty. If you want to keep
water in the tank you can disconnect the electrode wires and short
them together in the right order, but mark them carefully before you
disconnect them.
c) disconnect the green lead and connect to ground via one end and
middle of a 10-20 megohm pot, preferably linear taper, and adjust the
pot. With this method you can find the actual thresholds. A ten
megohm pot and a ten megohm meter will give you about 10.5V maximum
which is plenty. If you want to know what the actual applied
resistance is at each threshold, unhook the green lead, switch the
meter to ohms, write down the value and calculate Reffective = Rmeter
x Rpot / (Rmeter + Rpot). Same method if you want to know how the
meter is altering sender resistances in previous methods.
d) disconnect green lead and connect positive side of a variable
power supply to it, negative side to ground.
CAUTION: do not apply power with the panel turned off (if the panel
power switch is pulling out of the solder, fix that first to avoid
possibly losing panel power while your supply is driving the line).
CAUTION: Do not exceed the measured voltage present on the green wire
with panel on and nothing connected, and do not allow the voltage to
go negative more than a few millivolts. Some variable supplies may
go slightly negative when set to minimum, so check and beware. The
chip can handle a very slight negative input; that's how the flame
detector LED operates, the fridge thermocouple is the only voltage in
the van that's officially negative to chassis ground. But the
emphasis is on very slight.
e) connect various combinations of dry cells likewise. See CAUTION above.
f) connect individual resistors likewise ranging down from ten
megohms. This is the hard way for sure, but if you have a bunch of
resistors it works.
NOTE 1: with either method where you're actually supplying voltage,
adding about 5,000 ohms in series will prevent any excitement
resulting from shorting a wire to ground. The green wire itself can
be grounded safely.
NOTE 2: If you're not actively supplying voltage you will get most
useful results with a ten megohm (or higher) voltmeter. Most digital
meters have ten megohm sensitivity on voltage ranges, but some cheap
ones may be one megohm. A ten megohm meter will drag the green wire
down to about 11.5V all by itself, and a one megohm meter will drag
it to about 6.3V, severely limiting your testing.
Assuming the LED panel is being fed by 12.6V, the following voltages
at at the end of the green wire should have the following results
with a few per cent variation for differing resistor errors on the
LED board (if all resistors were x% off in the same direction the
thresholds would not change):
>12.6-9.45 --> No lights (no bolt jumper/not reachable with float
>sender. Signals </= one liter in the tank)
>9.45-7.35 --> Red (bottom sender bolt jumpered to next up/float
>sender at bottom)
>7.35-5.25 --> Yellow (add jumper to next bolt/float sender at middle)
>5.25-0.00 --> Green (add jumper to top bolt/float sender at top)
OOPS: Exact values of above numbers are suspect, I'll have to recheck
the spreadsheet.
Note that varying battery voltage will change the voltage thresholds
proportionately; but will not cause them to shift relative to the
operation of a particular sender. The circuit employs three of the
four amplifiers contained in the left-hand chip on the panel(the
fourth one is a spare). Each amplifier drives one LED, and
independently drives that LED any time the output of the voltage
divider formed by the various discrete resistances of the sender in
series with one million ohms becomes even microscopically less than
the output of a particular tap on a reference voltage divider (made
up of five resistors on the panel).*** Since both the sender-side
and reference-side networks are powered by the same battery voltage,
the ratio between them does not change.
***Ordinarily that would mean that the lights would show red,
yellow+red, green+yellow+red; but the LEDs are wired in trick fashion
so the red one loses its ground when the yellow comes on, and the
yellow loses its ground when the green comes on. This is done simply
by using the connecting LED- of the red and yellow LEDs to the
adjacent LED+ instead of ground. The amplifiers can hold the line
either high or low (to say another way they can either source or sink
current) so they can easily act as the "ground" for an LED.
Incidentally a different arrangement is used for the battery meter,
because there you *want* the result to change when battery voltage
does. There a very similar ladder of resistors becomes the sensing
part of the arrangement; and the (resistor+changing resistor) pair
used for the tank sender becomes instead a (resistor+zener diode)
pair which changes output voltage very little with any reasonable
change in battery voltage. As with the water side, lighting one LED
removes the ground from the previous one.
If you were to add the 500k resistor I refer to in my schematic, the
thresholds would change thus:
12.6-9.72 --> No Lights
9.72-7.56 --> Red
7.56-5.4 --> Yellow
5.4-0.00 --> Green
OOPS: Exact values of above numbers are suspect, I'll have to recheck
the spreadsheet.
I made this change on my '84 to prevent the panel indicating red when
it should have been showing yellow. Adding that single resistor
changed all the thresholds because it altered the overall value of
the string; to manipulate one threshold without altering the others
could be done in various ways. The simplest would be to leave the
original resistor string alone and simply re-route the input to that
amplifier to a separate pair of resistors having the correct
ratio. If the designers had envisioned a need to adjust the
thresholds they would have used a separate divider for each
amplifier, probably with a tweak adjustment included. It all costs
money though, and board real estate.
>2) The LM324 chips die a lot, at least on this board.
>
>Seriously.
If the battery lights work ok but the water tank lights don't all go
out when you unhook the green wire and the green (only) doesn't light
when you ground it (or if the battery lights start to misbehave),
your first action is to replace the left-hand LM324N with another
LM324; because it's highly likely that the chip If Radio Shack still
carries them that's fine, otherwise get one or a few from
wherever. Chip price used to be in the dollar range; I haven't
looked for a few years. If you aren't up to soldering (and this is
about as easy a board to work on as exists, see below for method) get
whoever does the job for you to install a socket on both sides
instead, so you can easily change or swap them yourself later.
UNFORTUNATELY I just took a look at my photo of the board and there
may be no room to put sockets in. What could be done is to add
individual socket terminals through the board after enlarging the
holes slightly. They would sit practically flush on top and wouldn't
raise the chips much at all. However I can't really tell from the
angle, and a low-profile socket might do fine. The controlling depth
is the plane of the top of the switch body and the shoulders of the
LED ends as they are what seats up against the rear of the metal
panel. If someone measures that clearance depth I can get a better
handle on this. Back to business:
The three troubles that afflict this setup (no special order):
1) Learning the hard way that the float-type sender as originally
built (and I hope and believe that Trevor has arranged for some slack
in the assembly of the new ones) is ABSOLUTELY INTOLERANT of being
bent the slightest amount in the direction away from the foil side
of the circuit board inside, where the reed switches are soldered
on. One or both of the glass reed switches will burst because their
leads will separate and glass doesn't stretch much. Repairing is
often practical and once you're inside you can put some slack in the
soldered leads so it won't happen again. Best way I found to open
them was with a plumbing-type tubing cutter used very carefully and
rejoined with a rigid oversleeve.** One of the reed switches is a
simple N/O type with two terminals and is easily sourced. The other
has three terminals and is a bit harder to locate. This circuit
carries extremely low current so if a lighter/smaller switch presents
itself it will be fine so long as the magnet in the float can operate
it and you're even more careful to put slack in the leads. Larger
ones the same except they also have fit into the tube which may not
accept anything larger. Note which way the three-terminal switch is
oriented since the N/O and N/C terminals must be correct. Test the
board with the LED panel or ohmmeter and a magnet before
assembling. The resistance changes should be in consistent order up
and down as you move the magnet. I can't remember exactly how it's
wired right now so I'm going to wave my hands vaguely. Anyone with a
good float sender can provide correct ohmmeter readings.
1a) Getting a drop or two of water inside said float sender, either
from a leaky joint or more likely through the top where the wires
come out. This causes troubles where the tank reads too high, but
the reading is likely to shift as the panel stays on for
awhile. This can be repaired by drilling a small hole next to the
bottom, breaching the seal around the wires if need be, and flushing
through several changes of 190 Everclear or Denatured Alcohol or
high-strength isopropyl alcohol dried with air or vacuum between
changes ; then over-cementing assembly seams as well as closing the
hole you made and the top seal.** But don't be surprised if you
break a switch in the process -- so really it's better to ever so
gingerly open it up as above and resolder one of the leads of each
reed switch so the board can flex a bit without popping them. Check
for corrosion/electrolysis eating up the copper and clean and solder
over it if need be. Then dry out the tube and circuit board and put
everything back together. And triple-protect the entry point at the
top as I think that's where the water usually gets in. I personally
would advise removing the wand from the tank and storing it safely
before any sort of cleaning activity inside or outside the tank for
both flexing and leakage reasons.
1c) Having to scrub off the older-type tank electrodes with green
Scotch-Brite, not the wussy blue stuff. Maybe adding a few pinches
of salt to a tank if the water's really pure.
** I'm sorry, you'll have to experiment or do some research for a
cement that will bond properly. Cheri or Trevor can no doubt supply
that info now; they weren't involved with the senders when I was fixing them.
2) Power switch working loose from the solder. It's a very small
switch supported only by its leads and it has a hard life from
bumping, vibration, something. I used to routinely re-solder mine
any time I had the panel out.
3) LM324 gone bad for no obvious reason.
That's it. I have never heard of any other difficulty with them ever.
Ok, I will check the threshold exact numbers and take another pass
through probably tomorrow some time. If you (anyone) notice any
problems in the mean time let me know. I'm a bit sideways on sleep
this past week and my proofreading has suffered.
>3) Some info and schematics here
>http://pws.prserv.net/synergy/Vanagon/LEDpanel.htm
d
--
David Beierl -- dbeierl@attglobal.net
