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Old 02-27-2015, 02:17 PM   #21
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I tested the monitor by connecting one channel and using a coffee cup with water to dip the probes into. I then measured the RED wire on the harness, which is the "sense" wire going to the monitor.

Button not depressed = stray voltage under 0.02VDC
All sensors out of the water = 8.6VDC - 1 LED lit.
Ground sensor only in water = 8.6VDC - 1 LED lit.
Yellow and Ground sensor (1/3) = 6.8VDC - 2 LED lit.
Yellow, Green, and Ground sensor (2/3) = 5.2VDC - 3 LED lit.
All sensors in water (FULL) = 1.3VDC - 4 LED lit.

Minimum current (no button pressed) = 0mA
Max current (button pressed 4 LEDs on) =31mA

I have no idea why there is such a change in linearity with all sensors in the water. Of course, it is because the unequal resistor values of the harness - but why was it necessary to do that. I suspect the answer lies in the configuration of the resistor ladder reference inputs to the comparators.

So more examination...

Also, it appears that there is no current drawn from the battery until you depress the switch. I think that powering down the capacitive sensor between readings will be the hardest thing to do. The existing resistor harness is passive, and any capacitive detection will be active.

But if the capacitive sensor could be made to operate in the uA range, then the coach battery's self-discharge rate would be higher, and such a small load would be insignificant.
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Old 02-27-2015, 02:54 PM   #22
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My plan is to use a one-shot to power the circuit off of the house battery when it detects power on the sense wire. The one shot will power the circuit for approx 10 seconds. Even if the tank is "full" (e.g. the circuit "grounds" all of the sense wires) the timer in the one shot will still provide power for the 10 seconds.
This way the regulator on the Arduino won't be seeing 12V all the time (and thus not be consuming current). At the moment I'm estimating the circuit to consume less than 20ma while active and am looking for a one-shot that consumes ~0ma when not triggered.
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Old 02-27-2015, 03:09 PM   #23
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My plan is to use a one-shot to power the circuit off of the house battery when it detects power on the sense wire. The one shot will power the circuit for approx 10 seconds. Even if the tank is "full" (e.g. the circuit "grounds" all of the sense wires) the timer in the one shot will still provide power for the 10 seconds.
This way the regulator on the Arduino won't be seeing 12V all the time (and thus not be consuming current). At the moment I'm estimating the circuit to consume less than 20ma while active and am looking for a one-shot that consumes ~0ma when not triggered.
Not a bad idea.

I was thinking of something like an opto isolator that could sense the sense wire voltage. When dormant, there is no voltage on the sense wire, so the opto isolator would be off. But when it sees voltage, the output of the opto isolator could be on the ground side of the capacitive circuit, which basically turns it on like a switch.

But it might be tricky to keep the led side of the opto isolator from pulling down the sense voltage.

The output of the capacitive circuit could also be an opto isolator, but the MOSFET type (typically called an electronic relay). But depending on the current going through the LED, you should be able to vary the "resistance" on the output side (which is connected to the sense line). Other options might be a programmable or voltage controlled resistor.

These days there are so many cool special purpose integrated circuits that most anything is possible.

On the other hand, a one-shot circuit would work very well, and I like it's simplicity... simple is always good. Even if the delay was much more than 10sec, it would not significantly discharge the coach battery.
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Old 02-27-2015, 03:14 PM   #24
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Something like a LM555 would work very well for a 1 shot. In quiescence, it can consume as little as 3mA according to the spec sheet. That is less than one standard LED, and probably low enough of a load to be insignificant. At a 3mA discharge rate, 100AH battery would take 1,300 days to discharge, and it's self-discharge rate is going to be a lot higher than that.

I have some 555s in my parts box, I might have to build a one-shot and see how much current it consumes in quiescence.
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Old 02-27-2015, 03:23 PM   #25
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You can get lower power versions of the 555 (like the 7555) which consume even less current (I've been shooting for single digit ua).

7555:
http://www.nxp.com/documents/data_sheet/ICM7555.pdf
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Old 02-27-2015, 03:39 PM   #26
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I popped a LM555 in my parts box on a breadboard to determine it's quiescence current. This is not a low power, but standard 555:



And I am measuring 1mA (minus any inaccuracies in my power supply's ammeter):



So, 30days x 24hr = 720 hours per month.

720 hours x 1mA = 0.72 AH per month.

Of course, any components that are added will add to the current demand, but it seems like we could just leave the thing on.

If you consider even a 3 month self discharge rate on a 100AH battery, that would be equivalent to a self discharge rate of 33 AH per month.

But a low power 7555 might be a good choice, just because we think that way. If I read it correctly, the spec sheet you linked to says 50uA.

Again, the "purist" in both of us says get it as low as possible, but the practicality of the situation is it will be well below the self-discharge rate of the batteries.
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Old 02-27-2015, 04:00 PM   #27
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Again, the "purist" in both of us says get it as low as possible, but the practicality of the situation is it will be well below the self-discharge rate of the batteries.
Very true! LOL but its the principle of the thing!
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Old 02-27-2015, 04:00 PM   #28
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I like that 7555. The spec sheet indicates the trigger current is typically in the pA range, which means it won't affect the reading of the sensor (since the same lead is going to be used for both the 1 shot and tank signal).
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Old 02-27-2015, 04:07 PM   #29
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Even better: they are only $0.88/ea:
ICM7555IPAZ Intersil | ICM7555IPAZ-ND | DigiKey

and, I think, pin compatible: Design the circuit with a 555 in a socket and then replace with the 7555 when it shows up.
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Old 02-27-2015, 06:23 PM   #30
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Digikey usually ships very quickly if they have it in stock.
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Old 02-28-2015, 02:03 PM   #31
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Update

I now have a "prototype" unit to work with:

The circuit I'm attempting to use looks like:

With this setup (a plastic cup wrapped with foil--about a quart in size) R1 needs to be fairly large for the CPU to be able to measure the time it takes to charge the "tank capacitor" (currently I'm using 20M instead of 800k as listed on the diagram). This means that it is pretty sensitive to changes in capacitance: When the tank is full waving my hands near the tank does cause a change (strangely enough when the tank is empty no such affect is observed).
As an aside these little Arduino Mini's are really cool. Took me about 10 minutes to get it up and running with the example blinky code.
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Old 02-28-2015, 03:23 PM   #32
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Doing some experimenting

With the original setup above I had two "plates" on either side of the "tank": One plate was grounded and the other was connected to the Arduino for charging/measurement.
In this configuration I could measure the fluid level in the tank although it was fairly inconsistent (and as mentioned above waving your hand by the tank when full could affect the reading). Here the fluid serves as the dielectric in the capacitor changing the capacitance with the level.
I've found a better configuration: Ground the fluid (in the camper this would take advantage of the fact that the bottom probe is already ground). In this configuration the fluid is the ground plate and the tank wall is the dielectric which also means there is very little separation between the plates. I also means I/we would only have to affix a single plate to the tank..not two.
With the fluid grounded (tap water in my testing) the readings are much more stable and accurate reflecting the level of fluid in the tank. In addition they didn't change all that much when I changed the conductivity of the fluid (e.g. adding salt).
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Old 02-28-2015, 04:19 PM   #33
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This is what I'm thinking the one-shot circuit would look like:

Note that the trigger input to the 555 is active low and thus an inverter is required. Here I'm using a FET (Q) for the inverter because its voltage triggered, not current (and shouldn't have any impact on the usage of the signal wire). The one I've found is here:
STQ3N45K3-AP STMicroelectronics | 497-10969-3-ND | DigiKey
According to my (dated 1984) Forrest Mims Radio Shack 555 book (yeah I have a few of these laying around! LOL) the 10M resistor & 1uf cap on pins 6 & 7 should result in a 10 second output pulse (10M * 0.01u = 10).
The 20k pullup on the trigger input is completely arbitrary as the FET can handle way more current than it will ever see in this circuit (regardless of that pullup's resistance).
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Old 02-28-2015, 06:36 PM   #34
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2/3 or 1/2 scale test

Scaling up the test to use a standard "Target Storage Tub" as my "Black Tank Analog":

(I figure 2/3 to 1/2 scale test as I bet this guy is at least 15 gallons or so--close according to Target they are 16.5 gallons).

Frankly I am quite pleasantly surprised at how well this worked. The values the Arduino calculated for the tank capacitance were very stable and repetitive--I did this test 3 times (first time to see if it worked, second time to add the 3 LEDs, third time to make that video) and each time returned the same values for the different levels.
The next step is to finalize the one shot circuitry. (I'm figuring I'll only be able to take this project so far until I have to wait for warmer weather and then continue the testing/designing/hacking at a full hookup campground so I can fill/empty the black tank several times LOL).
Putting the LEDs on gave me another idea: I'll have to mount the circuit so that I can see the LEDs from the outside and even include a momentary switch to activate it. This way when I'm using the Tornado to flush/back fill the tank I wouldn't have to run inside to check the level--I could simply push the button on the little black box next to the black tank!
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Old 02-28-2015, 09:07 PM   #35
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More experimenting...

Since regular household foil wouldn't really stand up on the side of the tank I went out to the home improvement store and picked up a 12" x 1" strap, and a 12" x 12" piece of steel.
The strap worked quite well but I really didn't have the resolution that I wanted (makes sense since its surface area is smaller it has less capacitance). The 12" X 12" piece of steel worked identically to the foil (which was also 12" x 12"). Another thing to figure out: The best way to stick the plate to the side of the black tank? Epoxy?
I did discover, though, that I could not power up the circuit with the plate connected to the output pin when the tank was full. For some reason it wouldn't read properly in that state. Disconnecting the plate and reconnecting (unclipping the alligator clip) would restore the proper values.
To have the circuit handle this I may use a small 5V reed relay.
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Old 03-01-2015, 10:06 AM   #36
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According to my (dated 1984) Forrest Mims Radio Shack 555 book...
I have not seen a new 555 book written in over 20 years.
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Old 03-01-2015, 10:10 AM   #37
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Since regular household foil wouldn't really stand up on the side of the tank I went out to the home improvement store and picked up a 12" x 1" strap, and a 12" x 12" piece of steel.
The commercial version of the capacitive sensors I have use aluminum foil tape that is commonly used in HVAC applications.

http://www.amazon.com/Scotch-Aluminu...=aluminum+tape
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Old 03-01-2015, 11:48 AM   #38
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A few random thoughts.

If you are going to interface the system to the installed monitor, why not use one of the Arduino's analog I/Os for the lead going to the monitor. Being analog, you can setup the output to be whatever voltage is required by changing values in the Arduino programming rather than having to swap out resistors.

This would make it very easy to calibrate the tank to the monitor by simply changing program variables.

I also got to thinking, a better approach might be to simply rip out the entire monitor circuit. If I recall, the circuit board is simply attached via stand-offs to the backside of the monitor panel.

It would not take much to construct a circuit board to match the one already there, having the same locations for the LEDs and switches, but with an arduino powered monitor. That way, the front-side of the panel would not have to be changed, but the backside would contain a new circuit board with the improved electronics.

But with my next thought, I start to go overboard...

Simply build a new panel. I have used Front Panel Express quite a bit, and a new aluminum panel could be professionally made, and an Arduino could drive a LCD display.

Here is a panel I had made for a camera control system a couple of years ago. I used an Arduino Uno for the processor. The panel was made by Front Panel Express and attaches to a Pelican Case:



It was not cheap - I think the panel was around $100, but it was a fairly complex one.

And finally, I like your idea about being able to see the tank level from the outside of the coach. But expand that idea to include the fresh water tank.

For the project above, I added a RF remote function. Rather than engineering and building a RF circuit, I simply bought one of those made-in-China transmitters/receivers from eBay for a couple of dollars.

I don't see the one I bought, but this one is similar:

http://www.ebay.com/itm/IC2272-4-CH-...item4addd0aea3

But the transmitters I used were circuit boards, not a finished module. But a little searching on eBay and you should be able to find up to a 8 channel (dry contact on/off) transmitter and receiver, or a digitally encoded version - which would require some encoding via Arduino board on the wireless receiver. Either way, it is not difficult.

These cheap little transmitters and receivers really work pretty well.

So I am saying, make a wireless RF tank monitor!!!

And even more crazy, you can buy a bluetooth kit for an arduino board. It would not take much imagination to create an app on an IOS device to monitor the tank levels. If course, we'd have to look into the IOS/Android SDKs, which I have not done.

Problem is, being retired - I am busier now than I was when I worked. I don't know how I ever had the time to work.
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Old 03-01-2015, 02:20 PM   #39
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If you are going to interface the system to the KiB, why not use one of the Arduino's analog I/Os for the lead going to the KiB monitor. Being analog, you can setup the output to be whatever voltage is required by changing values in the Arduino programming rather than having to swap out resistors.
The Arduino I'm using (ATMega328) only has A to D..not D to A.
Quote:
Originally Posted by FW28z
I also got to thinking, a better approach might be to simply rip out the entire KiB monitor circuit. If I recall, the KiB circuit board is simply attached via stand-offs to the backside of the monitor panel.
That is a possibility but one of my stated goals was to leave the panel as is--I want to be able to restore OEM functionality (This is mostly because I wasn't sure how well this would work and wanted a fall-back in case of failure LOL). In addition I was only really concerned with the black tank. Ripping out the KBB means making it work with everything (battery monitor, fresh water tank, grey, black).
Quote:
Originally Posted by FW28z
But with my next thought, I start to go overboard...
I agree: It is far too easy to go nuts with all of the hardware options available these days: Why not replace the panel with a color LCD touch screen and use a Raspberry Pi to monitor everything instead of an Arduino!

That is a cool case/panel!

Adding bluetooth wouldn't be that difficult and I already have a development environment for mobile (Android is much easier/less costly to develop for than iOS).

Lucky you being retired I'm still part of the working masses.
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Old 03-01-2015, 02:21 PM   #40
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Also, did you try the capacitive sensor with both ends attached to a foil outside the tank?

I suppose the point is moot since there is already a hole with a bolt in the tank for the KiB system.

The commercial sensors I had on my boat had both leads on the outside.



That does not mean you have to do it that way... any way will work, but it is a more elegant design for tanks that don't already have a hole in them.

The two aluminum tape strips can be as much as 18~24" high, and are the HVAC foil tape. The copper pads attach with adhesive backing to the aluminum tape strips. There is no direct connection from the copper pads to the aluminum - other than capacitive coupling.

I don't remember the exact restrictions to placing the aluminum foil tape. But if I recall there has to be a min/max separation and a min/max length. I am sure I could locate installation instructions.

The connector has VCC, GND, and an analog output.

The two white leads of course are the sensor leads.
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