Need help designing a simple(?) electrical circuit

[scolley]

I've been working on an automated tank replenishment problem for a number of months now. Many of you have helped... thanks! But it is still a problem unsolved.

I'm trying to create something that can detect water pressure so that I can automatically drain a bit of tank on a regular basis, and have the water pressure detection device shut off the re-filling process when it detects a "tank full" condition. I have the bottom of my tank bulkheaded, so I can mount pressure sensing hardware under my tank without having to put junk in the tank - that is a goal. But until yesterday, all the water pressure sensing hardware I found either did not work, or was insanely expensive. You can read all about it in this thread.

But yesterday I found the perfect device for sensing water pressure. I found it here. There are two models, a gage and a differential model. The gage model (sensing water pressure relative to ambient air pressure) would be perfect. But there is a hitch... it requires circuitry to do anything with it.

Can any of you (maybe EE's out there) help me?

I've been going to Radio Shack on a regular basis since I was 9. And I'm OK with a soldering iron. But I cannot design a circuit.

The pressure transducer in question will output varying voltage based on the pressure it detects. That varying voltage needs to, at some determined point, throw a switch to either open (or possibly close) a circuit that controls the re-filling of a tank. That predetermined point needs to be "user adjustable", like with a potentiometer or something. That way you can change the point (or water depth) at which the re-filling process shuts off.

Can anyone out there design such a circuit? Is this trivial or difficult? Expensive or cheap? Easily done by a rookie or requiring professional assembly?

Any and all advice appreciated. Thanks in advance!

[snafu]

i was going to suggest a simple float level sensor until i read parts of the referenced thread. heh. do you think you will be able to get the resolution (say less a cm accuracy) using a pressure sensing technique? will the normal variation in barometric pressure affect your control system? do the math before diving in and buying a high price pressure transducer... maybe someone has already suggested this, but have you looked into an active sensors for level sensing (e.g., ultrasonic ranger)? maybe use an ir-ranger for redundancy/backup? they are fairly common in the robotics world (meaning there are lots of schematics on the web), and, yes, it wouldn't require putting any more stuff into your tank. also, i believe you can pick up the needed parts at radio shack too.
-snafu

[scolley]

Originally Posted by snafu

Do you think you will be able to get the resolution (say less a cm accuracy) using a pressure sensing technique?

Yes, or close enough... read the specs on the sensor.

Originally Posted by snafu

will the normal variation in barometric pressure affect your control system?

For my original sensors yes, for the one referenced in this post, no. It reports not absolute pressure, but the differential between the pressure tested and ambient pressure. It's exactly what's needed.

Originally Posted by snafu

maybe someone has already suggested this, but have you looked into an active sensors for level sensing (e.g., ultrasonic ranger)? maybe use an ir-ranger for redundancy/backup?

No one has suggested that, but yes, I have looked into them. You can find stuff built to do that... IR, radar, sonic... all kinds of stuff. It's all pretty expensive, if it has any reasonable level of accuracy.

I appreciate the comments snafu. All good questions. Thanks. But they do belong in my other thread where I talk about doing this.

Here I very much would like to stick to the topic... needing help designing this (assumed to be) simple circuit. But unfortunately I've heard not a peep from an EE, or someone that can tell me how to do this.

[snafu]

Originally Posted by scolley

Here I very much would like to stick to the topic...

peace and good luck. (sorry for intruding on the thread)

might as well pick the sensor up from the source for about half the cost (and with a smaller pressure differential 0-1psid)...
http://www.omega.com/ppt/pptsc.asp?ref=PX26
(recall 1cm H2O ~ 0.014 psid)

product sheet
http://www.omega.com/manuals/manualpdf/M1608.pdf

-snafu

[scolley]

No prob snafu! You are just helping. Thanks!

Thanks for the link to the cheaper vendor too! I had already found that site, but was afraid to link to it because of all the other stuff they sell... ir, radar, and other cool surface detectors! I was afraid that the conversation would degrade into "Hey Steve, why don't you use one of these , or these, instead?"

Too late for that though I guess!


Back to topic...

Are there no electrical engineers or electroncs wizards out there that can help me with this circuit?

Any help would be greatly appreciated.

[g8wayg8r]

I looked at you sensor at Automated Systems and I can't tell if it's a static or differential gauge. First of all, 1 inch of water is 0.0361 psi. If you had 20" of water, your pressure would be 0.722 psi. To control within one inch of water you need to sense better than 0.0361 psi. The accuracy of the 5 psi sensor is 1% of full range or 0.05. Also, you're going to work at the low end of the sensor and I wouldn't bet that you are going to get a linear response. To me, it looks like it won't do what you want but maybe it will. Although this is simple in concept, the electronics is just a small part of what you want to do. You have to decide on a solenoid to turn the water off, the voltage and current type you want use for that and the electronics, insultaing and mounting the sensor and then making it work without drift and error. This is just my way of saying welcome to the club.

Here's your general circuit. Feed the output into an op-amp and wire it for hysteresis. That will take two or three resistors. Here are two references.

http://hyperphysics.phy-astr.gsu.edu...chmitt.html#c3
http://www.ee.ttu.edu/lab/robot/hyster.htm

The power handling part is relatively simple in concept. You will need and current limiting resistor and a transistor to turn on a solenoid or a power transistor. It depends on the solenoid valve you want to turn off the water. Here are some references to get you started with transistors.

http://www.kpsec.freeuk.com/componen...n.htm#choosing
http://www.rason.org/Projects/transwit/transwit.htm

I would recommned getting a breadboard from this sourse. It;s a good place to start that won't set you back a bunch if you decide it was nuts to get into this. Don't forget the wire and some jumpers.

http://www.futurlec.com/Components.shtml

Try this place for pressure sensors.

http://dkc1.digikey.com/us/pdf/Current.html

Here's one they sell.

http://rocky.digikey.com/WebLib/Meas...del%201220.pdf

Take a look at some the stuff above and get back. Maybe that shut off for the RO water units isn't so bad after all.

Enjoy

[scolley]

g8wayg8r - great response and thanks a mil!!!

So where do I begin in responding...

1) I'd be buying the differential gage. That way ambient pressure is not an issue.

2) Great point on the accuracy of the instrument. I'm assuming that if I pad by 1/2" from the top of the tank, and then then transducer responses swing by around 1.5", then at worst case I'm 2" from the top of the tank, best case 1/2". You are right in thinking that's not ideal. But I'm willing to take it to get out of water changes and toping off the tank.

3) The Schmitt Trigger looks way cool. I was worried about how to deal with "jitter" or noise, and this seems to solve that problem. One other answer is to employ a relay that has a delay built in, so you get a bit of "over-fill" by definition and thus get past that jitter.

4) Even still, with this circuit design, I still don't know how (where) to put a pot in so the desired level can be "dialed-in". That's obviously where I want to go... not a static circuit.

5) Thanks for the great links! Digikey is one in particular that I've made the mistake from ordering from in the past. Now I get their 10 lb. catalog. For me, that's 10 lbs. of paper of stuff I can't remotely understand! A waste of their money I'm afraid. They seem to think I have a clue. But for me the real problem with them is that their stuff is great, it's just not really not positioned for the EE newb, which I am the most extreme form of.

And finally, and my hat's off to snafu that brought me back to this website, I've started to wonder if maybe it might be better to forget doing a DIY of this circuit, and find something that does it all for me. This product looks like it might... assuming of course I hook it up to a relay that could do something useful with an alarm condition, like shut off the water filling. As an FYI, here is Omega's full set of such devices.

Thanks for the reply. It does indeed get me one step closer to a solution to this interesting problem.

[snafu]

fyi... simple reference design from motorola
http://www.freescale.com/files/senso...ote/AN1517.pdf

and remember... why buy? when you can SAMPLE! whee.
-snafu

[g8wayg8r]

Originally Posted by scolley

g8wayg8r - great response and thanks a mil!!!

Even still, with this circuit design, I still don't know how (where) to put a pot in so the desired level can be "dialed-in". That's obviously where I want to go... not a static circuit.

You will use two pots. The first is your reference that feeds into the positive input of the op-amp. You need a voltage divider so you can input a voltage setpoint. You will have to do a simple calculation and pick a combination that will give you what you need: Vref = V*Rpot/[Rpot+R1]. You may want to use a 16-turn pot to give you better control and put it in parallel with a fixed resistor so you can reduce the magnitude of the change in resistance per turn. You can always use a variable precision voltage regulator as well and zenor diodes to help select V. The equation above is the most elementary and you must have a stable voltage source.

The second pot is the negative feedback resistor for the Schmidt trigger. It will allow you to "dial in" you noise buffer.

[scolley]

Wow! Thanks Andrew. Thanks Snafu.

OK, I understand the need for two pots. That also explains why this has two pots. Maybe.

I really appreciate all the info you have given me. I'm sure that anyone with minimal skill with circuitry would be able to take it from here. But not me. I can look at a schematic and wire it, but that's the limit for me. Which is the same thing as saying I don't really "understand" circuitry. If I've got to figure out what value resistor (or any other component for that matter) in a particular spot, I'm lost.

So I figure I've got a few choices...

1) Talk to the Omega people and see if the aforementioned product will work instead.

2) Decide to invest the time to learn basic electronics.

I suspect I'll do the former, for I'm a bit hesitant to experiment with learning where the primary thing at risk is flooding my house.

Thanks.

[snafu]

that omega unit seems a little pricey...

the simple reference design should be able to guide you through the selection of resistor values for your intended application. There are only only 3 or 4 values/ratios of importance (all of which are very well documented).
a) R1/Roff to adjust the sensor offset voltage
b) R6/R5 to adjust the gain of the input signal
c) R7/Rth to adjust the voltage when the switch changes state (on/off)
d) Rh to adjust the amount of hystersis

per the recommended test/calibration procedure, you can use potentiometers for Roff, Rth, and Rh to either fine tune your circuit or provide additional flexibility later on. just sample a low power quad op-amp (from national), pick up some resistors and an NPN transistor (2N3904) from 'da shack', dig up a solid state relay, and away you go. you can play with the switch circuit before investing big money on the pressure transducer too

...and take everything i've said with a grain of salt, since i'm not a EE and have never taken a single electronics class. tee hee hee.
-snafu

[scolley]

Originally Posted by snafu

the simple reference design should be able to guide you through the selection of resistor values for your intended application. There are only only 3 or 4 values/ratios of importance (all of which are very well documented)...

Snafu - you seem to think I have a clue as to how to do this. I don't! If I have a detail schematic and a corresponding parts list, I can build it. But whenever it gets into something like "put the appropriate resistor here", where I actually have to have some understanding (vs. assembling parts) I'm lost.

I'm sure this is easy for someone with knowledge. But I'm not sure I want to, much less have time to, learn rudimentary circuit design when I can buy a working, tested solution.

[Naja002]

Snafu - you seem to think I have a clue as to how to do this. I don't! If I have a detail schematic and a corresponding parts list, I can build it. But whenever it gets into something like "put the appropriate resistor here", where I actually have to have some understanding (vs. assembling parts) I'm lost.

I'm in the same boat, so I'm going to chime in here tentatively. This may or may not be helpful, so take it for what its worth:

I talked to a Guy that I work with who did Electronic Aircraft Maintenance on Carriers in the Military. I didn't get into the whole Aquarium Auto change thing--just how to wire up that PX26 to turn on/off a pump or valve.

What he explained to me made perfect sense When he was explaining it--But I may not have it sorted out now...

Here is a basic diagram:




According to him, 1 or 2 resistors may be needed, but a pot can possibly be used to control the voltage into the Pressure Transducer (PTD) (10-16v) and another can be used to control the voltage out of it to the relay creating the setpoint. That may make things simpler.

Again, I may not still be understanding this correctly.

Any thoughts...?

[snafu]

To make sure we understand the operation of the differential pressure transducer... a) first, it requires an excitation voltage (specified at nominally 10V and up to a maximum of 16V). b) depending on the differential pressure across the sensor, it will output a very low (mV) response (for the 1 psid sensor with 10V excitation ~ 16.7mV at full-range. c) so at very low differential pressures, we are talking about something far less than 16.7mV, possibly a couple mV.

so, in the proposed design, we are going to take the voltage down even more with the pot before the relay switch? and the switch is able to change state directly based on mV-level logic? clearly, we are talking about a more sophisticated sort of relay switch with comparator logic built in? also, not sure about the pot before sensor, since the sensor output will likely scale proportionately based on the excitation voltage... meaning excitation voltages less than the nominal value (10V) will likely reduce the level of the output signal even further. basically, i'm not sure how the proposed design works.

the simple reference design (.pdf) has essentially two functions. a) signal conditioning (get the signal up to a reasonable working voltage), b) comparator (logic to switch based on a voltage set point to provide high/low logic to the relay, microcontroller, etc). something that only requires 1-2 ICs (or equivalent number of op-amps), an NPN transistor, and a handful of discrete components seems like a minimal sort of implementation. i should shut my mouth now and let the EE's speak because i'm way out of my league...
-snafu

[scolley]

Well folks, it looks like none of us have to understand it EE fully! I spoke to the people at Omega today - very helpful - and here is what they recommended.

Its "repeatability" rating is the degree to which it can be trusted each time. So that gives me a 1.6" variance. Not perfect, but for the price, I'm willing. The pressure level at which it switches state (water height at which it throws the switch) is dependent on a locking mechanical ring. It has a build in switch with three leads so it can open a circuit, or close it, depending on how you wire it. It'll handle wall current with either a 1 or 5 amp model, so I just need to match that to what ever I'm powering (filling pump, solenoid, etc.). And it operates "differentially", so it's not true pressure, but rather water pressure relative to atmospheric pressure.

It looks almost perfect. Wadda ya' think?