Extended Moisture/Temp Sensor

cdub

If I wanted to·detect moisture or temperature across a long surface, can a sensor be constructed that is connected to a long wire versus one point at the sensor?
This might run along a garment or rug. An exact measurement isn't needed, but a large change would be enough. What would be a good sensor to work with?

Franklin

Moisture could be done by resistance the temperature could be done with multiple sensors.

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- Stephen

FlyingFishFinger

What comes to my mind is, instead of using one long sensor, use several single ones spaced out over the distance you want to cover. Then compare the values at different points. That's unless you really need continuous measurements...
Rafael

cdub

Thanks for the comments so far.· One example of the project would be to detect a pet 'going in a hallway where·multiple sensors could work at a higher cost; So I was hoping there could be one long wire or grid.

Thanks,
Cdub

FlyingFishFinger

Hm...good point. However, I think long sensors would qualify as speciality, which would make them more expensive. I have no idea what exactly what you'd be looking for, but that's something you might want to keep in mind.
Rafael

cdub

Thanks for taking the time to consider the idea.· I searched the net and came upon a project that may belong to another list member (Allen?).

It has a picture of what looks like an extended sensor: http://www.emesystems.com/OL2mhos.htm

I haven't seen anything commercially available either, so I'll heed your comment that it may need a special application($) to drive its availability to hobbyist in the future.

Tracy Allen

The sensors could be a pair (or several pairs) of wires, running parallel through the sensing area. How long is long? I am aware that variations on that circuit have been used for applications as diverse as monitoring salt spay on the bottom of automobiles, condensation in aircraft deicer bags, wetness in baby's diapers, and the agricultural applications for soil and leaf moisture detection and greenhouse window control. The circuit uses a 555 with the resistance in the feedback path, with capacitors to enforce AC balanced drive.

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Tracy Allen
www.emesystems.com

cdub

Given your·reference to those real world applications, it sounds like I should begin by building something similar to the attached circuit-diagram. I don't have a ds1620 and only want to detect moisture and light an LED or use the sound piece in the diagram. As a basic user, what could I take off the homework board to just read when P10 receives a moisture signal as conductivity between the screws?· Also, can I attach a 5 foot or longer wire to each screw and lay out in a rug to detect light moisture (droplets at most) without modification?

Thanks,
Cdub

Kevin Wood

Conceptually this is easier to see from the schematic. The resistance that you would measure between the 2 screws is basically the same concept of bridging the air-gap with a conductive material, in this case water. Dipping the ends of the screws in water completes the circuit.

So, adding wires extending from the sides of the screws would mean that a drop of water contacting a part of the wire wouldn't affect the resistance in the air-gap between the 2 screws, since it wouldn't be completing the circuit.

Based on the schematic, it looks like you would use RCTIME on pin 10 to measure the charge rate of the capacitior, where the resistance value of the circuit will be inversely proportional to the conductivity of the fluid.

Bruce Bates

cdub -

I don't know whether this will be helpful for experimentation or not, but here is a water alarm kit for $ 8.00 US:
http://www.allelectronics.com/cgi-bin/item/MK-108/search/WATER_ALARM_KIT_.html

Regards,

Bruce Bates

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Tracy Allen

You can remove the DS1620 circuit and other stuff, and retain only the conductivity circuit and the sound maker. Then a simple snippet is:

'{$STAMP BS2}
'{$PBASIC 2.5}
result VAR Word
DO
  RCTIME 10,0,result
  LOW 10
  IF result then FREQOUT 0,7,3400
LOOP

No sound when dry, then increasing rate of clicks when wet and wetter. Activation will take a continuous film of water bridging the gap, not just a few drops here and there.

Sure go ahead and experiment with lengthening the wires. Long wires tend to make good antennas for AC and noise pickup, so there will be some limit. Please be careful! Maybe too increase the protection resistor from p10 to the circuit from 100 ohms up to 1000 or 4700 ohms.

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Tracy Allen
www.emesystems.com