Touch experiments and a question

[RonHackett]

I have been experimenting with the 18M2 touch sensors, and found something interesting. If I connect a simple jumper wire to one of the touch inputs, and dip the other end of the wire into a glass of water, the touch16 result immediately drops to 0. As soon as the wire is removed from the water, the touch16 result returns to its (approximate) original value.

So, here's the question I have for all the hardware experts on the forum: Is there any possibility that this procedure will damage the touch input over time?

If not, a touch input could be used as a very simple way to sense the presence of water (for example, in a basement). Also, multiple touch inputs may be able to measure the level of water in a tank. For example, five touch inputs placed at the appropriate levels in the tank could sense whether the tank is full, 3/4-full, 1/2-full, 1/4-full, or empty.

In the real world, tanks are frequently grounded, so I added a jumper wire from ground to the water in the glass, and re-tried the same experiment. The results were exactly the same, but the same question applies: Does grounding the water increase the possibility of damaging a touch input?

I suppose the answer to both questions may be "time will tell," but if there's an obvious reason not to use this approach, I would appreciate the feedback.

Thanks… Ron

 

[Technical]

You are effectively creating a 'touch pad' so large that the touch command gives up and simply 'times out' - hence the 0 value.

So yes, it should be pretty reliable - until the wire corrodes! This shouldn't damage the chip in any way.

 

[RonHackett]

Thanks Technical - as long as it's not doing any damage to the 18M2, I'll just clean the wire occasionally! Actually, I'll probably use a small copper pipe cut into segments and slipped over a PVC pipe with O-rings separating each segment.

Ron

 

[1968neil]

Water tank level

Another great and very reliable method is the An MPX-2010DP silicon pressure sensor (temperature compensated and calibrated)

there is a great project full of useful info on the following link.
http://kayno.net/2010/02/11/wireless-picaxe-based-water-tank-level-sensor/

enjoy

Neil

 

[RonHackett]

Thanks for the link Neil - that's a really interesting project.
I see Mouser carries the MPX2010DP, so I think I'll give it a try.

Ron

 

[westaust55]

Thanks Technical - as long as it's not doing any damage to the 18M2, I'll just clean the wire occasionally! Actually, I'll probably use a small copper pipe cut into segments and slipped over a PVC pipe with O-rings separating each segment.

Ron

If you consider how capacitive level sensors work using two parallel probes into a tank, etc then having one at ground (0V) and the other connected to a PICAXE "Touch" input may work.

 

[Dippy]

I'm confused (not too difficult).

If it's capacitive why do you need exposed 'pads' that will corrode?
I thought the whole point of going capacitive was that the electrode(s) could be protected with a non-conductive and inert coating or layer?

 

[Technical]

We think Ron is not using it as a capacitive sensor system (which won't work with 'wet' systems anyway).

He is just using the touch command in a novel way to get a cheap water sensor!

 

[Dippy]

Ah so. Sounds like an interesting app.

 

[slurp]

My first reading of Ron's experiment was the wire was in the water, not isolated from it - I'd be concerned about static and voltage potentials as well as pin to pin effects.

If the wire is in a tube isolated from the liquid then it doesn't appear to be such a problem.

regards,
Colin