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this is my first post here, thanks in advance for any help you can render. I am working on a project that turns on one 5mm white LED running on a 3.7v lipo battery when it comes in contact with saltwater. I want to mount this on my dock pole so when the tide comes up I can look out my window at night and know its high tide. I want it to be as small as a package as possible. I have a Suunto mosquito dive watch that has some sort of water contact in it that turns on automatically when I go diving. This is the same kind of approach I am looking for.

I have searched the internet for days looking for mini water contactor, micro water contacts, submersible underwater nano switches, etc. to no avail. What is the best way to make this happen. I don't want to have to use a micro controller. If I could somehow modify a mini led flashlight by adding a water contact that would be suffice.

So the question is what parts I could use to get it done and where to get them.

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  • \$\begingroup\$ Do you care if this turns on from water or just salt water? Also, my guess is that your watch turns on via pressure, (unless it turns on in just a tiny bit of water.) This is only a guess though. \$\endgroup\$ Oct 16, 2012 at 14:01
  • \$\begingroup\$ The canal I live on contains only saltwater. \$\endgroup\$
    – user15302
    Oct 16, 2012 at 14:15
  • \$\begingroup\$ I think the bottom time is measured by pressure, but the other function definitely from water. If it rains on the boat the function turns on. \$\endgroup\$
    – user15302
    Oct 16, 2012 at 14:18
  • \$\begingroup\$ @GarrettFogerlie Dive computers simply measures resistance between two separated electrodes and when it drops below a certain value, they "know" they're under water. The same thing the OP could do using two wires and a simple comparator. \$\endgroup\$
    – Axeman
    Oct 16, 2012 at 14:26
  • \$\begingroup\$ @Axeman thanks, I've never looked into dive computers but I figured it had to be derived from either pressure or conductivity. Also, for my info, will conductivity vary with pressure in water? My guess is it will, however like I said, I've never thought about it. \$\endgroup\$ Oct 16, 2012 at 16:51

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How about just using float on a rod. When the water rises it can trip a mechanical switch that's been sealed against the elements. Maybe a commercial one or just build one out of a spring. Or you could have a little IR sensor also sealed up that when it reflects off the float it sends you your signal.

enter image description here

Here's some waterproof switches I'm sure there are others:

You could make a simple switch out of two strips of metal, when the two strips of metal are in the water they'd conduct. You could hide them in a PVC pipe with a capped top and open bottom to keep out the rain. That would corrode over time though.

You might also try a hydrostatic pressure sensor like this guy: https://www.sparkfun.com/products/10221

enter image description here

There are other more expensive probably more reliable ones out there as well. The hydrostatic part of it will keep it from going off in the rain. It says it holds up in salt water.

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  • \$\begingroup\$ I kinda did that before but the barnacles and debris in canal from the current and tide caught on it. Wasn't very reliable. \$\endgroup\$
    – user15302
    Oct 16, 2012 at 14:18
  • \$\begingroup\$ One more solution for you above ;) \$\endgroup\$ Oct 16, 2012 at 14:47
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Use a FLOAT SWITCH like these. They're usually used in tanks, to activate pumps when the level is higher or lower than a set point. Mechanical ones are really simple to use, just hang them at the desired level. When water goes above that level, the switch close, and you can use the contact to switch on whatever you need.

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I've always been a fan of zinc and copper as a conductor which allows the circuit to close and conduct, (when introduced into saltwater.) Under normal circumstances it should not react to rain or tap water, however if it did; a current limiting resistor can be sized for optimal behavior.

You would want the Zinc and Copper electrodes close together. This can be made very small.

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