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From: jpexg@rice-chex.ai.mit.edu (John Purbrick)
Newsgroups: comp.robotics
Subject: Re: dirt cheap immersion sensor?
Message-ID: <12047@life.ai.mit.edu>
Date: 27 Nov 90 05:08:32 GMT
References: <1990Nov24.211134.28944@uunet!unhd>
Sender: news@ai.mit.edu
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Organization: MIT Artificial Intelligence Laboratory
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In article <1990Nov24.211134.28944@uunet!unhd> rg@unhd.UUCP (Roger Gonzalez ) writes:
>
>I need a cheap, fast, and small sensor to detect when its been submerged
>in (fresh) water..........

OK, how about:

Set up an oscillator, a 555 should be fine. Use a small capacitor; I'd start 
with 100pF and see how it goes. In parallel with this, arrange two plates where
water can get between them. Now, what will happen is that this forms 
(obviously) an extra capacitor. But the dielectric constant of air is small.
Water, however, has a large dielectric constant, so when the capacitor 
gets flooded, it becomes much larger and the circuit's frequency of 
oscillation will go down. If the water ISN'T pure, you may get more of a 
resistive than capacitive effect, and it's not clear what will happen. Some
change in circuit behavior, anyway.

--John Purbrick

ps Note that the water doesn't need to touch the electrodes--it only has to 
fill most of the space between them.