Activate LED when water not present

Thread Starter

srubino386

Joined Feb 29, 2016
6
Hey everyone

So I am having trouble with this project of mine. Basically, I need an LED to turn on when water is not present (dries out). So first I need this to happen with just a bowl of water (oh and I cannot add salt at any point). Then need it to also work with a bowl of wet sand, bowl of wet paper towels, soil, etc. So I took a 9v and ran a copper wire at each end of the bowl and then to a 9v normally open relay, however it only works when the wires are touching, it's not conducting through the water. Then tried it with a 3v relay, same problem... Hmmm.

Any help would be greatly appreciated!
 

DNA Robotics

Joined Jun 13, 2014
563
Until someone comes up with a better idea, do a web search for LM339 comparator circuits. I think it is sensitive enough. You may find a circuit that you can use. The output on or off depends on which input you use.
 

AnalogKid

Joined Aug 1, 2013
8,487
While regular water does conduct electric current, it does not do it very well. The current through the water is not enough to do much of anything directly. The signal has to be amplified and compared against a reference voltage to determine if there is enough current to equate to the presence of water (or damp materials). Because passing DC through water causes electrolysis and other chemical reactions, the signal through the water usually is an AC signal, either a sinewave or square wave. This makes detection a bit more complex, but not much. The innergoogle has many circuits for water detection. Modifying one to reverse the action of the indicator is a minor change.

ak
 

Picbuster

Joined Dec 2, 2013
1,008
Hey everyone

So I am having trouble with this project of mine. Basically, I need an LED to turn on when water is not present (dries out). So first I need this to happen with just a bowl of water (oh and I cannot add salt at any point). Then need it to also work with a bowl of wet sand, bowl of wet paper towels, soil, etc. So I took a 9v and ran a copper wire at each end of the bowl and then to a 9v normally open relay, however it only works when the wires are touching, it's not conducting through the water. Then tried it with a 3v relay, same problem... Hmmm.

Any help would be greatly appreciated!
Simple problem use an alternating current like a block two advantages:
a: no problem with electrolyse
b: pure water acts like a bad capacitor so it has an impedance.
take two metal bars one to gnd separate and isolate 1 cm approx.
generate a block feed this via resistor and capacitor into a metal bar not connected to gnd.
use opamp measure between resistor and capacitor.
This is a measure for water level. that's all.
 

Dodgydave

Joined Jun 22, 2012
9,203
You need to use an AC detection system like this...when water is present the led lights up, take the led from output to ground to make it on when no water detected.

circuit

Swap the opto-coupler for a led or transistor and relay.
 
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Thread Starter

srubino386

Joined Feb 29, 2016
6
You need to use an AC detection system like this...when water is present the led lights up, take the led from output to ground to make it on when no water detected.

http://www.simplecircuitdiagram.com/wp-content/uploads/2010/10/Optocoupler1.gif

Swap the opto-coupler for a led or transistor and relay.
Thanks Dave,
I've been trying to give this a shot and am still having trouble... I think this might be beyond my capability. Would you be able to walk me through this a little further? I hope we can work something out!

Thanks again,
 

Alec_t

Joined Sep 17, 2013
11,397
The post #9 linked circuit uses DC energisation of the probes. That will result in relatively rapid corrosion of the probes. Dave's suggested circuit in post #5 is better.
 

Thread Starter

srubino386

Joined Feb 29, 2016
6
Ok what have you got done at the moment?
Ok, I'm having trouble correctly placing everything in my breadboard... Ugh, I feel like I've tried this every which way now. My board goes from A-J, 0-60. Is there anyway you can help walk me through the placement of each item?
Thanks!
 
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