joecool85

I'm working on a really simple water activated alarm, and I want your input on it. First, will it work the way I have it set up? Second, with the circuit activated, but no water between the two probes, will it be draining my battery?

__________________
Life is what you make it.

ericgibbs

hi,
It really depends upon the resistance of the water.
A number of members have posted water resistance values, the values seem to vary quiet a lot.

It would be very easy to wire up a test circuit to check the resistance.

With no water path between the probes, the current leakage will be VERY small, minute battery drain.

Why have you put the ON/Off switch in that point of the circuit and not in the battery lead.

If the resistance of the water is high, then consider using a FET rather than a transistor.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Nigel's: www.winpicprog.co.uk/
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

joecool85

I guess I could put the on/off on the battery, I probably will actually. It doesn't matter too much, I just need it to mute the buzzer more than anything. What this will be used for is a cellar water alarm setup, so when my cellar gets more than 3-4" of water, it sets off the alarm and I know I need to do something about it. On that same note, compared to regular clear water, this water should be pretty low resistance. It is normally pretty dark in color and full of sediment, it should have enough salts etc in it that it would be relatively low resistance. I'm going to build the circuit soon, maybe even later today. I can grab a cup of the nasty water and check with that.

Thanks for the quick reply.

__________________
Life is what you make it.

ericgibbs

hi joe,

If you increase the surface area of the 'probe' plates, this will help with the water resistance. Two pieces of old copper pipe should do it.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Nigel's: www.winpicprog.co.uk/
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

Rolf

I have designed a water alarm that uses no electronics but a battery, push button switch and the buzzer (bell or whatever).
It works on the principle that paper is fairly strong when dry but not when wet. For example a half inch strip of dry ordinary 20 lb copy paper has a pull strength of close to 6 lb. But when the paper is wet the strength drops to less than 6 oz.
If you glue this paper to a common wooden cloths pin, in a way that holds it open, some way must be found to hold it while the glue dries; now you have a device that will easily detect water and can activate a switch if they are mounted correctly.
The only problem, it is labor intensive to reset.
Have Fun!!!!!!!!!!!

joecool85

Alright...well, I put the circuit together and it seemed to work when testing with a LED. The LED was off, then when I dipped the probes in water it was nice and bright. Well, I bought a buzzer, and it buzzes when the probes are out of water. I tested with the LED again and it worked fine. Come to find out, it is giving 0.523v when the probes are out of water! How can I fix this? I've tried swapping out the resistor all the way up to 10meg ohms and all the way down to 1 ohm with no luck. Am I missing something here?

__________________
Life is what you make it.

Rolf

Did you indeed use a piezoelectric buzzer?
Or your buzzer have a coil and did you put a clamping diode across it?
You might blown your transistor!

joecool85

I used this piezo buzzer from radio shack http://www.radioshack.com/product/in...entPage=search

__________________
Life is what you make it.

Rolf

That should have been ok without a clamping diode.
But is looks like the transistor is conducting without base drive current, which leads me to believe your transistor is now faulty.

philba

First thing I would do is measure the resistance between your probes. Use ohms law to determine the base current when there is water. Then multiply the base current by the Hfe of the transistor (I'd use 50 for the '3904). This tells you the current (Ic) that your transistor will draw from the load (piezo in this case). Compare this to the piezo spec. If it's low, you need to get more gain. I built something similar and measured around 100K through condensation water.

Based on 100K ohms and 9V, your base current is 90 nA and your Ic would then be 4.5mA. This would be enough to light an LED (though somewhat dimly) but probably not enough to make a piezo speak. You could use a darlington to get more current.

Secondly, I'd add a base to gnd resistor - a couple hundred K ohms or more to keep the transistor off when the input is floating (pun semi intended). You might need to experiment with the value.

Finally, I'd use a current limiting resistor in series with the LED to keep it's current at 10-20mA.

joecool85

I guess I'm confused, I added a 10k resistor from the emitter to ground and that seemed to help. From base to ground just made the piezo go off.

Am I wrong in thinking with a 2n3904 and a couple resistors I can make this work?

I do have some other transistors on hand including:

2n3704
2n4401
MPSA13
MPSA14
MPSA42

Would one of those be better suited for this application?

__________________
Life is what you make it.

philba

Your transistor is likely blown or incorrectly wired. 10K from base to ground should turn the transistor off.

this is a very general application, any NPN would be fine.

joecool85

I'll have to play around with it some more tomorrow. What values would you use for the resistors?

__________________
Life is what you make it.

joecool85

Maybe I could modify this circuit: http://www.discovercircuits.com/PDF-FILES/h2oalm.pdf to use 9v and a 2n3904 / 2n3906 combo?

__________________
Life is what you make it.

joecool85

Nevermind, I got it working perfectly...I had the 2n3904 in backwards lol.

__________________
Life is what you make it.