Simple Water Sensor - Alarm when removed from water

[Novix]

Hi all,

I want to make a simple and cheap sensor that will alarm when it is removed from water. I have found information on how to alarm when the circuit is completed when using a water sensor but i want something to switch power to the buzzer when the water sensor is no longer in the water. So rather then the sensor completing the circuit and providing power to the alarm i need a switch so when the water sensor is removed and the circuit is broken, it switches power to the buzzer and then back once the water sensor is put back in the water.

Thanks,

Mike

 

[davenn]

Hi there Mike
Welcome

cannot get much more basic than this one .....
http://electronics-diy.com/plant-water-alarm.php

 

[KMoffett]

Another simple circuit. Draws less than 0.5uA in the water. Manual switch to turn off when not in use. Use stainless steel electrodes. May have to play with R1 depending on the conductivity of the water.



Ken

 

[Novix]

Excellent. Thanks for the information. Very Very helpful

 

[KMoffett]

Out of curiosity...what is the purpose of your intended device?

Ken

 

[Novix]

Just wanted to clarify a few things that will show how new i really am.

First these are the parts i believe i need looking at the sample circuit provided by Ken.
Q1 BS108 = ON Semiconductor BS108ZL1G N-channel MOSFET Transistor, 0.25 A, 200 V, 3-Pin TO-92
R1 10M = Vishay MBB0207 Series Axial Thin Film Resistor 10MΩ ±1% 0.6W ±50ppm/°C
Buzzer = RS 3V ac PCB Mount Buzzer, 80dB

All from http://au.rs-online.com/

Also i was unsure about the physical connection to the Transistor.


Thanks again

Mike

 

[Novix]

Out of curiosity...what is the purpose of your intended device?

Ken

I'm looking to house this in the float for a crab pot. They get stolen and I'm thinking something like this will be an unexpected surprise for the would be thief. As soon as they grab the float out of the water to pull the crab pot i'm hoping the 80db buzzer sends a message to leave them alone.

 

[KMoffett]

Crab pot floats=Salt water exposure. This brings up a "boat load" issues. First the value of R1 will need to be changed...and even then may need to be replaced with a resistor and potentiometer. The issue is that salt water is very conductive, so when the electrodes are lifted above the water, any remaining film between them may keep the sounder off. A very loud sounder will probably require a higher voltage than I indicated. Also, the sounder will need to have an IP68 waterproof rating. And, the whole circuit will have to be encased in a waterproof container.

Ken

 

[kpatz]

Also i was unsure about the physical connection to the Transistor.

With the flat side facing you and the pins down, the pins from left to right are drain, gate, source according to the datasheet (I had to consult several datasheets from several manufacturers before I found one that actually had this info!)

On your schematic, the gate is the long flat part you drew the arrow next to, the source is the one with the arrow pointing in and the drain is the one going to the buzzer.

Salt water is going to create its own issues, due to the enhanced conductivity of the water and the corrosion issues.