Water level alarm

[mohsen 2012]

Hello , i build this car alarm circuit :






and i want a simple modification in this circuit to use it for a water level alarm .
please help me and thanks

 

[BradtheRad]

Question: Your schematic does not show a sensor. Could it be a capacitive sensor, perhaps implied as a track on the pcb? Or do you hook up an external sensor to the circuit?

 

[mohsen 2012]

yes i want to add external sensor to out of the circuit

 

[chuckey]

I think the circuit works by monitoring the 12V car battery, when some one opens a door the interior light comes on and takes current and the battery volts fall.
Q1&2 are a bistable, normally ON, when a door is opened the battery volts fall faster then the volts at Q1 emitter, so both transistors go OFF and allow the astable to squeak (Q3&4).
So connecting a 1K to Q1 base would be one probe, the other is the -12V (earth), the squeak would happen when the probes are shorted.
Frank

 

[mohsen 2012]

I think the circuit works by monitoring the 12V car battery, when some one opens a door the interior light comes on and takes current and the battery volts fall.
Q1&2 are a bistable, normally ON, when a door is opened the battery volts fall faster then the volts at Q1 emitter, so both transistors go OFF and allow the astable to squeak (Q3&4).
So connecting a 1K to Q1 base would be one probe, the other is the -12V (earth), the squeak would happen when the probes are shorted.
Frank

Thanks for explain , but i want to use this siren for indicate the water level , I have taken this modification, but did not give a good result :



when i put the 2 probes in the water , the siren sound level is very low !!!

 

[Vbase]

Put the 2 probes in series to the base of Q4. Because the current there is low the resistance of the water will not reduce the sound level.

 

[mohsen 2012]

OK, you mean like this :

 

[Vbase]

You got it right mohsen.

 

[BradtheRad]

I suggest connecting the base of Q4 to ground through a 2M or higher resistor.

I once made a water detector, which also made use of a darlington pair (because the multiplied gain was needed). I expected it to draw miniscule current from the 9V battery. However instead of working for months, the battery was depleted in a couple of days.

I believe the bare wires picked up ambient 60 cycle hum, and it biased the transistors on. (After I got an oscilloscope I was able to see this occur in devices.) Current flow was not strong enough to make the buzzer sound, but it drained juice from the battery.