Where in the world did you get that schematic?

 

From my professor who had it since 1975 so its hard to read when pins are not connected like they are supposed to be in schematics.

 

ok....

As far as the schematic goes.... if the wires are connected, you see the "dot" if they are not, you see the line "jump" over the crossing wire.

 

Yeah,I knew that.

 

When I close the circuit with water which raises the resistance it reflects on the base of the transistor ,which in return raises the voltage above 0.7V turning off the reset pin and turning on the alarm?Now does the electric current flow from base to collector or from emitter to collector to pin4?

 

When I close the circuit with water which raises the resistance ...

Water on that sensor "increases resistance"?

 

That's what they say in description.

 

I'll bet if you measure it, as you dip that board into water ... and the water contacts both strips of foil (+ and -), the resistance should decrease. But you would need a meter to do that.

The test described earlier, by connecting a 100k in series with the sensor and that combination across a power source, would come to the same conclusion.

http://www.lenntech.com/applications/ultrapure/conductivity/water-conductivity.htm has some figures for water conductivity.

 

OK,I guess they were thinking on pure water since it has good resistance when in pure form.

 

Here is the schematic.Its working but I'm not sure why its not making the siren noise it just makes a continuous siren sound it doesn't go up and down in volume?

The LED/Resistor/Capacitor connected to pin 5 will not affect the volume. It will affect the frequency. 555 oscillation is based on the trip points of two internal comparators. Current into or out of pin 5 moves both trip points, but one more than the other.

ak

 

Here is the PCB design in case someone wants to try it out on a PCB.You need to connect the wire from PIN 4 to the resistor's line because i wanted to keep the board on one layer.

Example:

PCB: