Hi,

I'm analyzing this water detection circuit and have a couple of questions.

As I understand it, the circuit is intended to drive the speaker when the two prongs detect water. A small current through the water should switch on the Darlington transistor (T1). Once T1 is on, there should be no current flowing to IC1's reset pin, allowing the oscillator to run and the output to turn on.

However, in my simulation, it seems to work in the opposite way. The output is on until water is detected.

1. Is there an error in my simulation, or is a NOT gate needed between R2 and IC1's reset pin?
2. I'm also confused about the purpose of R4. Is it simply there to limit the current into T1?
   

Thanks in advance.

 

Hi X,
Your circuit will oscillate because 'B' Reset is HIGH, T1 is OFF.
Who designed the circuit?
E

 

For the 555 to run, the reset must be held high. For that to happen the darlington MUST be non-conducting.
And that only happens when the probes are dry.
You have to invert the polarity with another transistor

EDIT: Wait, there is a resistor missing on the transistor base to Vcc

 

Eric;
The drawing style appears to be from Elektor magazine.

EDIT: upon closer inspection, R4 should go to the base instead of the collector. That is it

 

Hi X,
Your circuit will oscillate because 'B' Reset is HIGH, T1 is OFF.
Who designed the circuit?
E

Using the circuit to search on Google then you can get these sites.

 

An arduino+ a compatible water sensor module could make the task easier.

 

Hi X,
Your circuit will oscillate because 'B' Reset is HIGH, T1 is OFF.
Who designed the circuit?
E

I found this circuit from the book 307 circuits.

 

Eric;
The drawing style appears to be from Elektor magazine.

EDIT: upon closer inspection, R4 should go to the base instead of the collector. That is it

Why should R4 go to the base? Would that not inhibit T1 from switching as the current is likely to be very small.

 

Hi X,
T1 is a Darlington transistor, it has a high input impedance and a high gain.

T1 needs to be ON conducting to set the RESET low to stop the 555 astable circuit

So it needs a bias resistor from its Base to the VCC supply

E

 

hi X,
Checkout this LTSpice simulation.
E
Reversed the change in water level, makes it easier to follow.

 

Project : Pet Water Dish Alarm Sensor
Completed Projects Index

 

EDIT: upon closer inspection, R4 should go to the base instead of the collector. That is it

We have a winner!

ak