Have built this simple low liquid alarm as attatched-my problem is when probes are in the liquid the buzzer is still sounding albeit at low level. It is working as when the liquid level drops the buzzer goes loud but how can I make it silent when liquid is present? I know this is all very basic but hey I've got to start somewhere.

 

Where did you get this circuit from? There are some wrong assumptions made here.

A lot depends on the electrical conductivity of the liquid.
Ordinary tap water is not a very good conductor of electricity. The 1kΩ pullup resistor is way too low in value for this circuit to work.

You will need two stages. One stage to detect the liquid and the second stage to drive the buzzer.

 

OK MrChips, Thanks for your prompt reply. Circuit was from the website shown on the attatchment, any idea where to get a better one?

 

try this circuit, its a Dry sensor circuit for plants
buzzer sounds when dry

http://www.circuitlab.org/2008/08/circuit-water-sensor-for-your-plant.html#axzz26wK05YR7

 

Hope I'm not being stupid here, but I want to detect lack of liquid not prescence.

 

Hope I'm not being stupid here, but I want to detect lack of liquid not prescence.

http://www.circuitlab.org/2008/08/circuit-water-sensor-for-your-plant.html#axzz26wK05YR7

 

Thanks DodgyDave

 

You didn't state the purpose of you water system, but using direct current between long-term, submerged electrodes will cause the electrodes to corrode.

Ken

 

Maybe you can add a Zener between 100Ω and transistor, this can be increase the Voltage detect level, and also increase 100Ω to 1K~5K, it depends on how do you use the value of Zener, and how many current does the Buz needed.

If you want to detect the water, the resistance of water maybe over 10K, you better use 2 transistors to do the detection job.

 

Decades ago I built a water sensor for my Mom (she liked to hang the clothes outside to dry). Just a quad CMOS NOR or NAND gate, 2 gates as a gated astable, 1 gate to detect the water, and I dunno what the last gate did (maybe needed an inverter?). The astable output drove an NPN transistor to squalk a speaker.

The thing ran for years and years off the original 9V battery,

Sensor input was a gate input with something like a 1M pull down resistor, other probe was the 9V with a series 100K resistor.

 

To echo Ernie, I've used variations of this circuit for long term water level monitoring: http://www.electroniq.net/other-projects/detectors/fluid-level-sensor-circuit-using-common-parts.html
Very sensitive and AC coupling, so no corrosion of the electrodes. Note, you do need to use stainless steel wire or plates for the electrodes.

Ken