Simple water sensor

177

01:58:14

Switch and 50k resistor simulates gap between wires that act as the water sensor.
 Open swich =no water
 Closed switch =water

published 10 years ago

Robert_Kidd

10 years ago

Not good to leave transistor base 'floating' when water level is low - add a pull down resistor between base and ground, perhaps 15k. More importantly, when water is present the transistor will turn on and short out the battery - the transistor will burn out! There is no indicator to show water level - add a LED & resistor (say 390 ohm) in series with collector. Hope this helps :-)

Robert_Kidd

10 years ago

Reduce 390 ohms to 100 ohms & I ncrease transistor gain from 100 to 200 for better LED brightness.

Robert_Kidd

10 years ago

Alternatively, leave transistor gain at 100 but increase base to earth resistor to 100k.

Richardc

10 years ago

@Robert_Kidd the led isnt part of the sensor itself this is just the 'sensor unit'. I forgot to add the resistor between negative and the emitter (i think i have my terminology right haha) it didnt blow the transistor w/out it but the transistor got hot and would likely have failed w/out resistor; with it transistor is cool to touch. Why is it bad to leave base floating when no water is present?

Richardc

10 years ago

P.s. I will add resistor tomorrow when I can see what i used. And thanks for your review and advice Robert!

Robert_Kidd

10 years ago

Hi! It's never a good idea to leave the base 'floating' (or a logic gate with no connection) because you can't guarantee what voltage it will be and, therefore, how the transistor will behave. It's possible the base will pick up noise, especially if it is connected to a PCB track or length of wire which is effectively an aerial.

Robert_Kidd

10 years ago

I'd go for a 100k rather than 15k by the way.

Robert_Kidd

10 years ago

I meant a logic gate input. Must be tired :-)

Richardc

10 years ago

Good to know Ill take that into account on my build, thanks again.

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