What type of water sensors there is and what should one use for the following:
when drop of water is on sensor it should activate a motor for x amount of time.
Take a search engine of your choice and ask the WWW for
'rain sensor +arduino' to collect some data to be sorted out to get the needed information.
Thanks; I have found this and that and both seems what I needed. Any suggestion for choosing one over the other?
That depends on your unknown system basic design.
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What do you mean?
I would like that if amount of water is greater then x it will activate motor. if less - motor is Off
So this:
and that:
I don't think either of them will give you any indication of the amount of water?
Maybe look at one of these:
https://www.weatherstations.co.uk/hydreon-rain-sensors.htm
Or this:
Another approach is to have a cup that collects the rain, and a sensor to detect when it's filled to the required level; eg,
What about a standard rain gauge...?
both have Analog pin which mean continuous data? it not need to be very accurate just to make a motor start working if water was detect
You said you wanted it to depend on the amount of water. 
Shows the importance of giving a proper specification!
You'll just have to test them to see if they do give enough variation for you to get your required level of accuracy...
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There are all sorts of devices around, the most common are on car windscreens for detecting how heavy the rain is.
You can use the pcb track type placed at an incline inside a rain catcher to determine the quantity of rain at any time by setting a timer up.
Given this is a capacitive device then the output will depend on how much water is on the surface at any moment in time. If this is an analogue device then the amount of rain will be depicted by the Voltage in.
I think those tend to be optical which, I think, is also the principle of the Hydreon sensor.
The Grove one says it uses conductivity.
The Amazon one doesn't say, but it looks very much like it also senses conductivity:
Note that it's applying a constant DC across the sensor - so there's going to be electrolysis happening ...
(Groves schematic view is broken, so can't see that one)
what does it means?
Which part are you unsure of?
You can see that one side of the sensor is permanently connected to VCC (via the 10k resistor), and the other side of the sensor is permanently connected to GND - in other words, it has a constant DC across the sensor.
Electrolysis is what happens when an electric current passes though a liquid
This causes corrosion:
Its just another type of sensor that can be used
Thats because it conducts when rain water bridges the electrodes, however parallel tracks also form a capacitance, therefore it can be considered as capacitive.
Rain water running over the surface will impart a charge onto that type of device, given rainwater carries a charge.
But what's actually driving the outputs of those boards is the conduction - that is what's being detected.
To work capacitively, you'd need to insulate the electrodes - so that the water drops don't short them out!
I'm sure capacitive ones do exist - but these ones aren't.
Does it really matter what you want to call it. If you don't consider a board to be an insulator, you can tell millions of PCB designers they have it all wrong because interspaced tracks are not capacitive.
The problem here is to count the rain drops, it does not really matter upon which method is used does it.
The question was to determine the amount of water, which this type of board can achieve, with a little calibration.
I use these same boards as rain sensors and I also use fibre washers attached to a switch to stop an irrigation system from overwatering. Both methods work effectively, however I don't use them to count rain drops.
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