I have a Toro automated sprinkler system that can take a rain sensor so that if it rains it will not water the garden.
I have been trying to find one but they have never had stock so I made a plan with some stuff that was lying around the house.
The thought came to me while watching the rain water flowing out of the gutter one evening.

It was very simple and it works very well as all the Toro needs to not water is an open circuit across the rain sensor connection that it has.

Step 1: What You Need

1 x Marlboro cigarette tin, or a square tuna tin, anything that can be use to hold water
1 x brazing rod or hard wire
1 x lever switch that will give you the option of "off" when pressed
1 x small spring
1 x piece of wood
1 x some glue, epoxy worked for me

Step 2: Start Putting It Together

I cut a small piece of the brazing rod so that I could make a adjustment lever for the spring and the amount of pressure that would be needed to activate the switch
I stuck the rest of brazing rod onto the lever of the switch with some help of some copper to hold it together while the epoxy dried
I then bent the rod around the tin so that it fitted under the little edge the tin has got. I clamped and glued the rod to the tin with the epoxy.
I started by screwing the adjustment lever into the wood with a small screw and leaving space below for switch to mount
I then placed the switch so that there was adjustment between the lever and the spring to adjust the level of pressure of the water that the tin would hold and activate the switch

Step 3: Almost There

Her I just checked to make sure that the lever, spring and switch were all functioning properly
I tested the whole think with some water and realized that once the water fills the tin is has to go somewhere otherwise you will not have any watering done until it's evaporated.
I made a small hole in the middle and the back so that the water would slowly trickle out and the switch would reset

Step 4: Fitting It Next to the Gutter

I mounted the tin slightly in front of the gutter as the water can really come down when there is rain, I was not worried if there was only a drizzle as that would not water the garden very well anyway.
Notice the holes, do this before you fit it.

It works well, and has been operational for a few months, I sprayed the switch with Mr Mechanic so that it would not corrode quickly and do that ever y now and then just to avoid fitting a new switch.

I hope this helps someone, and gives someone pleasure tinkering in the garage like it gives me.