Water Tank Float Switch



About: Electronics Engineer by trade, general bodger by nature. Always making things, preferably from what-ever bits I have in the garage. You will find more interesting things on my personal web page rather than t...

This explains a quick solution for a float switch.

I wanted to replace an existing pump supplying the greenhouse watering system and found a Fountain Pump which is very cheap but does not have an inlet port as they are supposed to be completely immersed and, in fact must not be operated "dry" as the water is the only lubricate and cooling medium. I needed a switch that would stop the pump so the water would never get too low.

It took me less than to 30 minutes to make after I had a plan.

Step 1: The Theory?!

Naturally, I did not want to buy a switch that probably costs more than the pump so I wondered if I could use left-over electrical conduit, maybe with a cork inside....

I wanted the switch to be remote from the water to avoid any electrical problems and it has been my experience that mould grows freely in the greenhouse environment and it tends to clog things when given a little water.

So, into the kitchen to borrow the scales...

Step 2: Bouyancy Test

... where I found that the cork could resist 18g which was not enough for the "slider" of 31g that I had in mind so I tried an empty (except for air) plastic pill dispenser which pushed back a suitable 149g, enough to overcome the weight of the slider and the switch action with a lot left over to overcome any stiction.

Step 3: The Concept:

The basic parts that will become a water tank level switch are:

The pill bottle

Plastic channel = 11.5x 11.5mm conduit

"Slider" = 10mm square aluminium (or plastic) tube

and a micro-switch - the roller type shown above would be best for this application.

(I love micro-switches as they usually come with both Normally Open (N/O) and Normally Closed (N/C) contacts.)

A strip of plastic taken from a disposable milk bottle

not forgetting a selection of self-tapping screws 3off and M3 machine screw and nut to secure the micro-switch 2 off.

Some right-angle for securing the micro-switch, in my case it was 10x20mm but ideally would be 5x30mm.

(You can see how I made-do with the 10x20mm in a later step.)

Step 4: A Little Cutting and Drilling:

I had to fabricate a bracket to mount the switch but it must not compromise the channel or slider:

I cut a couple of right-angle brackets from 10x20mm aluminum strip and cut them as shown which clamps the micro-switch in place using the micro-switch mounting holes; if I had had 10x30mm strip then it would have been perfect.

Step 5: Putting It Together:

Using some plastic channel and a piece of aluminium square tube as the slider it was a simple matter to put a self-tapping screw through the lid of the pill container to secure it to the tube, sealing the lid with a gasket of glue before closing it forever but not before fashioning a collar to keep it in the channel and checking for free vertical movement. Some weight can be inserted inside the pill container if necessary e.g sand, old coins etc.

I cut a piece of plastic milk bottle to cradle the micro-switch to provide spacing and hold it out of the channel whilst it's securing screw and nut provide the clamping action; this method also allows it to be moved along the channel for adjustment. I gently scored the plastic before bending to fit it to the micro-switch dimensions and the channel width.

The other end of the slider has to actuate the switch and, whilst you can buy many styles of micro-switch with suitable actuators, I only had available the basic micro-switch so I simply formed a ramp by using one side of the square tube as shown in the photo.

Step 6: Finally:

The pill bottle is screwed to the slider (with the plastic collar around channel) and set in the channel so that the micro-switch carrier can be slide down into position and clamped with it's securing screw.

A couple more self-tapping screws through convenient mounting holes on the pump body and now only the wiring has to be done to finish the job.

So, the picture shows the pill bottle screwed at right-angles to the slider which runs up and down the channel secured to the pump where the water level actuates the micro-switch as the slider moves past and, err, that is a tie-wrap through the other mounting hole of the micro-switch as my bracket was not deep enough. :)

Step 7:



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