How to make a swimming pool sensor to detect water movement?


The swimming pool alarm devices are very, VERY expensive. Like $300-$600. Take a look on In Brazil it costs the double.

So, I want to build a simple swimming pool alarm. If some water movement are detected it sounds.

I have no idea how to start this. Any idea?


Wagner Sartori Junior

A home built capacitive level sensor will detect mm sized waves. Two half submerged conductive insulated plates 200mm x 20mm, 5mm apart, a 555 timer and a divider to give a reasonable frequency that the Arduino can handle. The timer circuitry will need to be adjacent to the plates and waterproofed using epoxy or silicone.
By experiment you will need to determine what digital filtering is required to distinguish wind and rain induced readings from those produced by people/animals.

The difficulty with pressure sensors is finding one that has sufficient resolution and is rated for water immersion. The type of sensor used for air speed pitot tubes will do, but you will have to build an "air bubbler level sensor" which means providing a continuous supply of compressed air. Cheap aquarium air pumps require 6 monthly maintenance cycles.

I will build this and see what happens.

I keep you informed on my results.

Thank you very much for the help.

It is worth noting that the swimming pool environment is very hostile to electronics and plastics with high humidity, salt, chlorine, extremes of temperature, UV radiation and foot traffic.
Treat the project as if you are building something for use on a boat. Fibreglass epoxy/polyester, uPVC and marine quality stainless steel are about the only materials that will survive 10+ years.
If you can remotely locate the Arduino away from the pool under shade and away from the splash zone then it has a far better chance of long term survival.
Make the sensor part self contained, totally waterproof and strong enough to survive being kicked into the pool.
Put cables inside uPVC conduit or 'poly pipe'.

It seems you already found a solution, just one more idea:

If you only want to detect larger waves you could take a vertical tube with a floating ball inside and then either have the ball connect or disconnect a circuit which would trigger an interrupt on the arduino for example.
This might not be the most precise setup but it could be very cheap to implement, easy to setup and maintain and is possibly more than enough for what you want it to do.

Just a thought... :wink:

People have been using 'stilling wells' for hundreds of years.
The deeper it goes the greater the rejection of high frequency surface noise.
The sensor (whatever type it is) lives inside, either above or below the water line.

This is something I came up with some time ago, although I haven't built one:-

Mike, its a good idea.. Thanks for sharing:)

Umm... at the risk of being "boring".... why not PIR sensors around the pool to detect intruders to the pool area before they even get in the water?

Not nearly so fun, but do you want to protect your liability or have fun with home made electonics?

If the latter... is this an indoor pool, or outdoor?

Do you have a solution to the problem of the trip hazard if you run a wire across the ground around the pool?

Can you live with a device which lives in the water when the alarm is set, but is "fished out" when the pool is to be used?

It depends on why you want a pool alarm. Most are not for intruders but for toddlers or pets falling in the water. They are fine by the pool side.