A clean-contact float switch, a tank, and the wish for your phone to tell you when it empties or fills. The Shelly Plus Uni is made for exactly this: the float’s two wires go to one of its inputs and that is it.
How it connects
The Plus Uni is a module for reading sensors. It has two digital inputs that take a potential-free contact directly — which is exactly what a float switch is. The module sees whether the contact is open or closed and alerts you when it changes. Shelly lists it as compatible with float, reed, moisture and presence sensors.
The thing to bear in mind is that it is low voltage. It does not connect to 230 V: it runs on 9-28 V DC, 8-24 V AC or 5 V DC, so it needs a small power supply — a 12 V adaptor is more than enough — and it goes inside an enclosure or the equipment itself.
One float, one alert
A contact float gives a single level: it changes state when the water crosses the point where you place it. With one you get “empty” or “full”, not both.
For both you need two floats, one high and one low. That is where the Plus Uni fits: with its two inputs, one float on each, it tells the two states apart with a single module.
The alert on your phone
- Add the Plus Uni in the app — it pairs over Bluetooth —, connect it to Wi-Fi and enable the digital input.
- Create an action on that input: when it changes state, send a notification.
- With the device online, the alert reaches you wherever you are.
With Home Assistant it can also run locally, without the cloud.
What you need
- The Shelly Plus Uni.
- A 9-28 V DC power supply. Remember: not 230 V.
- One float per level you want to watch.
What about a Shelly 1 Gen4?
It can be done, and we cover it in the article on a tank-level alarm with the Shelly 1 Gen4. The difference is in the installation: the Shelly 1 Gen4 has one input and, powered at 230 V, the float switches mains voltage and has to be rated for it. The Plus Uni always works at low voltage and reads two floats at once. For a tank with empty and full, or somewhere you do not want 230 V, the Plus Uni is the more convenient tool.
Frequently asked questions
Can it also start a pump?
Yes. The Plus Uni has two dry-contact outputs that can drive, for example, a pump’s relay or contactor according to the level. They are low-current outputs for control, not for powering the pump.
What if I want the exact level, not just full or empty?
The Plus Uni also has an analogue input: with a suitable level probe it reads the percentage full. That takes a small script.
Do I need more than two floats?
For more inputs there is the Shelly Plus i4 DC, which reads up to four potential-free contacts, also at low voltage. For one or two floats, the Plus Uni is the more complete choice.