Turn any tank into a smart tank: with a Shelly 1 Gen4 and a float you can get a phone notification right when the water reaches the alarm level. The float activates the SW input of the Shelly and the Shelly Smart Control app alerts you instantly, wherever you are.
Why this automation with the Shelly 1 Gen4 is worth it
A tank that overflows because nobody switched off the pump in time, or a cistern that empties without warning, are expensive and annoying problems. With a Shelly 1 Gen4 and a simple level float you turn any tank into one that alerts you on your phone when the water reaches the critical point.
The key is the Shelly's control input, marked as SW. This input reads the state of a switch, and a level float is nothing more than a switch that opens or closes as the water rises or falls.
What you need
| Item | Detail |
|---|---|
| Shelly 1 Gen4 | Model S4SW-001X16EU. Smart relay with Wi-Fi, Bluetooth and Zigbee, potential-free contact and SW input. |
| Float switch | Of the dry-contact (potential-free) type. It's the level sensor. |
| Shelly Smart Control app | For iOS and Android. Where the scene is created and the notifications are received. |
| Wi-Fi + Shelly Cloud account | The Shelly connects to your 2.4 GHz Wi-Fi and the cloud sends the alert to your phone. |
| Power supply | 110-240 V~ or 12 / 24-48 V⏞. The Shelly 1 Gen4 needs neutral. |
How it works, step by step
The float acts as a mechanical switch. When the water reaches the alarm level, the float tips and closes (or opens) its contact, wired to the Shelly's SW input. That physical change translates into an electrical state change the device reads instantly. From there, a scene saved in your Shelly Cloud account watches that input and, as soon as it activates, runs the action of sending the notification.
Wiring the float to the SW input
The terminals involved, according to the Shelly 1 Gen4's official datasheet, are:
L — Live N — Neutral SW — Control input (float) I / O — Output contact (optional here)
Setup in the Shelly Smart Control app
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Add the Shelly to the app
Open Shelly Smart Control, log in and tap "Add device". Press and hold the Shelly's button for 5 seconds to enable its access point and Bluetooth, and follow the wizard to connect it to your 2.4 GHz Wi-Fi. Leave the cloud enabled: that's what lets you receive alerts away from home.
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Set the SW input as "button" / detached
In the device settings, choose a detached or button-type mode. That way, when the float activates, the SW input only generates the event for the notification and doesn't trigger the output relay.
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Create the scene
Go to Scenes → Create scene. In the "IF" condition, select your Shelly 1 Gen4 and its SW input with state "Activated". This is the float's signal when the tank reaches the alarm level.
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Add the notification action
In "THEN", choose "Send notification" (push). Write a clear message, for example: "⚠️ Tank full — check the pump". Save the scene.
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Test the system
Manually lift the float (or fill the tank to the alarm point) and check that the notification reaches your phone in seconds. Adjust the float's height to set the exact alert level.
Ideas to extend the automation
| Goal | How |
|---|---|
| Alert and cut the pump | Wire the pump (or the contactor) to the output contact O/I and create a scene that, when SW activates, switches off the relay as well as notifying. |
| Alert to several people | Share the device with other Shelly users; each one gets the notification on their phone. |
| Integrate with Alexa / Google / SmartThings | The Gen4 is compatible with Matter and with the main assistants. |
| Advanced automation | It supports webhooks (20, with 5 URLs each), MQTT and scripts to send alerts to other systems (Telegram, your own server, etc.). |
Frequently asked questions
Are floats powered at 220 V?
No. A switch-type float draws no current and has no power source of its own: it's a dry contact that opens or closes a circuit. What matters is the voltage passing through its contacts. With the Shelly powered at 230 V~, the SW input is referenced to the live, so the float switches ~230 V and must be rated for that voltage. If you power the Shelly at 12/24 V⏞, the float only switches low voltage.
Can the float be any type?
It must be a dry-contact (potential-free) float, of the level-switch type. Choose normally open or closed depending on whether you want the alarm to trigger as the water rises or falls.
Do I get the alert even when I'm not home?
Yes, as long as the Shelly is connected to your Wi-Fi with internet and you have the cloud enabled. The notification travels through Shelly Cloud to your phone.
Do I need neutral?
Yes. The Shelly 1 Gen4 requires neutral. It accepts 110-240 V~ and also 12 V⏞ or 24-48 V⏞.
Does it consume much?
No. Its consumption is below 1 W, so it can watch the tank permanently at almost no cost.
Specifications based on the official Shelly 1 Gen4 datasheet (kb.shelly.cloud). The exact app menu names may vary depending on the version. Indicative content; 110-240 V installations must be carried out by qualified personnel and in accordance with local electrical regulations.
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