shelly Plus 1 as 240v Sensor only

[chammock]

Hi, I have a few shelly Plus 1s acting as smart light switches and in 12v mode to activate a door so I am farmiliar with the wiring.  However I am attempting something new and having issues.

I have a gas boiler which is on standby constantly with 240v.  The boiler fires with a switched live signal from my heating system.  I want to monitor that switched live (240v) so I know when my boiler is fireing.

I have wired the Shelly Plus 1 with a constant supply from the L and N terminals and I have split the switched live from the heating system so it connects to the SW terminal.  I am not using the I and O terminals as I don't want to control anything.

I find in this configuration the input (SW) always reads on regardless if there is 240V live on the terminal or not.  If I remove the cable from the terminal the state changes to off.  If I put a meter accross the N and SW terminal when the switched live is off I get a reading of about 34V.  Is this enough to trigger the shelly SW input and why would I be seeing this?

Thanks for any assistance

[thgoebel]

Due to the fact, the SW input of every Shelly device powered with mains supply is activated both by live and neutral potential, you're perceiving this effect. Remedy is given by (a) an intermediate relay with NO contact. The contact is wired between SW and L, the coil of the relay is wired in parallel to the appliance to be monitored.

Or, (b) with a resistor in the wire from the appliance to terminal SW. This resistor (500kΩ to 1MΩ) prevents the Shelly from activation by live potential. The Shelly is now activated by neutral potential only. This inverts the switching logic: Input is activated if the appliance is switched off, and vice versa. This can be amended by setting "reverse input" in the settings menu of the Shelly device.

Such a resistor, ready for installation, is shown here:

https://www.smarthome-forum.eu/attachment/22379-image4-jpeg/

Some details about the functionality:

https://www.smarthome-forum.eu/thread/10990-konstruktive-lösungen-für-einen-widerstand-an-klemme-sw-in-x/

I gladly offer you my help in sourcing such a resistance wire.

 

Edited August 3 by thgoebel

[chammock]

thanks @thgoebel, thanks for the quick and clear response.  I am keen to go with option b and place a resistor inline.  I must confess I don't have the time right now to read through the entire link post but will do.  I am keen to understand the math required to determine the correct type and size of resistor hopefully this is in the post.  I have say that there seems to be an issue with the sites registration activation email so I am unable to see the images on the site.  

[thgoebel]

Sorry for the inconvenience - didn’t remember the restrictions of the „smarthome“ forum, which are implemented to raise the user count only…
Thus, I‘ve added two pictures above.

The value of the resistor is pretty insignificant. Did some measurements:

Explanation: „Shelly schaltet“ => Input is activated, „dito“ => same as above, „schaltet verzögert“ => „activation delayed“, „schaltet nicht“ => no activation.

I use two MELF resistors with 380kΩ each. Gives 760kΩ, which allows a current of 180μA - more than 4 times larger as the activation current for live potential (36μA). 

 

 

[chammock]

thanks for the detail and the explanation.  I will give this a try