Monitoring dry contact?

[OPGA]

TLDR: Can I use a Shelly to monitor a dry relay and log anytime the circuit is closed vs open? Which model(s)?

I have a parking lot gate motor that sometimes stays open longer than it's supposed to. Unfortunately it tends to happen sporadically in the middle of the night, and I'm trying to determine what's keeping it open. I'm looking for a device to "listen" to the safety terminals (dry contacts) to monitor how often the safety sensors are closing that circuit and for how long. Is Shelly suitable for dry contact monitoring? If so, which models do you recommend? If not, any ideas for other products that would do this (with Wi-Fi)?

[thgoebel]

Shelly UNI plus, Shelly plus i4DC. Or a Shelly 1 Gen3 or Shelly plus 1 - both devices can be supplied with 12V DC, which may be essential for the dry contact. Alternatively any other Shelly which is suited for 24V DC supply.

[wooly]

If contacts are "dry" then the supply you use on the other side is quite irrelevant, even if 5-24 is the most manageable.

[thgoebel]

The issue is the isolation parameter of the dry contact. This is unknown. Thus, we have to propose SELV compliant solutions.

[OPGA]

I'm new to this... If I wire the Shelly i4DC to the safety sensor's dry contact in parallel with the gate (meaning two wires from the gate's motherboard's safety contacts to the safety sensor, and two wires from those same contacts on the safety sensor to a Shelly i4DC's input terminals), will that interfere with the communication between the sensor and the gate?

My mental model: the safety sensor's contacts are potential free ("dry"), so they're just connecting the wires together. The gate motherboard's terminals must have some potential on them (right?) to detect when the circuit is closed. Is that all correct?

But ... If I connected the safety sensor only to the Shelly, could it detect contact closure? If yes, does that mean Shelly has potential between the input contacts? And if that's the case, and Shelly is wired to the gate motherboard (because it's wired in parallel), wouldn't that mean you'd have two devices putting potential/current on the circuit?

I'm sure I'm missing something... Thanks again for your help. 

[thgoebel]

You‘ll have to ensure that the Shelly is supplied with a „floating“ and SELV compliant power supply. Under these circumstances the Shelly does not interfere the proper operation of the gate control.

Alternatively, the Shelly could be supplied by the gate motherboard. But this demands an in-depth study of the functionality of the motherboard.

[OPGA]

The Liftmaster board as an auxiliary 12v DC output that can be used to power peripheral devices like sensors, so I was planning to tap into that. 

Can you clarify how the Shelly 1 Gen 3 could be used to monitor a dry contact for closure? Does it not require voltage at the SW terminal?

[thgoebel]

With the exception of Shelly UNI (Generation 1), all denominated devices use the same circuit diagram at pin SW/IN-x:

The input is triggered by a very small current (approx. 35μA) flowing from pin SW to pin L/GND. The current is driven by 3V. A diode inhibits reverse current.

 

[OPGA]

Oh, I see - super helpful, thank you. So if I were to take a Shelly 1 Gen 3 and power it with 12v from the gate operator's auxiliary power output, what other connections would I need to make to attach it to the gate sensor's circuit?

Currently, it is:

Wire 1: motherboard safety terminal 1 -> safety sensor dry contact relay terminal 1 

Wire 2: safety sensor dry contact relay terminal 2 -> motherboard safety terminal 2. 

Do I just attach safety sensor dry contact relay terminal 2 to "SW", and that's it? Sounds like it would detect anytime any voltage is present relative to the motherboard's ground... Am I understanding correctly?

[thgoebel]

It is necessary to check if one of the two wires is (a) connected with GND of the board or (b) with +12V DC (supply voltage of the board. With case (a) or (b), it will be possible to supply the Shelly with the boards 12V DC.

If neither (a) nor (b) is true, Shelly must be supplied with a floating DC supply (12V or 24V DC). Pin SW of the Shelly has to be connected to the most positive of both wires (1 or 2). Pin L/GND of the Shelly has to be connected with the other wire (2 or 1). This ensures proper detection of the contact closure by both devices (board and/or Shelly).