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I'm currently trying to automate my intercom system. My overall aim is to have it so that I can both detect when the buzzer has been pressed and then also to have it so I can open the door. I am using a NodeMCU as the main controller. I'm fairly familiar with DC but have had virtually no experience with AC. Therefore, it has been easy for me to simply wire a relay triggered by the NodeMCU to open the door when requested.

However, I have been stuck trying to work out a method of detecting the buzzer press with the NodeMCU. This is due to the fact that the intercom system runs off 12V AC. After some searching around, my options were narrowed to either a Current Sensor or a Relay. The Current Sensor seems to be too much detail for what my purposes are; leaving me to fall once again on Relays. This time, however, due to the fact that the thing triggering the relay (the buzzer in this case) is using 12V AC rather than DC it has left me in a slightly awkward position as I am unsure exactly what I am looking for.

I have found myself upon this relay which seems to be what I am looking for. I would really appreciate it if someone could:

A) Let me know if this is the right sort of relay for my use case. And if not then what is (and costs little)?

B) Point me in the right direction of exactly how to wire this sort of relay.

This is because, unlike DC relays, I expect that AC relays only take 2 inputs (compared to 3); i.e. the relay gets triggered when electricity flows into the module. Is this correct?

Thank you in advance for any help.

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Both AC and DC relays should have two terminals for the coil - apply the appropriate voltage between those terminals to operate the relay. For the relay you linked to, you would apply 12 Volts AC to those terminals.

Any relay will have two or more contacts for the contacts (the switch part of the relay) Although the Amazon listing doesn't say, with 8 terminals that relay must be a DPDT (Double pole, double throw) type.

A schematic symbol for that relay would be:

schematic

simulate this circuit – Schematic created using CircuitLab

Circuitlab won't let me place the labels where I want - hope you'll understand things.

To operate the realy, you apply 12 volts between terminals "Coil 1" and "Coil 2".

The contacts for one pole of the relay are "COM1" - the moving contact, "NC1" Normally closed contact - connected to "COM1" when the relay is not operated, and "NO1" - Normally open - connected to "COM1" when you apply power to the coil (similar for COM2, NC2, and NO2).

You don't have to use both sets of contacts - only connect the terminals that you need for your application.

I don't understand what you mean by "This is because, unlike DC relays, I expect that AC relays only take 2 inputs (compared to 3); i.e. the relay gets triggered when electricity flows into the module. Is this correct?", but I hope I've explained that relay's operation sufficiently.

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  • \$\begingroup\$ Thank you for your reply! I apologise as I am relatively new to this sort of circuitry and I have only just gotten into electronics. To confirm, this relay can be triggered with 12V AC? I suppose my main query was with the fact that all the relays I have had experience with have 3 pins (VIN, GND, IN1) and that IN1 is responsible for actually triggering the relay. However, in my case, the circuit is "dumb" so I simply want to relay to activate any time 12V AC is applied and as I don't have the relay to hand, I don't see exactly how I would wire this. Surely there would just be 2 inputs? \$\endgroup\$ – Rocco Aug 29 '18 at 0:32
  • \$\begingroup\$ The relay you linked to is just a relay with a mounting socket. The common relay boards or modules sold for use with Arduinos and simiilar microcontroller boards include an optoisolator and driver transistor, so they can be driven directly by an Arduino GPIO, and require a DC control signal. \$\endgroup\$ – Peter Bennett Aug 29 '18 at 1:21
  • \$\begingroup\$ @Peter: The text component has a "snap" on / off dropdown. \$\endgroup\$ – Transistor Aug 29 '18 at 6:13
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What you're trying to do is actually very simple, you're right that a relay would be a better fit than a current sensor or current sensing relay.

A relay is just a switch; compare it to the doorbell button, which is also a switch. The doorbell button is a momentary switch. The switch contacts are closed when someone's finger tip applies mechanical pressure to the button, and opened when they take their finger off the button.

A basic relay is also a momentary kind of switch. Instead of finger pressure, it operates when power is applied to the coil, closing the normally open contacts.

So you'll want to simply run the relay coil in parallel with the bell. The doorbell button (momentary switch) powers both the bell and the relay coil when pushed. The coil of the relay is just another 12VAC load - if you choose the correct relay.

When selecting a relay, you want to select one with a coil that operates on the power you're working with. In your case, you want a 12VAC coil because your doorbell operates on 12VAC. You'll find dozens or hundreds readily available.

Now most relays have double throw (-DT) contacts - there is a common terminal and a normally open (NO) terminal and a normally closed (NC) terminal. This is referred to as "Form C." You can get relays with one pole, double pole, or more poles - SPDT contacts, DPDT contacts, 3PDT contacts, etc. That would be referred to as 1 Form C, 2 Form C, or 3 Form C contacts, etc. If you just want to switch one load based on doorbell power, a single pole would be adequate - 1 form C contacts. However if you're like me, you often find it's handy to have more poles later, so you if you need one now, you buy 2 Form C just in case.

Like other switches, the contacts of the relay are rated for voltage and amperage they are capable of switching. You'll want to select something suitable, although with your project most relays will probably handle much more current and voltage than you have.

The common ice cube relays are inexpensive, reliable, and readily available. They relay part, the ice cube, plugs into a socket with screw terminals and mounting holes. You refer to the product's documentation to determine which terminals on the socket are the coil terminals, and which are contact common / NO / NC terminals.

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    \$\begingroup\$ Thank you for your response! I really appreciate the detail included in your answer, it has been very helpful! As for the actual choosing of the relay module, would the one I linked in my original question be suitable? Thanks once again. \$\endgroup\$ – Rocco Aug 29 '18 at 15:42
  • \$\begingroup\$ @Rocco - From the linked page: "Coil Voltage : 12VAC; Contact Capacity : 5A, 240VAC/28VDC" - coil voltage suitable? check. contact capacity 5A at 240 VAC or 28VDC? - IDK - is that adequate for the load you want the relay to switch? \$\endgroup\$ – batsplatsterson Aug 29 '18 at 16:05
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    \$\begingroup\$ The max load will be 550mA (@3.3v) so I am more than sure that the relay would be able to handle it. My only query was with the actual activation voltage. \$\endgroup\$ – Rocco Aug 29 '18 at 16:10
  • \$\begingroup\$ @Rocco I think you're good to go ... only reservation, it doesn't look like there's online reference for what goes to what on the socket terminals - you could figure it out easily enough when it's in hand but I like brands with easy access documents so there's no guesswork. \$\endgroup\$ – batsplatsterson Aug 29 '18 at 16:15
  • \$\begingroup\$ Perfect! Thanks for the heads-up about the documentation but I am happy to give it a stab at finding the pins. As for the integration with the intercom system, would something like this be okay? Thanks once again \$\endgroup\$ – Rocco Aug 29 '18 at 16:53

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