Author Topic: Detecting push button in AC dorbell/intercom circuit  (Read 158 times)

Lukapple

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Detecting push button in AC dorbell/intercom circuit
« on: July 31, 2020, 06:50:04 AM »
Hi,
I have a home smart doorbell/intercom system, which is powered by 12 V/AC and connected to the chime. the intercom system has only 2 wires - it doesn't have separate wires for push button and power.
What I want to do is to detect an "intercom button" press with the Moteino.
I've measured voltage on the circuit and there is about 3.9Vac when intercom is in stand by, and once you push the button, it spikes up to 12Vac.

Any idea how to detect push button event with Moteino?
I was thinking about using an AC relay or something, which will be triggered by that 12Vac spike. I can detect closed relay switch with Moteino then ...
Will that work or is there any simpler solution? If using relay is the way to go, then which relay is appropriate for this job... DC relays probably can't be triggered by AC?

sparky

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Re: Detecting push button in AC dorbell/intercom circuit
« Reply #1 on: July 31, 2020, 01:34:53 PM »
How about voltage drop across the switch.. when voltage goes to zero button pressed is detected.  Not sure if 12v can be applied to the Moteino pins though.

Regarding the relay.. generally you can use a DC relay with AC but not the other way around. 

Lukapple

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Re: Detecting push button in AC dorbell/intercom circuit
« Reply #2 on: July 31, 2020, 01:42:15 PM »
How about voltage drop across the switch.. when voltage goes to zero button pressed is detected.  Not sure if 12v can be applied to the Moteino pins though.

Regarding the relay.. generally you can use a DC relay with AC but not the other way around.  Windings are heavier on DC relays..  AC coils has inductance to help limit the current

Approximate multimeter reading are:
voltage supplied to the circuit by the transformator(rated 12VAC): 15 VAC
voltage drop across the switch (intercom): 14 VAC
voltage drop across the chime (which has additional chime module(some kind of resistors) wired in parallel): 1 VAC

Once you push the button, voltage drop across the switch is approx 0V and 15 on the chime.


 

Lukapple

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Re: Detecting push button in AC dorbell/intercom circuit
« Reply #3 on: August 02, 2020, 04:25:18 PM »
I've tried to use Optocoupler H11AA1SR2M for this job.



I've connected pins 1 and 2 to the doorbell circuit, with 2k2 resistor, so when you push button, it triggers that optocoupler.
For test I've wired a led diode with separate power supply to pins 5(collector) and 4(emitter), and when I pressed the doorbell button, LED blinked... so it worked fine.

Then I removed that LED circuit and tried to wire Collector and Emmiter to Moteino pins 16 and GND, so I could detect button press with Moteino, it didn't work. I've used sketch for button press detection (if I connect pin 16 and GND directly, button press is detected fine).
Any idea what am I doing wrong?

Lukapple

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Re: Detecting push button in AC dorbell/intercom circuit
« Reply #4 on: August 03, 2020, 06:54:51 AM »
Wiring diagram is something like this: