Author Topic: Doorbell (AC current) sensor design for alarm system door/window sensors  (Read 2849 times)

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Offline sequoiaTopic starter

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I've been playing with design to implement simple sensor that detects when doorbell button is pressed. Goal was to design sensor that could be plugged into (wireless) alarm system sensor as external door/window "switch". And also avoiding to having to modify doorbell wiring other than hooking up the detector between the door bell and the transformer. Also, I wanted to avoid using mechanical relay to mimic real (magnet) switch...

Any feedback on the design would be very welcome (any obvious mistakes?, can it be simplified?, etc..)   :)


Detector works by detecting "significant" A/C current, by using simple current divider and when enough current is flowing through the smaller (ohm) power resistor it ends up triggering the optocoupler.  My doorchime uses about 850mA and and the lighted button about 40mA, so with some trial and error I ended up choosing resitor pair 2.2ohm/200ohm, this seems to work for this purpose. But it would be nice if someone could explain how to calculate the "trigger" current needed to trigger the optocoupler (H11AA1 in this case)   :-/O (It's been too long since I took my electronics course...)
Since it might be good to choose the "trigger" current to be rather low, so that this would work if ever change from mechanical door chime to electronic one, that presumably would use significantly less current...



After I added indicator LED to the circuit to show when door bell button is pressed I had trouble triggering the alarm sensor, so I ended up adding second optocoupler which seemed to work (but I suspect this could work withouth it?)

I'm using this with Interlogix NX450 wireless window/door sensor that has connector for external switch.  Unit works on 3V and after hooking it up to scope it was clear that the sensor sends tiny 3V pulse every 250ms (apparently to save battery) to test if "switch" is closed.


[EDIT: wrong schematic was initially attached, now should be "correct"...]
« Last Edit: September 30, 2015, 07:04:46 am by sequoia »
 

Offline Seekonk

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Re: Doorbell (AC current) sensor design for alarm system door/window sensors
« Reply #1 on: September 30, 2015, 04:38:42 pm »
I always use a current transformer.  Have a whole carton of very small 120/9V transformers that are wound on a split bobbin.  Pretty easy to cut out the 120V side with razor knife then replace that with a couple turns of hookup wire.  That gives a lower voltage burden than an opto.  Speaker transformers would also work and used to be everywhere, they are now hard to find.
 


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