I have started a project using my Arduino that notifies my iPhone whenever a switch in a separate circuit is closed. The plan is to connect this to our home doorbell so as well as the usual ding-dong we also get an iPhone alert. Software-wise this works great using a Wifly shield to enable the Arduino to talk the the Prowl notification service on the Internet over http. It’s kind of a wifi version of the project described here http://blog.makezine.com/archive/2010/12/snail-mail-push-alerts.html.
Now the problem I have is how to properly connect the Arduino to the doorbell circuit. The doorbell circuit is powered by a transformer that deliver 5V AC at 1mA. Note AC not DC. I tried measuring the voltage across the transformer using my meter but it read 13.8 so I’m not sure why that is. I want to detect a connection in the AC circuit and notify an Arduino digital input pin. I have been trying to use a H11AA1X optocoupler transistor to keep the two circuits separate. The H11AA1X is designed for AC input as it contains two leds, one for each AC current direction. I connected the H11AA1X in series with the doorbell circuit along with a small 470ohm resistor (to protect the leds in the optocoupler). I also have a couple of leds in the circuit so that I can see when something happens. I have tested the prototype away from the doorbell circuit using a separate (admittedly DC) power supply and simulated the doorbell switch and it works fine. But when I connect it to the doorbell circuit I can not get it to respond to the doorbell switch and the bell itself does not ding-dong.
I now have a working prototype, although I am still a bit worried about the current the H11AA1X is handling.I realised that my original “test” led on the AC side of the circuit was probably a bad idea and removed that. I created a simple circuit with just the H11AA1X and the smallest resistor I had in series with the doorbell circuit. I put 5v across the collector and emitter with an led and a resistor across the DC side of the H11AA1X. When I pressed the doorbell the DC led lit up but the dinger did not dong. I obviously had too much resistance on the AC side to trigger the solenoid. It occurred to me that the bell solenoid is already providing sufficient resistance and that I could probably run the H11AA1X without an additional resistor. So I tried that and it worked. My test meter can’t do AC current so I have no idea what current is actually flowing through the H11AA1X. I did find a web site that described the transformer as 1amp, so fingers crossed it will be OK. Here’s my working circuit diagram.
I also got it to work by building a different circuit with the H11AA1X and a resistor in parallel with the bell solenoid. Unfortunately that would entail extra wires and the Arduino to be installed in our hall way which would not pass the wife-acceptance-test. The bell transformer is in our garage so well out of sight but it does only leave the possibility of connecting my iDoorbell Arduino in series.