Monday, October 4, 2010

Phone Activated Door Hax

This was a quick hack to buzz myself or guests in without using my swipe card or pressing the buzz button on the panel. The Idea was to use the ringing signal on a phone line to trip a relay, therefore opening my door. Ringing phones were not an issue as there are no land line phones connected in the apartment.
After taking some measurements and talking about the project with the phone tech at my school, I figured out what I was going to do. After some research, I determined that phones typically ring in the 90 to 120 VAC range. I measured mine to be around 200, sometimes over 300, Still not a big deal. The other thing though is that there is always 10VDC sitting on the line for when the phone isn't ringing but you're talking etc... To deal with that, I used a pair of non polar capacitors in parallel, as a single .5µf capacitor wasn't passing enough current to trip the relay.

From here, I connected a bridge rectifier chip, which in turn was connected to my 12V relay. I soldered a 47µf 35V cap across the terminals to smooth out the DC. It may not have been necessary, but I had it laying around and I figured at 25Hz I might as well. 

Diagram of the circuit I used

Hodge Podge of parts and hot glue. I love that stuff, plus, I don't have any perf board.

My pretty (yeah right) wiring job

The intercom/door thingy, easy enough to deal with.

Final packaging of the opener. It's crammed in the top of a ranch salad dressing neck with lots of hot glue. I used a chunk of Cat5 to connect to the phone line. My apartment has RJ45 connections so it works fine.
I packaged it all up and stuck it in the wall and its working great. Everything tucked away and connected. I cut a small hole for the cable in the bottom of the plastic so it looks better/involves fewer broken intercoms.

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