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Doorbell Project

My home/office situation has my office and shop in the basement.  The problem was I couldn't reliably hear the doorbell especially in the shop.  The challenge was to add an additional annunciator to the doorbell without creating an "unsightly" situation, or fishing wires through finished walls.  My situation is fairly typical in that the doorbell is on a finished wall in my foyer, the doorbell button is outside next to the front door, and the transformer is in the basement.  Electrically the most desirable place to connect to this circuit, would be at the doorbell, but there is no 110VAC power and "out of sight" space to add a circuit is very limited.  The easy place to connect to the doorbell circuit is in the basement, but there are no switched signals available there.  The idea of using some radio wireless signaling system at the doorbell was abandon due to complexity, and a lack of "building block" circuits to accomplish the task.  My second idea was to use changes in current to detect doorbell events.  Employing existing home automation technology and building a simple interface circuit solves the problem.  A small interface circuit is required to convert the doorbell circuit current into a "dry contact" output for an X-10 transmitter.  The beauty of this interface circuit is it requires no power supply and uses only 4 readily available components.  Small, simple and gets the job done, my kind of circuit.  The X-10 components I selected are a PSC01 Powerflash Burglar Alarm Interface (using Input B, Mode 3), and a SC546A Remote Chime.  The PSC01 was selected for its "dry contact" input.  Note that the final system connects to the doorbell system at the transformer, and the annunciator(s) can be anywhere in the house were there is an electrical outlet.  Additional outputs can be audible or visual, pretty much anything that can be X-10 operated.  Additionally I have the opportunity to data log the doorbell events through my home's control computer running my Powerhome software.  This system with a lamp module could also be used as doorbell extension for the hearing impaired.  

I had some difficulty with my particular doorbell in that it was 1/2 wave rectified and had an idle current.  I adjusted the sensitivity by adding and subtracting loops on the current transformer.  7 turns worked well for me.  The trick was to find enough turns to reliably detect the doorbell press, but not so many that the PSC01 triggers on the background current.  The PSC01 has hysteresis so if the system continues to chime after you press the doorbell button, the interface is too sensitive and you should remove some turns.  Since I lost my wire stretcher years ago, and you probably don't have one either, you should start with too many turns and remove a few at a time until you get to a good balance.  It is far easier to cut off some wire then it is to rewind a new piece for each test.  Also pay attention to the polarity when connecting the interface circuit to the PSC01 box.  A reversed connection will cause a continuous output condition (it won't harm anything, but constantly ringing chime probably isn't what you had in mind for this project).

Interface Circuits Parts:

These parts are also available through Radio Shack, and Allied Electronics.

During the beta test of this project, I noticed that sometimes the add-on would ring and the doorbell wouldn't. . . surprisingly enough there was always someone at the door.  After a short period of experimentation I discovered my brand of doorbell is sensitive to the length of time the button is pressed.  Very short (quick) actuations would fail to ring the doorbell, but the add-on circuit would pick-up the button presses every time.  The only "false" indications I've observed are at the end of a power failure, the add-on will ring once, when the power first comes on. . .