I am looking to find a way to wire up a contact closure trigger or 12VDC trigger to an old-fashioned doorbell that runs on 16VAC. I have run 2 wires to the doorbell which is also lighted. The two doorbell wires that I have access to are under constant 16VAC and go low (maybe around 3-5VAC) when the bell is pressed. What is the best way to add a trigger to this system that activates when the doorbell is pressed? Are there any sensors that will trigger when an AC voltage drops below a certain level? Thanks.

The doorbell button is only providing contact closure. The voltage drop is caused by the coil for the doorbell when the current flows through it. Is this the kind of doorbell with the metal bars, or a round bell with a striker that repeatedly hits the bell?

ELK is your best friend for these situations:

[Link: elkproducts.com]

It is a standard residential doorbell with the dual metal bars. Depressing the button provides the "ding" and releasing the button provides the "dong." I have seen some interfaces such as the Russound DIM-1 and Niles DBI, but these appear to need to be placed after the bell and trigger from contact closure. I only have access to the wires prior to the doorbell right after the transformer which are under constant 16VAC.

A brief look at the Elk device shows it might be just what you're asking for. In order to keep this post at less than a page, I'll just add that making this is not such a difficult electronics project.

Yes, the ELK-930 looks exactly like what I need, and gets wired inline prior to the door bell button right after the transformer. It is also powered by the 16VAC transformer instead of an additional power supply like the other interfaces I mentioned. At $50 it seems like a good deal, well worth the time it will save me.