Think I've got some of the answer:
http://docs.google.com/viewer?a=v&q...qUopGM&sig=AHIEtbRwpRBQvnzXRfk1BlSLljnVm3BgsQ
Blow Your Horn With A Wireless Doorbell
by Scott Gould ? August 2008
If you?ve got a sound system that uses magnets on the track to trigger a whistle or blow a horn, you may get tired of hearing the same sound at the same place every time around. Here?s an inexpensive way to blow your horn whenever you get the urge, or whenever Bossy wanders onto the track. All you need is a reed switch and a wireless doorbell. Wireless doorbells are available at Radio Shack or from various Internet sites. I bought the one pictured on Amazon.com for under $8.00, plus shipping. The doorbell comes with a pushbutton that will trigger the bell from a distance of up to several hundred feet. To make it operate your horn, you will need to use a reed switch. (Radio Shack sells a 5VDC/0.5 amp version that should work for $3.00. Part no. 275-232). If track magnets trigger your horn or whistle, your sound board probably already has a reed switch, and you may be able to use it with the wireless doorbell
instead of purchasing a new switch. Reed switches are a type of relay, in which contacts in a glass tube are surrounded by a coil. The contacts close in the presence of a magnetic field, as
when a magnet passes near them. In our case, we?ll use the current from the pushbutton on the doorbell transmitter to the close the contacts on the reed switch, thus triggering the horn or whistle.
First, take apart the doorbell and locate the leads that attach to the
doorbell speaker.
Clip these leads (unless you want to hear a doorbell coming from your
engine) and solder them to the terminals attached to the coil on the reed
switch relay. (The wiring diagram that comes with the Radio Shack switch
will show you the correct terminals.)
When you press the doorbell transmitter, the current which would
ordinarily flow to the doorbell speaker will now flow through the coil.
Connect wires from the two remaining terminals on the reed switch relay
to your sound board. To do this, look for leads on the sound board that go
to whatever device triggers your horn or whistle.
Solder the reed switch contacts in series with these leads.
To save on space, you can remove the doorbell circuit board and battery
pack from its housing, and install the parts adjacent to your sound board. I
put my doorbell in a tender.
If everything works, every time you press your doorbell buzzer, your whistle or
horn should sound. The same device could just as easily control a turnout or a
signal. It will also operate a doorbell. So have a blast!
Endnote
MGRS member Mile Silsby comments that he has used a similar doorbell circuit. He
indicates that since the above circuit, depending on the sound board, may be replacing a
relay operated by a magnet with a powered electromagnetic relay, a ?protection diode?
could help prevent damage to sound board components from brief voltage surges.
