The points typically ground something (like the coil) when they close. This
is the key- use a transistor that switches on when its base is taken low. A PNP
bipolar power transistor would be a good solution.
Connect the base of the transistor to two resistors- one is a fairly high
value one, like 2.7 K and goes to the 12 volt supply. The other is perhaps 120
ohms and goes to the contacts. What will happen is this. As the contacts are
in the open state, the base is pulled to the 12 volt side and the transistor is
turned off. When the contacts close, the base is drawn to the low side (ground)
and the transistor switches on.
Now, the emitter of the transistor will be connected to the coil to be
driven, and the collector will be connected to ground. It is a good practice to
place a series resistor (like 1 or 2 ohms) in series with the emitter to keep
the current limited.
The coil, incidentally, is an inductive device and will likely blow out a
transistor the first time you fire it this way. The solution is to place a
diode (sometimes referred to as a "snubber") in parallel with the transistor-
across the emitter and collector.
Use a fast rectifier for the diode and wire the cathode directly to the
transistor's emitter. The anode of the diode will go to the collector.
This will give you a circuit to play around with and see how it will work
for your application.
Cheers!
Chip Shults
My robotics, space and CGI web page -
http://home.cfl.rr.com/aichip