This is an open circuit system. Momentary power is supplied only when a switch closes.
January 30, 2007.
February 3, 2007.
May 9, 2008.
Here's the wired doorbell switch. The cover is a 
lid from some kind of container in Peggy's kitchen.
Thanks, Peggy.
This is the magnetic switch on the gatepost and its magnet
on the gate. The magnet causes a momentary closure when it passes the switch.
A closer look at the magnet and the switch. There are two switches shown. First I tried the round recessed switch but it wasn't as dependable as the surface mount switch. Maybe because it's outside in the weather. I left both of them in the circuit, wired in parallel.
NOTE: I just found something important. I built this system during the summer when temperatures were hot. The first part of out winter was warm. When our temperatures dropped to near freezing and below, the system became erratic or quit altogether. Then it would begin working fine during the daily warmup. The problem was alkaline batteries. A component of alkaline electrolyte is water. Near freezing and below, the electrolyte water becomes ice; there was no electron flow until warming occured. My battery pack is located on a back porch where the temperature rises and falls as mother nature dictates. I had two choices: move the battery pack indoors where it's warm or use an AC to DC power supply. I had a120VAC to 12VDC power supply, which I'm now using..    
Left, is the plastic box that holds the 120VAC to 12VDC  power supply and transmitter. Above the box is the on/off switch which places the gate in or out of the circuit while leaving the doorbell active.
Below, in the upper corner of the window, is another addition: 
a red LED which comes on when the gate switch activates - lets me know the system is working.
Using wire to extend the range of a wireless doorbell.