ROBERT SCHWORM says:

A reed detector is a small glass cylinder in which is a pair of reeds ( points ) that act a a switch to complete an electrical circuit. The other half to this is a magnet, installed most likely under a car.

As the car passes over, the magnet pulls in the 2 reeds completing the circuit. The wiring on this reed assembly goes off to a processing unit such as a occupancy detector in your dcc system, or perhaps to a sensor array on an arduino running jmri.

On the RPI with JMRI, this can translate into occupancy and when properly configured in your panel, can cause the track section to go from black to red. This red moves around your panel as the train moves around your track.

Another form of “sensor ” is the IR (infra red) sensor. This is comprised of a IR led next to a detector led, embedded below your rails in the center of the bad, and flush or slightly below the members. Same principal. A closer to “ground” is completed and sensed by your supporting detector system. Sensors can be used in various ways. It can even cause an audio file to play that you are approaching a station, for example.

Go to youtube and read up on videos by GEOFF BUNZA, an electrical wizard in these matters.

Good Luck.

Bob S

Kevin Ching says:

I use a reed switch in my track to flash an LED but it does not stay on as once the loco passes the switch open again I used this as a flashlight on a person taking a photo I just had the circuit of 3 volts to make the light flash I also used an SMD LED that is very tiny The kids believe the figure is taking a photo of the train as it passes