The BD-20 block detector module is normally used to indicate the presence of a locomotive, caboose or other rolling stock in a track section by sensing electrical current drawn by that rolling stock. Locomotives will naturally trigger the detector because they draw current through their DCC decoder. Unpowered rolling stock will not trigger the detector unless equipped with some means of drawing current from the rails such as lights or resistor wheel sets.
The BD20 needs to be as close to the section being detected as possible to avoid false detection triggered by long runs of cable. For the BD20 to work correctly ALL feeders for the section being detected must go thru the BD20. The best practice is to have one BD20 per feeder, and only one feeder per section of track. That section should be isolated from all other sections by using gaps or insulating rail joiners.
The BD20 acts like a normally open switch that gets triggered by sensing current going thru the coil. In a typical application you connect the outputs of the BD20 to something (Logic + 5 to 12vdc) that will react to a ground short. we use it to trigger the inputs of the AIU. Most signal systems work the same way.
The BD20 is a detection device and cannot drive a signal directly unless you want to use the external LED connection in some fashion. An AIU is an input device that detects a 5v line being shorted to ground, such as the output of a BD20. The AIU then puts this information on the NCE Cab Bus. Neither device (BD20 or AIU) are directly related to signaling. They do detection and notification. NCE does not make a signaling product. My recommendation would be to take a look at the Team Digital Signaling decoders. Some very cool stuff!
The Track Feed is wrapped through and around the transformer. The track feed must be the main wire that all the track feeds are tied into. Stated another way, all the track feeder wires for the rail must be reduced down to a signal wire before it passes through and around the BD20.
Current transformers work just like normal transformers in that there is a primary and secondary winding. However, the primary winding is built by you, the installer, who must run a loop or wire through the hole. Each loop of wire through the hole is called a turn. The wire is wired in series with the track block to be monitored by the detector. So any current consumed by any rolling stock or engines on the track must pass through the primary winding of the transformer. The more turns (loops) made in the current transformers hole, the more sensitive the current transformer will become. In other words it will detect lower levels of current.
When current is flowing through the primary, it will generate a magnetic field in the current transformer’s magnetic core. The secondary winding which is also mounted on the same magnetic core will be forced to establish a current flow in the secondary winding wire. It is this secondary current flow that is used by subsequent circuits to implement detection.