IN STOCK & SHIPS OUT IMMEDIATELY, Unlike Other Dealers Who Take 5 Days or More to Order it Then Ship Out to You a WEEK Later !!!!!Up for your consideration is a Lot of 2 Brand NEW, NEW Stock – ANY Scale NCE – Item No. 205 Ready-to-Use – BD20 Block Detector Each pack contains 1 for a total of 2. Isolated so that there is no electrical connection to the trackNeeds no external power for most operationsSuitable for DCC power up to 20 AmpsCan light up LED or operate relay with external DC power (5-12 Volts)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. *** ALL ITEMS Will be Shipped out in a Secure Box, NO Padded Mailers like some irresponsible Dealers ***I am an Authorized NCE Dealer and I carry the Full Line of NCE Items. If you don’t see it in my eBay Store, just contact me for availability. Check out the NCE website for additional details and information.SHIPPING Outside USA Through eBay’s Global Shipping Program ONLY !!!!! Plus FREE SHIPPING In the USA !!!!!I am Constantly purchasing items DIRECT from the Manufacturers, Wholesale Suppliers, Hobby Store close outs and Overstocks and I am passing the savings on to you. Several of these items are Discontinued and SOLD OUT at The Manufacturers. Don’t miss them again. I have been in the Hobby and Retail sales in this Hobby along with Sports Cards, Coins & Other Collectibles since 1991. I own and operate a Retail Store. Please check my feedback and purchase with confidence. Payment methods accepted are PAYPAL ( Immediate Payment is Requested ). S/H for this item will be FREE by USPS First Class Mail. Thank You for viewing my Listings and I hope you find something you like for the price you want to pay. Good Luck and God bless !!!!! Please confirm your shipping address is correct before making payment. For all items with a manufacturer warranty, if your new item is damaged or not working, please return to the manufacturer as directed on their warranty statement. If you need a purchase receipt for this purpose, please contact us.