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 needs to be as close to the section being detected as possible to avoid false detection triggered by long runs of cable.
You may get false triggering of the detector if the block is too long. The capacitance between the rails actually conducts a small amount of the DCC signal and can cause the detector to trigger. If you think you have problems with this see the section on desensitizing the detector.
Desensitizing the detector:
If you have problems with false triggering due to humidity, ballast glue, track oil, etc. providing a conduction path for the DCC track current, a resistor or potentiometer may be installed on the PC board to desensitize the detection circuit . Figure 3 shows the location of the desensitizing resistor mounting holes. The resistor or pot should be 1/4 watt or more. The value is determined by
adjusting the pot. You can use a pot to determine the correct resistance for the block then substitute a fixed resistor of the same value for the pot. The value is typically in the range of 500-10K Ohms. We suggest a 6mm 5K-10K pot. Digikey (1-800-DIGIKEY - www.digikey.com) offers suitable pots. Part number X262R103B-ND is under 50 cents.