Using the NCE BD20 occupancy detector
As I have mentioned I’ve been installing NCE BD20 block occupancy detectors for use with my signaling system on the Piedmont Southern. For the signal logic board I built a circuit that Jeff Scherb designed and published in MR back in March 2001. It uses two 2N3906 transistors and a few resistors so is a quick and easy build even with perfboard, which is what I used. I provided a circuit diagram and parts information for this circuit in my book “Wiring Your Model Railroad”.
With the circuits built and the BD20s installed I powered it all up and when the detector inputs were pulled to ground the LEDs lit up just as described. Then I made the final connections, put a loco on the tracks and the LEDs lit up. Next I got out my resistors and tried using a 10K across the tracks–no lights. Then I dropped down to a 4.7K–no lights. Two 4.7K did give me just a hint of light in the LEDs. Finally a couple 2Ks did the trick but I don’t like the idea of using resistors that low so I started experimenting with the sensitivity of the BD20.
I went up to four turns but no deal, and with a DCS200 putting out 8 amps it didn’t seem to be a good idea anyway so I went back to three turns. Finally I dug into the literature on the NCE website and found a pdf by the HUB Division, NER, NMRA describing a similar problem they had. Their solution for using BD20s with CMRI boards was to feed +5 volts into pin 4, the ground into pin 1, and used pin 3 for the logic feed and got good sensitivity using a 5K or two 10K resistors. Sounded just like my problem so I hooked mine up the same way and sure enough it works!
I ended up using two 10K resistor equipped wheeltsets on each caboose which gives me good detection. The other difference is that my signal board uses a 12 volt power supply so I fed the 12V+ through the BD20 on pin 4 and the ground or negative wire to pin 1, with the output from pin 3 going to my signal detect in connection. Here is a link to the pdf file describing the wiring used by the HUB Div NMRA for their CMRI based signaling system. A tap on the link should open the file or allow you to download it: DetectionSensitivityandNECBD20sonCMRI