Electronics

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Current-Sensing DCC Detectors
	I've been tinkering around with DCC detectors, trying to get Wayne Roderick's Optimized Detector to work on a breadboard when I came across this design on the web. This is about as simple as it gets as far as raw part count is concerned. This design, from Richard Napper was made to control relays on the output. I haven't shown them, instead, I show a possible way to get an open-collector output that can be used as an input to the CMRI interface. I love this design because of its simplicity and minimal parts count on the board. With five turns through the transformer, this can detect 20K ohm resistor wheelsets. This circuit also contains the fast-attack, slow-decay feature in Bronson's and Roderick's detectors. Sadly, MR may buy the rights to an article based on this detector, so I cannot show the circuit here.
	I still wanted to know if the overall complexity of the these circuits could be simplified even further. A 555 timer contains two comparators and a flip/flop, so Richard's circuit, although simple to build with minimal parts count, is actually more complicated with more circuitry. Wayne R. was able to take Dick Bronson's detector published in MR awhile back and reduce it from three comparators down to two. If it can be reduced from three to two, why not one comparator? The circuit to the left shows just such a solution. Now, four detectors can be built from just one LM339 quad comparator chip and four discrete bipolars. The nice thing about this circuit is that it doesn't rely on resonance to work so that means it can work with other current-sensing transformers that have different output inductances without trying to figure out the correct capacitor to use. The Schmitt trigger remains and is formed by the LM339, 470K resistor, and the voltage divider resistors. The divider values are somewhat different than Wayne's. I believe four detectors can be built for under a dollar total (minus the current- sensing transformers)!!! How it works: When a load is applied across the tracks, the transformer amplifies the DCC signals. The transformer output serves as the input to the base of the 2N3904 bipolar transistor. The diode clamps this output to 0.7V which is still enough to the turn the bipolar on. The bipolar collector node pulls down on the 220K and 1K resistors thus charging up the 10uF capacitor rapidly. This in turn pulls down on the positive input of the comparator and the output goes low thus turning on the LED. When the load is removed from the track, the bipolar turns off which slowly discharges the capacitor until the comparator output goes high. The 470K resistor adds some positive feedback and thus hysteresis to the circuit for added noise immunity. Simple and elegant. I'm thinking that it would be possible to reduce some more if a hex Schmitt trigger CMOS chip was used, but that probably wouldn't have an open-collector output suitable for a CMRI application. That would then yield 6 detectors per chip. This is a rather odd number of detectors to put on one card!