Model Forum / General / Railroads / June 2008

To expand on this a bit.
The only 'magnetic fields' involved, are the magnetic coupling of the
'track feeder' passing through the 'sensor coil'. And those, you need,
and they seems to be working. (Too well at times.)

The DCC signal is 'loaded' (current drawn) to indicate occupancy.

Something is loading things, the problem is WHAT?

The available DCC signal, can be loaded (some current drawn) by either
capacitance or resistance.
Capacitance can be as simple as the two wires paralleling each other
over a long distance.
Resistance can be as subtle as DAMP roadbed.

We need to determine what the cause of the problem is.

Chuck

Well, not in dollars, but my circuit (see my previous posting) can
be made as cheaply as this:

1/4  quad optocoupler eg TLP521-4            L1.08    27p
1x   bridge rectifier (4 diodes) eg Fairchild DF08M        15.2p
2x   resistors eg 2% 0.1W                L0.001    0.2p
                                                               -----
                                42.4p

which is a little under $1.  I'm assuming that you're having lots and
lots of these - at least a hundred - so I used Farnell, a serious UK
electronics supplier, and assumed (for example) the 50-off price for
resistors and the 25-off price for quad optocouplers.  You need to
budget for some board to mount it on too which will at a guess will
add 25-50p to the cost per block.  Also these prices don't include VAT
(UK sales tax).

Resistor values I use are 56R on the optocoupler input, but this will
depend on the desired sensitivity and the voltage drop provided by
your actual bridge rectifier.  I use a 100K pull-up on the output.

In my setup I then feed the outputs to PIC18F458 microcontrollers for
scanning and multiplexing; the PIC has quite good properties on its
inputs which means I can feed the output from my circuit straight into
its general IO pins as they have an excellent high input impedance of
1MR or so, don't seem to mind wavering levels, and which I can program
to debounce the incoming signal before reporting detections.

In my setup, if you count the cost of the microcontroller too, and the
custom PCB to mount it on, and all of the gubbins for reprogramming
the microcontrollers, talking to them from the computer, etc. then it
works out as a lot more than $1 per block :-).

But I don't know what you want to do with your output.  If you need an
output which remains constant rather than flickering between `present'
and `not present' at 10-20kHz (due to the NMRA DCC squarewave, with
the detection circuit only working during one half of each cycle) you
need to at least add a capacitor (say, 10nF at 5.3p) and you'll
probably need to use something to provide a hysteresis, eg a schmitt
trigger 74HCT14 (10.5p for 10-off but you get six to a chip) - that
will give you a TTL-level output for another 7.05p per block if you
don't bother to include a decoupling capacitor.  Total: 49.9p which is
pretty close to US$1 :-).

Of course all of this more or less assumes you have a suitable 5V
power supply and want an output you can feed to something resembling a
TTL input.  And as I say I haven't included the cost of some matrix
board for mounting, which will be a significant fraction of the cost.

I think the real cost of anything along these lines depends on what
you're going to do with the output from the detectors.

There's actually a US patent on this idea/design. the abstract is vague
enough that it would cover your design, too, I think. I didn't look at
the details.

http://www.freepatentsonline.com/5752677.html

For a survey of occupancy detection ideas, look at
http://homepages.paradise.net.nz/~rdmurg/clinic/detectit.htm

HTH