OK, here is another idea.
Back in the heyday of DC, there was a circuit for constant lighting for locomotives. This may work for your detection scheme, but I am not sure although it seems logical.
Three diodes were connected in series and a low current 1.5v light bulb and a single diode in series with the bulb was connected in parallel with the two diodes. This small circuit was put in series with the motor of the loco. When the train would run, the current would pass through the diodes. The three diodes dropped 1.8v and could be read with a meter, this small voltage would light the bulb through the other diode (which would drop .6v bringing the voltage to the bulb down to 1.2v). Now, this would only allow the loco to run in one direction, so three other series diodes were added in parallel with the first three, but backwards. The result was that the light would only light in one direction, and the diode in series with the bulb prevented the bulb from lighting in the reverse direction.
Since diodes will pass a DCC signal (Digitrax uses them in it's BDL-168) this cheap circuit may work for your detection. You also may be able to put an LED in place of the 1.5v light bulb.
The circuit would be put in the main feeder to the track and all other feeders would have to be connected to it so there is only one path for the current to the track.
---|<--|<--|<--- D1, D2, D3
---|<--O------- D4, Bulb
At this time, I am not sure about the polarity of D4.
--->|-->|-->|--- D5, D6, D7
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
ruderunnerThat said, what about option 2, bridging the rails with the LED?
No, sorry. That would just keep the LED on all the time.
Have you considered using an infrared LED and an infrared detector? These are relatively cheap, although you'll need some resistors and a power supply to support them. Use the detector to turn on a panel LED. Infrared will be less sensitive to room light than visible.
It takes an iron man to play with a toy iron horse.
That was kind of what I was afraid of, figuring the powwer would bypass through the other feeders. Maybe the others were saying the same thing, you just put it more succinctly.
That said, what about option 2, bridging the rails with the LED? Or asked differently what kind of resistance would a decodr provide since the resistor for the LED can be changed within a range that may still work.
Modeling the Cleveland and Pittsburgh during the PennCentral era starting on the Cleveland lakefront and ending in Mingo junction
No, the current will only flow through the feeders without the LED. They have effectively zero resistance. You will need at least a few hundred ohms in series with the resistor to keep the LED from becoming a DED (Darkness Emitting Diode.) With a zero-resistance path in parallel with the LED and resistor, there will be no current going through that path at all.
Another way of looking at it is to observe that the feeder is effectively a jumper across the LED-and-resistor combination, so the voltage across them will be effectively zero, so that no current will flow.
True that the red rail doesn't change to black. And if no loco is present there is no current flow between the red and the black. But when a loco "completes the circuit" between red and black rails, current begins to flow through the feeders (LED included hopefully)
All the red feeders are parallel circuits controlled by one switch (the loco being present or not) This is why DCC can work with just 2 wires connected to track, but works better with multiple feeders, each feeder carries some of the load.
As an alternative, what if the LEd/resistor bridged the rails? The LED would be lit if no train was present, would it go out if a train was present? This I believe would be dependant on the resistance of a decoder/lighted car/wheelset versus the resistor needed for the LED.
I'm not particular on wether the on or off aspect means occupied, as long as the other aspect means unoccupied
Think about it for a second, or better yet, draw it out. You have a red buss wire and a black buss wire, both of which are attached to their respective red and black rails.
You are wanting to attach another feeded from the red buss wire, through an LED to the red rail. At no point does that red rail ever become the black rail in order to light the LED.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Ahh I see I wasn't terribly clear abut the feeders, the LED feeder would be an add on to the primary feeders, not the only source of power for the loco. Though it'sa good point about the voltage passing through it.
LED are diodes, they just light up. From my understanding of diodes, no power passes through in one direction (no light) but does in the other (lights up) A check valve for current flow if you will. Therefore the A/C like DCC siginal "should" be acceptable with current flow through it approx. 50% of the time.
I'm also a cheapskate, a pack of LED's and some wire should cost less than 1 premade detector
That NCE BD20 that maxman suggested is a simple, reliable device and not all that expensive. I use them to trigger my crossing signals wherever the track intersects with a road. You gap the rails and loop a wire through the ring on the BD20 and you are good to go for occupancy detection.
Rich
Alton Junction
You would really need one of these or an equivalent: http://www.ncedcc.com/component/virtuemart/?page=shop.product_details&flypage=flypage.tpl&product_id=63&category_id=16 (NCE BD-20)
Another way would be to utilize some sort of infrared circuit.
My initial guess would be that the current draw of the train would be much more than a LED in series would be able to handle.
Brad
EMD - Every Model Different
ALCO - Always Leaking Coolant and Oil
CSX - Coal Spilling eXperts
As they say a little knowledge is a dnagerous thing. I'm no electronics genius but I do understand some basics of wiring.
Due to some revision of my layout plans (redesigned and reloacted stagng yard) my staging will no longer be in plain sight. I need some sort of track occupancy detection.
Would it work if I wired an LED into a track feeder for a staging track? The thinking goes like this: the rails, when properly gapped, have no power passing through the feeders when the track is unoccupied. This would keep the LED off. Now if a loco, lighted car or somethigs with resistor wheelsets bridges the gap between the rails, it completes the circuit causing power to flow and hopefully lighting the LED. This is for DCC since the bus has power all the time.
Or, would the electricity bypass the LED and just pass through the other feeders?