ruderunnerTransistors huh? Hadn't really thought about those but that's certainly a possibility. Refresh me on those, IIRC they act kind of like a relay don't they? Meaning using a very low current siginal to control a higher current load.
your idea about placing an LED in series with the track is novel. Presumably you're interested in electronics. But as others have stated, most LEDs can't handle the current through a locomotive. The twin-tee circuit is very similar to your idea except you need a way to know when current is flowing through the diode. And that way is the voltage across the diode (~0.7).
you can read more about transistors on the web. The relay analogy isn't quite right. In general, the current through the base-emitter junction controls a larger current (x50) across the collector-emitter. The diode voltage in the twin-tee circuit is presumably large enough to supply this current into the transistor. A resistor in series with the base is probably needed otherwise you might burn-out the transistor.
an LED-resistor can be in series with the collector of the transistor and when there is sufficient voltage across the diode to "turn-on" the transistor, it should sink current through the LED. The twin-tee may needs an isolated supply (try a 9v battery for starters).
greg - Philadelphia & Reading / Reading
Part of the reason older circuits liek the Twin-T fell out of favor with the advance of electroncis technology is that they aren't terribly sensitive. A powered loco, no problem, a lighted car, no problem, but they need fairly low resistence wheel sets to detect unpowered equipment left standing. The lower the resistence, the more current each resistor wheelset draws. The transformer type used on our club layout work with as much as 15K across the rals, we put 10K on each car (me, I put a pair of 4.7K, two out of 4 axles, one on each truck, others just do 1 out of 4 axles with a single 10k resistor). Most of ourse are the ones from RR-CirKits.
Rob Paisley has this one: http://home.cogeco.ca/~rpaisley4/DccBODvt5.html
You can buy the pc board and all parts, except for the transformers. The transformers are cheap, overall this would cost less than $5 per block. It doesn;t get much cheaper than that. Maybe if you source the parts yourself and point to point wire them on perfboards, but under $7 for the pc board with all the parts is a pretty good price. The other similar circuit is the Chubb DCC Optimized Detector, works on the same pricinpal but incorporates some delays in the final output so the indication doesn't flicker ack and forth if there are dirty spots on the track. Problem is, even in the MR article on building these, the specs for the transformer were never given, so you'd have to order them direct, at a premium. The RR-CirKits BOD-8 and set of transformers comes in at $6.48 per block, even though it's meant to work with their Tower Control interface device, it can be used as a standalone detection unit and just drive LEDs.
Doesn't get much cheaper than this. Considerign there are some other clowns out there offering block detectors that they calim have all these superior features and are as much as $25/block - and still use diode drops. None of the ones I mentioned here will cause voltage drop to the rails.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinkerPart of the reason older circuits liek the Twin-T fell out of favor with the advance of electroncis technology is that they aren't terribly sensitive.
I found a description of the twin-tee detector in the Thorne book and ralize that my description isn't quite right. The diodes across the base-emitter junctions are not necessary if the currents are low (HO or N) and the transistors can pass the expected current. This would mean all the current through the tracks passes through the transistors and detection should be relatively sensitive. While this may not work for one truck with resistors, it may work fine with several in parallel. (With DCC, the track voltage is always on, which should make detection easier than DC).
The diode would provide some protection by limiting the voltage across the transistor and may be needed for O gauge locomotives. So one circuit may not fit all needs.
Another aspect is what the OP hopes to gain: reliable operation quickly in order to build more scenery or operate, or an opportunity to gain some understanding of electronics to better understand all the gadets used on model railroads today.
The EASIEST option is quite low-tech - just hang a mirror. A cheap camera and display would also be extremely easy, if comfort level with model railroad electronics is the deciding factor.
Remember the Twin-T goes way back - having the 75 year MR DVD has allowed me to read the initial articles when it was first presented. At the time, transistors available to the typical person were all germanium, and ones with any sort of current capcity were very expensive. Most systems up til then used relays, which were super cheap as war surplus, but war surplus can't last forever. May sound silly when today you can buy 100 pack of transistors for $5 on ebay, but ina time when $5 was almost a day's wage, blowign a $5 transistor because you goofed blew the hobby budget for a week.
DCC pretty much just eliminates the need for the bias supply and bleed resistor, since the tracks are always powered, unlike a block system where unused blocks have no power in them at all. The other thing it buys you is a great simplification of the circuit - if you put enough capacitance in the output stage so it holds up during the time the waveform is going the opposite direction, you really only need to detect current flowing one way. Worst case this does is delay the detection by the maximum allowed zero stretch - a loco entering the block right at the satrt of the negative half of the signal won;t be detected untilt he positive half, and likewise a loco leaving the block right at the satrt of the negative half won;t be 'forgotten' until at least the positive half starts. Most signal detectors incorporate longer delays than that anyway, to prevent flickering detection with dirty wheels, and in the case of Dr. Chubb's circuits, to "simulated the slow moving and ponderous relays of the prototype". Remember CTC machines sent coded signals to tell a signal or turnout to change state over a SLOOOOOOW serial line - formerly used for telegraph communications. One of the CTC sites has recordings of the relays all clunking away sending out signals, change ONE turnout lever and hit the code button and it clunks away for a significant amoutn of time to transmit the required commands.
A mirror is an idea but this staging is under my layout, about 20" off the floor. If one has to get down to see the mirror, there really wouldn't be a need for it.
I looked into these transformer units, and see they are the base of lots of ready made detectors. They look to be what I remember beinig called "step up" transformers, that is they take a low voltage a/c current and multiply it to higher voltage a/c. So a 1mV drop through a feeder gets converted to 350mV right? I know still a small number but do I get the gist?
Modeling the Cleveland and Pittsburgh during the PennCentral era starting on the Cleveland lakefront and ending in Mingo junction
Greg, no I'm not terribly interested in electronics or building lots of little gizmos. But you are correct that I'm looking for reliable detection in an easy to use form that won't break the bank. Therefore if I can build some small circuits quickly at a savings over off the shelf products I'll learn and do it.
For the record this is HO/DCC and consists will be 1-3 locos (depends on train length)
I scrounged through some stuff that I inherited and found an "amp clamp" meter. This is the type that clips around a wire to read current. So I fiddled a bit with a loco and an isolated section of track, results were interesting. train parked current read 10mA, as the loco ran down the track (about 15 ft) and got closer to the feeder I was reading the reading climbed to almost 250mA. Like a Doppler effect. This may or may not be useful info but it goes to show how much resistance there is in nickle/silver track. I attempted to get a voltage drop reading but the batteries in my DMM were dead.
Sort of - they are more like one of those clamp-on ammeters - the kind you just close the jaws areound the wire and it tells you the current flowing in said wire.
Still the same principle as any transformer. Inside those blocks are many many turns of fine wire. You run your feeder wire through the whole, with 1, maybe 2 turns. A small voltage flowing through that wire causes a larger voltage to flow in the coil, which is detected by the additional circuits attached to it.
As for a mirror, the idea woudl be to place the mirror such that the operators can see without bothing to bend down and peer underneath. A workable position may or ma not be found, deending on how your benchwork, fascia, scenery, etc. are set up. I've seen them used for this where the staging is either way up high, or hidden behind a row of scenery. Using it for staging near the floor might be trickier. But it is relatively inexpensive, low tech, nothign to fail, and requires no modification to any wiring or rolling stock to work. It does, however, require operators to pay attention.
For hodden staging though, IR detection can work, especially if the light level is fairly constant. It could even be completely automatic, unless you use mid-train or rear helpers - have the sensor near the very end of the track, but have a gapped section longer than any loco consist, or, if the staging tracks are guaranteed to hold the longest train you run, then just gap each siding. A simple IR dectector would trip a relay to cut power to the track when covered, so it's just a matter of selecting the proepr track and runthe train in, it will stop when it gets to the end, befor it hits the bumper. To get traisn back out, a momentary button or toggle bypasses the realy contact to apply power to the track, until the IR detector is clear. With that sort of a system, even an inattentive operator won;t smash into the bumper.
Ah clamp on ammeter is that tool I fooled with. Mechanics call them amp clamps.
Yes my staging will be hidden, as in not normally seen when operating. But one side will be open to the aisle, just close to the floor. It's going to be double ended, no stub track though and entered from 2 distant points on the layout (about 36 ft from one end to the other) I'm figuring 5 or 6 tracks plus a through track (access dependent)
So IR detector or light sensor can operate a relay without additional circuitry? I can get lots of relays for free (used but heythey're free) 12 volt and rated from 10 to 30 amps. I'm probably oversimplifying this too...
Just as an fyi, I also have about a dozen trainset powerpacks to use for power supplies if needed
Noo, the IR sensors will need some circuitry to drive a relay. Again I direct you to Rob Paisley's electronics site, he probably has the very thing. There are also commercial ones. Though I doublt they'd be as cheap as the $5 or per block for the Paisley transformer detectors.
The difference here is that the transformer (and the other current-sensing detectors) show when a loco, lighted car, or car with resistor wheels is within some area of track, an IR detector activated when a train gets to a specific spot and blocks the detector. Beam breakers like the IR detectros are better suited if you want the train to stop when it hits a specific spot, or at least to turn on a light when the train gets to a specicif spot that you don;t want to go past.
I priced some camera sets, Yikes. Especially if I need more than 1 monitor.
I'm liking the transformer detector idea better. So I see how a feeder wire gets looped around the sensor and I do undertsnd induced current. To clarify though, all the feeders need to go through the sensor/transformer correct? Do all feeders need to loop around it as well? This could end up making some long feeders.
Or could a "sub-bus" be made and 1 wire of that looped through?
I've got my main DCC bus of 14ga and lots of 22ga feeders installed on the layout so far. If I were to go from the 14ga main bus to set of 18 bus wires (one pair for each staging track) then runs my 22ga feeders to the rails.
I should be able to run a loop of 18ga through the sensor and have the same effect as running a bunch of 22ga feeders through it, right?
The sub-bus is what you do - see, once you think about it, it becomes pretty obvious. This is why the transformer option is fairly easy to retrofit. Gap one of the rails in the detected zone (both ends, just one rail though), cut the bus wire that all feeders in that zone connect to, and make a jumper that loops through the transformer to connect the bus and this new sub-bus. #18 should be fine.