Basic track detection

[Great Western]

I'm looking at a very basic track detection system for a new layout.

 

All I want is a typical signal box red light for each section of occupied track, displayed on a traditional signal box diagram.

 

The layout is a basic two track circle, with say three "sections" per line, as the train moves along I want the track circuit to show occupied and unoccupied etc.

 

The scale is O and DCC power feed.

 

Thanks

[Crosland]

As this is in the non-DCC forum can you confirm your last statement to avoid misunderstanding.

 

Lots of ideas here http://home.cogeco.ca/~rpaisley4/CircuitIndex.html if you want to DIY.

[DCB]

You should probably look at Protocab before you start a new layout.

Taking the power away from the track simplifies track circuiting immensely.

Full track circuiting was quite common in the model railway press in the 1950s for 3 rail layouts. Simply shorting the wheels on your locos and brake vans would give indications of which sections are occupied by trains or parts of trains.

 

Look at onboard power and Radio control before spending a lot of money on brand new obsolete equipment.

[uax6]

I found a couple of articles on track circuiting of two rail layouts from mags back in the 60's. Basically it appeared that a standing higher voltage was applied to the rails that operated relays that worked at the higher voltage. I did read them, but can't remember the finer points, but the articles have been scanned for future use!

 

Andy G

[cliff park]

I think uax6 is referring to a sytem where a short length of one rail, say 2", is totally isolated (ie 2 cuts and 2 isolating rail joiners) and connected to a relay, the other side of which goes back to the other rail, or in some cases another supply. As the engine passes along the isolated section the relay operates. There are several areas to consider. The relay needs to reliably operate on about 6v and the length of the isolated section is critical. Too short and the relay may not have time to operate, too long and 0-6-0 and similar will stop on the section. If you are in to electronics you could use a transistor or op-amp to switch the relay, increasing the sensitivity. This is easier if the trains only ever pass in one direction.

[uax6]

Nope, What I recall reading was proper full track circuiting, I'll see if I can find the scans and work out how they really worked...

 

Andy G

[Satan's Goldfish]

If DCC rather than DC and all you want is a red light (LED) to illuminate on your panel, then simply add an LED to either your 'red' or 'black' feed for that particular section of track. The led will only light when a loco is in that section to complete the circuit.

 

So, for example, have a common 'black' rail with no breaks along your layout, but on the 'red' rail have an insulating fish plate between each section and add your LED to the 'red' feed wire to that part of the layout.

 

Bit more complicated for DC, depends on controller type used. I'd need to draw it.

 

Edit: also, if you want it to do more than just light an LED (I.e. tell something electrical there's a train in that section) it'll be a little more complicated again. Transistor time!)

[Suzie]

If you don't mind a bit of kit assembly then the MERG DTC is a very effective current detector that will do what you require.

[Gordon H]

If you are using DC rather than DCC (still to be confirmed either way?) MERG also has kits for track circuits that will work with this too.

In fact the predecessor to the DTC (which uses a current transformer, so works only with DCC), was the FTC (Floating Track Circuit), which used a diode array for current detection of either polarity, and included bias voltage options for detecting stationary trains too. As such, it worked (and still does, by the way) with either DC or DCC - but with the inherent diode voltage drops.

The DTC, on the other hand, has effectively zero drop because it is just a piece of wire (the track feed) passing through the middle of a transformer coil. Not only that, but in principle you can pass several wires through it, feeding different pieces of rail, which can help making detection over a complex junction simpler.

[Kiwi]

If DCC rather than DC and all you want is a red light (LED) to illuminate on your panel, then simply add an LED to either your 'red' or 'black' feed for that particular section of track. The led will only light when a loco is in that section to complete the circuit.

So, for example, have a common 'black' rail with no breaks along your layout, but on the 'red' rail have an insulating fish plate between each section and add your LED to the 'red' feed wire to that part of the layout.

cellular data icon in the status bar on your device.

Sounds like a great and simple solution for my dcc layout. But my novice brain can't figure how / why it would work. Would you mind drawing the wiring diagram for it?

[uax6]

I've found the article now. The basic method is to have a relay with 3-4000 ohm resistance wired in parallel with the running rails. The relay should operate on 18+ volts, but not at 12 volts. The power feed to the track should be from a 24v dc supply via a 1000 ohm resistor. When the track is clear the relay is operated, but when a loco is on it the motor shunts the relay so that it drops out. The motor won't be effected by the 24v supply as the 1k ohm resistance reduces the current to less than 200 mA. (A fuse for that value should be put in circuit from the power supply.)

 

If you would like to see the full article please PM me.

 

Andy G

[DCB]

You need to decide whether you want a prototype track occupied track circuit or a locomotive detection system.   Onboad power will let you have proper indications that a train has separated and left its tail in section.    Track power lets you see which sections a train is in and taking power, and a magnet and reed switch system can show when a section starts and ceases to be occupied.

My magnetic system gave an indication when the brake van left the section,  operated the signals and points and applied the power to the electrical sections.  It was great, and totally unaffordable.....

 

My preference is for onboard and non insulated axles on all stock.

 

Incidentally prototype Track circuiting failed in its safety role as it would not reliably detect Railbus and lightweight shunting engines, and BR had to ban Railbuses from some lines, notably Swiindon Station, and had to Take shunters to and from yards with a larger loco just to activate track circuits and in some places axle counters replaced track circuits.

[Satan's Goldfish]

Sounds like a great and simple solution for my dcc layout. But my novice brain can't figure how / why it would work. Would you mind drawing the wiring diagram for it?

Not a problem, can you wait until next week?

[kevinlms]

My preference is for onboard and non insulated axles on all stock.

 

Well that puts you at a distinct minority. Non insulated axles, means modifying every single item of rolling stock.

[Suzie]

A ready made equivalent of the MERG DTC is the NCE BD20 which might be a useful alternative.

[DCB]

Well that puts you at a distinct minority. Non insulated axles, means modifying every single item of rolling stock.

Non insulated axles is the cheapest quickest and easiest method of making track circuits work properly, if you merely want loco detection then the Merg system is fine but for detecting a parted train you would need resistors across axles in at least the brake vans or end coaches of each rake.  Added to this I have a drawer full of Hornby Dublo 3 rail wheels and a lathe to re profile them with...

[Kiwi]

Not a problem, can you wait until next week?

Of course. Thanks.

[Satan's Goldfish]

Ok, in simplistic terms DCC is an AC signal constantly fed to the track with the DCC information encoded onto it. So for simply making a lamp light up when a loco is on particular section of track, split your track as below and add an LED and a standard diode to each track feed (checking the components for suitable current ratings!). the full AC signal is still fed to each bit of track, but the circuit to light the LED is only completed when a loco is in that section. LED will still light even if loco is not moving.

 

 

Caveats! I've not tested this, but it shouldn't have any issues, just check the polarities of the LED and diode wired in the correct way round or you'll lose half of your DCC waveform.

 

Be aware of any train tail lamps or other accessories fed from that particular section of DCC bus, these could also illuminate the 'track occupied' LED.

 

This will not work effectively with analogue/DC control. There's potentially 4 different circuit diagrams required here, DCC indication only (as above, that's the easy one), DC indication only, DCC occupation signal, and DC occupation signal. Those last 3 are a bit more complicated, i'll draw them up if anyone wants them but it may take me a while to find the time.

[Kiwi]

Ok, in simplistic terms DCC is an AC signal constantly fed to the track with the DCC information encoded onto it. So for simply making a lamp light up when a loco is on particular section of track, split your track as below and add an LED and a standard diode to each track feed (checking the components for suitable current ratings!). the full AC signal is still fed to each bit of track, but the circuit to light the LED is only completed when a loco is in that section. LED will still light even if loco is not moving.

Thanks. I shall certainly be experimenting with this. Hopefully, if I correctly understand the original post in this thread, this is the answer that person was really hoping for.

[Suzie]

...

 

DCC Track detect.JPG

 

Caveats! I've not tested this...

 

It won't work if your trains draw more than the 20mA typical rating of a LED (the LED will be overloaded at more than 20mA) and will induce an asymmetric signal causing some trains to brake. You can use diode drop detectors but they are a bit more complicated than this. The transformer based detectors (MERG DTC or NCE BD20) are a bit easier to understand and work with.

[Satan's Goldfish]

Hence the bit in brackets at the end of the second sentence. Higher current options than 20ma are out there (link below), balance the voltage required for the LED against the Diode (or number of) opposite it to balance it out if needed. It's a squared AC wave form, the peak to peak may be reduced slightly but it will still be operable. Remember, DCC systems that can still operate a single DC loco on the same tracks do it by altering the +/- peaks.

 

http://uk.rs-online.com/web/p/visible-leds/7877087/

[Gordon H]

 Remember, DCC systems that can still operate a single DC loco on the same tracks do it by altering the +/- peaks.

 

No they don't.

They operate by varying the duty cycle of '0' bits by stretching the length of one half cycle or the other, depending what direction you select. This alters the average DC value away from zero, causing the motor to turn.

[Crosland]

Hence the bit in brackets at the end of the second sentence. Higher current options than 20ma are out there (link below), balance the voltage required for the LED against the Diode (or number of) opposite it to balance it out if needed. It's a squared AC wave form, the peak to peak may be reduced slightly but it will still be operable. Remember, DCC systems that can still operate a single DC loco on the same tracks do it by altering the +/- peaks.

 

http://uk.rs-online.com/web/p/visible-leds/7877087/

 

You simply cannot put the DCC traction current through a LED, in any sensible way. Think about the current required by a double headed train with sound in both locos, carriage lighting, etc., etc., and the sort of LED you would need to buy that would survive possible well over an amp.

 

DCC, whether runing DCC or DC locos works by varying the pulse width. For DCC data both half cycles are the same length and the net DC voltage is zero. To run a Dc loco one half of the 0 bits is lengthened to introduce a DC bias.

[Satan's Goldfish]

My apologies, thinking of a different unrelated system (trying to do 60 things at once today, i wouldn't want to hook a layout up to the system i was thinking of this morning!!!), DCC is 0 stretching for DC control.

 

The original query was for a simple* track section indicator for a circular layout, the above with 1A rated LED fits the bill (edit, thought it was OO, just seen it's O. ######). If the user is looking at multi heading with sound, then simple has been left behind already. For simplified balancing, then rather than the diode, use an identical LED. If current handling is an issue, double up in parallel to 2x 1A LEDs in each direction (although on the 1 linked you're over £5 at that point). I believe NMRA standards allow a track voltage down to 7v for DCC to allow for connection losses between supply and track, so even if your supply is kicking out 12v, a 2v diode will still leave you at 10v assuming all your connections are good. Whilst it may not be the ideal, it is 'simple'.

 

One thing i've found from speaking with others at shows is everyone has their different methods for achieving results with electronics that they prefer. The above wouldn't be my prefered solution in this situation, but simple was requested, not bomb proof with future expandability.

 

*Thinking about it, simple can mean different things to different people in this context too, to some simple would be what box can i buy that does this for me (an example from the points world is frog juicers), or simple could be what's the cheapest way of doing this because i just want a bulb.

[Crosland]

Can you point us to the data sheet for a 1A LED?

 

It might be simple, but what does it cost?