How to build a train detector?

[jxtrrx]

I've got a 45-degree crossing on my layout.  Never fails that when I take my eyes away for one monment, two trains arrive there at once, and --- well, you know.

I'd like to install some kind of detector so that if a train is in the crossing, the perpendicular track power is killed.  I assume I use some kind of block detector??  Recommendations please.  (By the way I run DCC, a Digitrax Zephyr)

[jamnest]

Set up the junction with an interlocking plant.  Power both sections of track through a 4P4T switch so that only one section of track can be powered at a time.  Use the other set of poles of the 4P4T switch to power signals to indicate which section of track is powered.  The powered section signals will display green, while the unpowered section will display red.  You will not need any detectors for this set up.

[Anonymous]

The only problem with that is you either need to have an operator stationed there all the time, (which would be pretty cool during op. sessions but bad for 1 person ops) or only run one train. (although running one train would solve your problem anyway...)

DC would probably be easier if you don't want the "on/off" control method, for smooth stop you'd have to hook your layout up to a computer. 

[gandydancer19]

There are three ways you could go with this.

Manual - as Jamnest said in the first post.

Full blown computer control - Use a BLD-168 or smaller block detector connected to LocoNet and computer controlled.

Simple Automatic control - Use a simple photo detector (as available on eBay most times), and relays to shut off the track power as needed. No computer required, but you do need to know how to design (figure out) how to install and wire the parts together.

[richg1998]

The simplest and cheapest solution is have engineers keep their eyes on the track. You might add train detectors with signals. That is prototypical.

Rich 

[Texas Zepher]

I'd like to install some kind of detector so that if a train is in the crossing, the perpendicular track power is killed.  I assume I use some kind of block detector?

I would use a very simple block occupancy detector on both "legs" of the crossing.  If there is a train present it trips a relay cutting power to the other leg.  Depending on the exact design there could be problems with either flickering (optical dectection) or train length (current detection).  Neither are really hard to overcome.  The other thing to be aware of would be that the "dead" section would have to be long enough to cut power to all the locomotives of a lashup. Otherwise ones in the rear would continue to push the front ones toward the crossing.

Almost all the DCC vendors make track occupancy detectors based on current draw.

[lvanhen]

I thought all the "techies" were supposed to be at the DCC forum

[dinwitty]

It doesnt matter if your DC or DCC, a detector on both will work, just be sure the DC detector doesnt filter the AC signal much.

Linn Westcott designed a Twin-T detector which uses 2 power transistors cross connected to make the detection. Bruce Chubb has a detector design also.

You would need a relay that kills power to the "other" track, if a train enters the "other" track it can't get power to detect and drop the "first" relay, so you get a lockout feature.

The most important thing is you should set up a rule that any train that has to get thru this crossing has to stop and get clearance before proceeding. Something your dispatcher has to qualify. Running by train order only solves everything and you don't need detectors.

 

[Texas Zepher]

I thought all the "techies" were supposed to be at the DCC forum

Nothing techy about this old stuff.   Here is a simple diagram of something that should work.  Like I said in a prior post the insulated rail joiners (or gaps) need to be far enough back so multiple unit locomotives won't push others onto the crossing.   The detector  device could be a twin-t, or any of the various ones from the vendors.

as normal - click the image to enlarge

This detector is they type that senses current on the rails so only a locomotive, lighted passenger car, or caboose will be detected.  Resistor wheel sets are highly recommended to protect the center of the train.    

One of these circuits would obviously be needed to protect the other track as well. When they are both installed, as presented, if the one track is automatically powered down, a train over running the block could not power down the other.

One could also make the power cut off block a different size from the "detection" block.

Another option would be to put a warning buzzer or bell into the circuit to get the engineers attention. 

[jxtrrx]

I know you'll say I'm a weenie... but I was thinking of something I could just buy and wire up.  Anything like that?  I see some "detectors" on websites, but don't know if that's the same as something that will provide a switch-type connection for power.  (You start talking relays and transistors, and I get out of my comfort zone quickly).

Oh, and sorry if I posted in the wrong section.

[cwclark]

  I built Lynn Westcotts "Twin T"  before on a MR i built in the early 80's and it is a whole lot easier to just buy one. DALLEE electronics sells a very nice train detector that uses an AMP draw indication to set off a N/O - N/C relay.  If a train is present in one block than it is wired to  kill the power to the rails of the other block and with two detectors the opposite can be achieved. there are enough connections on the DALLEE relay that it can also signal the trains if you want to go that far. Heres a picture of their train detection circuit. See how easy it is?. The block wire goes thru the hole and instant block detection. (Of course, you'll have to wire the relay in those screws on the left side of the detector)   A relay is easy to wire too. It's sort of like wiring a light switch. It's either closed (light on) feeding power or open (light off) to disconnect the power.

[Texas Zepher]

I know you'll say I'm a weenie... but I was thinking of something I could just buy and wire up.  Anything like that?

The diagram I drew isn't far from that.

You start talking relays.. and I get out of my comfort zone quickly

I think you are making it too hard.  A relay is nothing more than an electrical switch that is thrown remotly with electricity rather than with your hand.   Think of it like a remote control turnout for electricity.  Relay power-on then the remote electricity flows one way (straight). Relay power-off then the remote electricity flows on the diverging route.

DALLEE electronics sells a very nice train detector that uses an AMP draw indication to set off a N/O - N/C relay.  If a train is present in one block than it is wired to  kill the power to the rails of the other block and with two detectors the opposite can be achieved. there are enough connections on the DALLEE relay that it can also signal the trains if you want to go that far. Heres a picture of their train detection circuit. See how easy it is?.

That is bascially the diagram I drew on one circuit board.   Nothing automatic gets much easier than this.