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IRDOT Detectors

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  • Member since
    July 2006
  • 5 posts
Posted by sfupbn on Friday, February 15, 2008 1:01 AM


Welcom from DownUnder 

As you are no doubt aware the detector with the ATLAS signal system 'holds' the signal driver to red whilst the a loco or lighted car is in the signal block. When the loco or car leaves the block the signal goes yellow etc.

Whilst an IRDOT will detect the train and is separate from the train power your problem will be that once the train no longer covers the IRDOT the ATLAS signal driver will 'think' that the block is unoccupied and proceed to clear the signal. In other words the IRDOT will only work for your installation if the signal block more or less EXACTLY = the train length which is simply not practical. ( Or you use multiple IRDOTS which would be somewhat expensive)

When the train is in the signal block and no longer over the IRDOT you will need the IRDOT when it first 'sees' the train to drive a latch - (RS flip flop). This latch in turn 'locks' the driver (DRIVER #1), on to the red signal aspect.

This is the only way the signal will hold at red if your signal block is longer than the train.

You also need the IRDOT at the next signal, (IRDOT #2), to 'inform'  the latch  holding signal driver #1 to release so that the driver #1 can go to yellow. The only alternative to this is to manually clear the latch.

I have just done this kind of installation, though using the ATLAS detector instead of an IRDOT on an 'O' scale layout where the owner had the same wiring issues as you appear to have, namely that he had multiple feeders into the signal block and did not have a signal sub buss.

(If you have multiple feeders and no separate signal sub buss you will get false detection because the current drawn can bypass the detector which depends on current draw to operate.) The 'O' scaler  did not wish to rewire so that is why we used a latch system.

You you have some understanding of electronics you will be able to do what I described quite easily but I'll admit it is somewhat complicated. I am writing an article on what we did and propose to offer it to 'O Scale Trains' in the next month or so. 

By the way if you use the system I described use the ATLAS detector and save the cost of an IRDOT. Just make your signal detector block a little longer than one diesel truck in this way you'll avoid the 0.7 volt drop which is inherent in the ATLAS detector because it uses an Opto Coupler.




John L 







  • Member since
    May 2005
  • From: Burlington, Washington
  • 196 posts
Posted by PHARMD98233 on Friday, February 8, 2008 7:32 PM

You might take a look at this site

He has a pretty neat product line.  We use his detectors for our club layout. 


  • Member since
    July 2006
  • 290 posts
Posted by steamnut on Friday, February 8, 2008 5:48 PM
I have used the IRDOT detectors for driving semaphore signals. They work fairly well if they are correctly installed - the tops of the emitter and collector MUST be slightly above the ties / ballast. The gaps between cars can be a problem. As another commented, they do need a clean, filtered DC power source. The IRDOTs have an output sufficient to drive a relatively efficient relay and more than enough for any signal lamp.
  • Member since
    January 2001
  • From: SE Minnesota
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Posted by jrbernier on Friday, February 8, 2008 12:41 PM

  I have used Logic Rail 'Block Detector' signal units and they have all the logic to drive a pair of signals.  They have a tech sheet on how to use IR detectors instead of the light sensors provided with the product.  This units are completly seperate from the track wiring, and will still detect a train after it has uncovered the entrance sensor.


Modeling BNSF  and Milwaukee Road in SW Wisconsin

  • Member since
    October 2006
  • From: Northeast OH
  • 883 posts
Posted by jktrains on Thursday, January 31, 2008 4:15 PM

I've used infra-red detectors to control a grade crossing controller.  They are mounted between the ties so they are well hidden.  The ones I used were from TCH Technology.  They work well but do need a good, filtered DC power supply or they start to act funny.  Search and find their website and the instructions for the detector.  IIRC, they simply pull to ground when activated.  They also have a timer that can be adjusted which would help prevent false drops as couplers pass over.


  • Member since
    December 2004
  • From: Pa.
  • 3,153 posts
Posted by DigitalGriffin on Thursday, January 31, 2008 2:06 PM

Short answer is: Yes.


Don - Specializing in layout DC->DCC conversions

Modeling C&O transition era and steel industries There's Nothing Like Big Steam!

  • Member since
    November 2003
  • From: Rolla, Missouri
  • 8 posts
IRDOT Detectors
Posted by trainpilot1 on Thursday, January 31, 2008 2:00 PM

I am looking for alternative block detectors to be used for controlling 3-color signals (Atlas). My DCC layout is redundantly wired in such a way that power or current pickup for the signal system is impractical from a re-wiring standpoint. I need a detector which has absolutely no requirement for any connection, either inductive or physical, to the track power system.

Circuitron has some optical occupancy detectors, but I'm also considering the infrared detector systems (IRDOT). Is there anyone out there with some experience with these types ?  do infrared systems have detector outputs which can be used to drive signal systems as indicated above.


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