Help please with photo transistors

Discussion in 'The Projects Forum' started by icadj, Apr 9, 2013.

  1. icadj

    Thread Starter New Member

    Jan 2, 2010
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    1
    I have built for my model railway the attached curcuit a bistable 555 and it works (lol) . This is brilliant for signalling BUT can anyone help with replacing the magnetic reed switches with photo transistors. As i want to use it on a reversing loop.

    I have asked on model railway forums and they only gave me ideas , but nothing that I could buy or solder. I can build stuff but have no idea about parts or what replacements or part numbers.

    Any help in what to buy (part number etc) and how to put it together would be brilliant, many thanks in advance,, steve
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,439
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    Good. Nice of you to change the title from "Help please" to one that is more descriptive otherwise you would have received a citation from the title cops.

    The words "help please" are not necessary.

    A better title would be "photo sensors to switch model railroad".

    This would catch the attention of model railroad buffs.
     
  3. icadj

    Thread Starter New Member

    Jan 2, 2010
    5
    1
    ok i c what u mean ,, but how without starting a new thread , how do i alter the subject title please
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,439
    3,360
    You are new to this site so you need to learn the rules.
    Don't create a new thread. The current title will do for now. Next time you will post a better title.
    So now you know a few things:

    1) Choose a descriptive title
    2) Don't open multiple threads
    3) Don't use shorthand or textese such as "i c what u mean". It is rude and shows laziness.

    Sorry steve, I don't have any immediate suggestions for solving your problem. Just making sure you have an educational and enjoyable experience here. Do stick around. This is the best website of its kind on the web.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1,605
    Hey Steve. I think as a new member (under 10 posts) your editing abilities are restricted. But we got it now.

    Unfortunately, I can't read your schematic to see what the reed switched do, so I can't suggest anything beyond you posting again with a clearer schematic where we can read the pin numbers and such.

    And welcome to the forums!
     
  6. icadj

    Thread Starter New Member

    Jan 2, 2010
    5
    1
    Sorry about the schematic but thats the quality of the editor,

    ok i'm using a 555 as a bistable switch ,, in my words a 555 in an "on" or "off" state switch (not a timer circuit), controlling a relay which in turn controls signalling (green or red) and track condition (go or slow to stop) on my model railway.

    the reeds are connected via negative , separately to pins 2 and 4 ,
    pins 2 & 4 are connected to positive via a 33K resistor
    pins 5 & 7 N/C
    pin 6 to negative

    I want to replace if possible the reeds with photo transistors as i want the train to auto control the signal/track not the length of the train

    Hope this simple explanation helps and that its understandable

    thanks in advance ,, Steve
     
  7. icadj

    Thread Starter New Member

    Jan 2, 2010
    5
    1
    PS ,, thanks ErnieM
     
    ErnieM likes this.
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Steve: the thing about schematics is they convey a huge amount of information words cannot contain... plus it's a way of thinking for those of us who do this a lot. Damn that editor!!!

    Anyway, the reed relay switch part just goes from pins 2 & 4 to negative? If so, then yeah, a photo transistor is a definite possibility.

    A photo transistor (PT next time) will act as a short like a reed relay switch when a bright enough light shines on it.

    Possible candidates perhaps easy to get are from Radio Shack:

    Catalog #: 276-145 Infrared Phototransistor
    Catalog #: 276-142 RadioShack® Infrared LED Emitter and Detector

    Each needs an infrared light source (one comes with a diode). Visible light transistors are also available but would be a mail order thing.

    The devil in the details on this is it's very "touchy-feely." I mean there isn't any way to predict what the circuit does, so it needs some kind of a sensitivity knob, either a real knob or some testing & adjustments (possibly playing with the value of the 33K resistor).

    So just a PT will act as a closed reed when the light is on it, and stay that way until it goes back into darkness.

    Does that sound useful to your model world?

    (the back of my brain is trying to tell me someone has already done this for model railroads)
     
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