Substitute electronic trigger for pushbutton

Discussion in 'The Projects Forum' started by Circuit Breaker, Dec 16, 2010.

  1. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    As suggested, I am posting a new thread to an earlier one from tonight.

    Attached is a schematic of a time delay circuit that is activated by a pushbutton switch. I am looking for a way to activate this circuit from the the output of a different circuit. I was thinking of using an NPN transistor and another member mentioned using a Mosfet IRF540. Another member replied that neither of these would work in my situation.

    The circuit that I would like to use to trigger the time delay circuit is a commercially built current sensing detectors output, which is (-) negative 12 VDC. The output configuration is a push-pull and can sink and source current and has a maximum output of 400 ma.

    So what I'm hoping for is if someone can tell me what kind and size of device I could use to trigger the time delay circuit and what leads would get wired to what on my two circuits to accomplish this.

    Thanks again, for any help and advice.

    Circuit Breaker
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Can you change your attachment to a .PNG? Some odd program wants to open .BMP files. PNG display inline with the browser.
     
  3. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    I can sure try. It doesn't appear that I can add another attachment to this thread. Having never done this before I presume I wouls start a new thread with the .PNG file attached to it?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You can edit your post and upload a new attachment.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    [​IMG]

    I looked at the properties of the file and got the internet address. {img}{/img} will take it from there.
     
  6. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    Thanks Bill.
     
  7. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Can you explain the purpose of the circuit you are trying to build?
     
  8. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    The purpose of this circuit is to automatically stop and start a model railroad train that is controlled using DCC (Digital Command Control). I would like for a current sensing detector employed in an electrically isolated section of track to activate my time delay circuit when it senses a train is present. The time delay circuit would then activate a DPDT relay to switch track power from DCC to DC in another electrically isolated section of track. By introducing DC power to a DCC decoder equipped locomotive will cause the locomotive to come to a slow smooth stop until DCC power is reintroduced to the isolated track. That would happen after the time delay relay de-energizes and the locomotive will slowly resume its preprogrammed speed.
    I suppose I could use a SPST relay in place of the pushbutton to be activated off of the current sensing detectors output but I was hoping for something perhaps a little bit more sophisticated and/or compact.
    Any thoughts on this concept would be appreciated.

    Circuit Breaker
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Here's his pic.....
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Is this 555 being powered from the RR power supply? If it isn't and if they don't share a common gnd, an OptoIsolator (as the interface), might be in order. You would use the collector to pull the trigger pin to gnd, in place of the switch. ;)

    FYI: At nearly 65 years old I still get giddy when I see a really good model railroad!
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    I just had a brain fart... Since the polarity of the tracks are not constant you might be better off with a 'Current Sensing Relay'. These relays have very low series resistance and are placed in series with the load.

    EDIT: I did a quick search and didn't find any suitable DC models. The AC models work on inductive principles like a clamp-on meter. Others may know though.
     
    Last edited: Dec 16, 2010
  12. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    No, the 555 and all of my electronic gizmos are powered from a highly filtered, regulated and dedicated supply. Your suggestion of using an OptoCoupler certainly has merit. Unfortunately I have to plead ignorance on having any knowledge of what type of opto to use, where to hookup all the pins to my circuits and what resistors, if any, may be needed and where.

    As a sidebar to your FYI, I am pleased that you enjoy a well built model train layout. I've been modelling trains for over 20 years and am a stickler on detail. It brings life to the layout in my estimation.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    First off let me say that I've been down with the Flue or possibly even walking pneumonia for the last two weeks. As a
    consequence my brain is missing on a few cylinders. So please bare with me if I repeat myself or get something wrong.

    Anyway, I did a quick read at the Digitrax site because I had no idea what DCC is. After the quick primer I believe I have a basic understanding.

    Please correct me if any of the following is incorrect.

    (1) The voltage to the rails remain constant.
    (2) The polarity of the rails remain constant.
    (3) Digital control packets are superimposed on the DC that powers the rails.
    (4) Each Loco has a decoder module that controls speed and direction of said Loco.
    (5) If DCC is lost the decoder puts the Loco into a controlled (realistic) stop.

    Your 555 circuit:

    (1) This track will be electrically isolated from the main line.
    (2) Prior to triggering the 555, DCC powers this track through one pole of your SPDT (K1) relay.
    (3) When a Loco enters this track some sort of current sense or other means is required to trigger U1.
    (4) When U1 is triggered K1 will switch this track from DCC to pure DC.
    (5) After U1 times out K1 switches this track power back to DCC.

    Do I have anything wrong here? If not, is there any reason that you couldn't employ a IR Transistor and Diode hidden in
    something creative along the tracks? It will probably be easier than current sensing.
     
  14. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    Sorry to read of your bout with the flu. Nasty stuff so I'm glad you are on the mend.

    I am most impressed and must commend you for actually going to the trouble to research DCC. You went to the right source for your research. Digitrax is one of the pioneers in DCC technology. You are absolutely correct on all of your counts except that the digital packets are superimposed on AC rather than DC. You now possess more knowledge about DCC than probably 50% of the model railroaders who use it.

    I see no reason why an IR transistor and LED would not work. An excellent suggestion! And you are correct when you say IR would be better than current sensing. I employ current sensing in other areas of the layout and it will false trigger on occasion, especially in high humidity. The area where I would install them across the rails is hidden by a "mountain". The only question left for me to ask is which leads of the IR transistor would replace the pushbutton switch leads?

    Now I see why you electronic engineers get the big bucks.
     
  15. CDRIVE

    Senior Member

    Jul 1, 2008
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    Oops, I just assumed it was HO .
    Uh, sports figures and actors make big bucks. In comparison, Engineers make a barely respectable living. ;)

    I'll get back to you soon with a proposed circuit. I'm not going anywhere today.:(
     
    Circuit Breaker likes this.
  16. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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    Thank you in advance for your kind offer...
     
  17. Circuit Breaker

    Thread Starter New Member

    Dec 15, 2010
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  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Here's the basic idea. The components used are a PhotoTransistor, LED and a 2N2222 to provide the proper logic (collector high prior to loco detection) for the trigger pin of the 555. C1 is required to prevent the trigger pin from being locked low which will prevent the 555 from timing out. These components should be available at Radio Shack and are not critical because the distance between the LED and the PhotoTransistor are close.

    A note or two here:
    We have members here that have extensive experience with Opto Electronics, so if you desire Infrared TX/RX please post that. Those members can recommend an IR Diode-PhotoTransistor combo better than I can.
    Also, you will note that this scheme positions the LED and PT on opposite sides of the track. I believe that there are IR modules available that are mounted in the same package and work via reflection. The advantages would be two fold, in that the Q1 would be eliminated and the module would be positioned on only one side of the track. Oh yeah, it would also be IR, so it wouldn't be visible. Since I'm too sick to do anything else I'll probably post again after searching my files. :)
     
  19. CDRIVE

    Senior Member

    Jul 1, 2008
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  20. CDRIVE

    Senior Member

    Jul 1, 2008
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