Add a delayed release to IR circuit

Discussion in 'The Projects Forum' started by numbskull100, Nov 25, 2010.

  1. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    Hi,

    I want to add a delayed release to the attached circuit diagram.

    I want to keep it as simple as possible and ideally just add a capacitor between the sensor and diode. However it is obviously not that simple.

    What modifications would I need to make, I am looking for a delay of about 2 seconds?

    Thanks in advance,

    Kev
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    What is the purpose of the circuit? Are you trying to make it activate level crossing lights?

    If so you may want to think about removing the LED and using the transistor to trigger a 555 timer to flash the lights. You can incorporate time delays within that circuit. I have been in the process of designing something like this that works on all the level crossings. One flasher circuit that is switched to the required crossing as a train approaches.
     
  3. bertus

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    Apr 5, 2008
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  4. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    That was trying to make it work, now it needs some modification!
     
  5. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    I am trying to have a block detection system which will change a signal to red. I have inserted a relay with diode across instead of LED to act as a switch. The next stage would be to have a time delay so when a train leaves the detection area the signal stays on red allowing the train to clear.

    Any ideas?
     
  6. bertus

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  7. windoze killa

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    Feb 23, 2006
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    You could also use a flip flop circuit with a second detector to switch it off. That way it won't matter what speed the train is travelling.
     
  8. Dream-

    Active Member

    Nov 26, 2010
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    Actually there may be just a very simple solution. If you add a capacitor at the LED, it could keep the LED on but barely, as the capacitor will quickly discharge and the LED will fade out. This is not what you want.

    But if you add the capacitor between the transistor base and ground, you will have the cap switch the transistor on after there is no current flowing into the base from the circuit. Since the current needed for turning on the transistor is much less than that needed to drive the LED, you will get no fading (the transistor is operating in switch mode). You may need a resistor after the capacitor, but that's about it. You will need a capacitor that keeps the LED on for 2 seconds.
     
  9. thatoneguy

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    Feb 19, 2009
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    Due to the 10k resistor, the cap wouldn't have have enough time to charge to supply base current for any useful amount of time, it would only slow down the turn on time, possibly making the circuit not work at all.
     
  10. Dream-

    Active Member

    Nov 26, 2010
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    The 4.7K resistor is also a problem as it will discharge the cap quickly.

    I am sure there is a redesign solution to this that won't require a timer. Just need to think about it.
     
    Last edited: Nov 27, 2010
  11. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    Could a second transistor be used?

    i.e. the current transistor is used to switch on/off a circuit with a capacitor which then switched (via transistor) on/off the led?
     
  12. thatoneguy

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    Feb 19, 2009
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    For a 2 second delay, it would be better to use the pulse from the transistor to trigger a 555 timer for the delay, possibly 2 if blinking LEDs like a crossing light are desired.
     
  13. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    Blinking is not needed, just the ability to hold a light on after a train has left a "zone" to allow it to clear.
     
  14. thatoneguy

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    Feb 19, 2009
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    555 Monostable posted above, take wire from collector of your transistor to the "B" input of the schematic in the link.

    Do you have a breadboard? Bill has made excellent graphics showing how to connect the breadboard. The components will give a 3 second delay.
     
  15. windoze killa

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    Feb 23, 2006
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    How about considering my suggestion of having a second IR detector at the end of the block. If you ever drive the train in the opposite direction you will need this detector at the beginning of the block. Also as I said the speed of the train won't be a problem. If the train was moving very slowly the timer/light will change before it leaves the block. I also posted a circuit for something similar a month or so ago and the OP seemed to like it.
     
  16. numbskull100

    Thread Starter Member

    Dec 29, 2008
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    Yeah, sounds sensible, will give it a go tomorrow, would save much fiddling!

    Looking at this example flip-flop, am I right in thinking the best way to employ this would be to use the IR sensors as switches, and rather than leds, use a latching relay which would cut down on the number of sensors needed?

    [​IMG]
     
    Last edited: Nov 27, 2010
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