Circuit for flashing LED when connection broken

Discussion in 'The Projects Forum' started by MountainGuy, Jan 15, 2017.

  1. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    I need to figure out a way to do the following. (I’ve tried 555 circuits, transistors , not gates and arduinos with transistors etc and can’t seem to figure it out.) The problem is that the led doesn’t flash then go off. It’s the ability to turn off the LED after about 1 millisecond that is the problem. Can anyone help?



    (1) My (switch) needs to be a break in ground.

    (2) Turn on a 12v LED when the switch opens (breaks ground)

    (3) LED flashes for about 1 millisecond and turns off

    (4) the switch will then close and the LED should stay off



    So in other words I need a 12vdc LED to flash about 1 millisecond when the ground circuit opens up then goes off until the ground opens again. I'm open to using transistors, timers, arduino in combination with other components. Thanks for reading.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    What is the voltage across the ground switch when it opens?
    Can you use a DPST or DPDT switch to break the ground connection?
     
  3. djsfantasi

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    As a follow on to crutschow's question, what causes the ground circuit to open up?

    Using an Arduino (or a smaller version like the Trinket), it be done with only a few added components (maybe). Like a couple resistors and a transistor. I could help with the code if you need assistance. .
     
  4. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    There is no voltage across the ground when the wires are open.
    I can't use any type of switch. What I'm using is like a set of automotive points that open and close. When they first open I need the LED to flash briefly.
     
  5. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    I was looking into using the arudino, but I need it to allow a 12vdc source to turn on the LED. Any help would be appreciated.
     
  6. djsfantasi

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    Ok, what is the voltage that is connected to ground when it is closed? What is the load on this part of the circuit? Where does the voltage come from? (Driving the 12V LED is easy).
     
  7. crutschow

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    I doubt that's true unless the ground is carrying no current when the points are closed.
     
  8. MountainGuy

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    Jan 15, 2017
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    I looked at this diagram, but don't know if it will work. The only voltage I'm trying to use is 12vdc unless an arduino is used. The voltage would come from either a wall wart or a 12vdc battery setup. I assume I could hook an arduino where the Vin is located and then control the 12vdc coming out the other side.

    points open --| |-- Led flashes 1millisecond and goes off. Contact is made again light is out.
     
  9. AnalogKid

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    A 1 ms monostable is easy to build with all kinds of different technologies, and Powering an LED for 1 ms can be done with a relatively small holdup capacitor, so that if the "ground" that is "breaking" is part of the LED's power source it still will blink. But a 1 ms blink is barely perceivable.

    Why so short?
    What is the "ground" that is breaking? Is it the return for a 440 VAC 3-phase industrial power distribution system within an jet aircraft factory? If so, that could be a problem. Is it the ground return cable from a starter motor in a car? etc........... And, what breaks it?

    Your problem probably is very simple, but the description needs a lot of work. Can you post a sketch of the overall system, including the power source for the LED circuit and the mysterious "ground" "break"?

    ak
     
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  10. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    Sorry for the description problems. Basically I'm open on the circuit. It is a standalone system. When the part (switch) first opens up the LED must flash briefly and turn off. I want to use a 12vdc source either battery or wall wart will work. I'm not locked into 1millisecond, but that is what I understand is the speed of an electronic flash. Maybe a pot would help in determining the optimum duration of flash. The picture shows the part that serves as the switch and it is hooked up to ground.The problem I've run into is that the part (switch) does not flash the light quick enough (stays on until the part closes). The light must stay off until the part (switch) opens again. The light is never on except during the brief flash once the part (switch) first opens. Thanks for reading.
     
  11. AnalogKid

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    Still not clear. An image of a set of contacts tells us nothing about the circuit, the voltage, the current being switched, any current limiting components. Basically, what is it you want to do - from an overall point of view, not the micro-detail of one switch. What is the switch switching - and don't say "ground". It isn't. It might be connecting a part of a circuit to ground, or a part of a power source to ground, but can you see that even those descriptions tell us nothing? Are you trying to trigger a photographic flash when power to a device is interrupted? Or use an LED as a photographic flash? How many amps does the LED need to function? So far, all we really have is that you have a switch and need a circuit. That's not much.

    ak
     
  12. Alec_t

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    What is it that you are disconnecting from ground with the mystery switch???? (We get the bit about the LED, the 1mS period and the 12V supply.)
     
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  13. KeepItSimpleStupid

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    You probably can't see a LED flash for 1 mS?
     
  14. AnalogKid

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    To put your lack-of-information in perspective, it is entirely possible that a circuit to do what you want can consist of exactly one capacitor. Or not.

    ak
     
  15. MountainGuy

    Thread Starter Member

    Jan 15, 2017
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    I'm thinking you are probably correct in that a capacitor needs to be used in order to make the bulb flash quickly then go off. Sort of seems like maybe the capacitor might charge while the points are closed then releases the charge (flashing the bulb) upon the points opening. Somehow a transistor is used to do allow power to the bulb. I'm trying to use a 12vdc supply to flash a 12vdc 10watt led. This is an example of the type of LED I'm wanting to use. The points don't really do much other allow the LED to flash, once they are open. Since there are no other external components involved there should be no other voltages involved, just the 12vdc power supply. (The points could be replaced with two wires even. One wire is connected to ground.) When held together there is no light or flash, when not held together there is a brief flash at the point the wires are first opened. So after the brief flash the LED stays off. Then the cycle repeats. As far as the flash duration it should be able to stop motion. From what I've read a camera flash operates at 1 millisecond.
    (1) when the points are closed there is no light
    (2) when the points open there is a brief flash
    I hope this makes sense. I have read and tried so many things I have brain fog.
     
  16. Alec_t

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    How often do you want to open/close the switch ?
    Are you trying to make a stroboscope?
     
  17. MountainGuy

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    Jan 15, 2017
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    Not trying to make a strobe, but I want the LED to flash quickly like a xenon bulb might. I would guess it does operate similar to a strobe in that the duration of the flash is short.

    It opens and closes anywhere from 800 times per minute to about 40000 times per minute. Basically the duration of the flash stays the same, but the frequency (by frequency I mean how often it will open and close) can change quite a bit. So I don't believe the frequency is really part of the equation because the operation has less to do with frequency and more to do with duration of the flash. The frequency can't be set at one particular frequency since the frequency can vary so much, it is variable. If I have a lobe on a shaft and the switch opens and closes based on when the switch hits the lobe, the LED would flash every time the switch opened. If I speed up the shaft the switch would hit the lobe say 800 times. If I speed up the shaft again to hit the switch 10000 times I still need the same flash duration to illuminate a point which shows it stopped. The LED has to flash when the switch first opens rather than closes.
     
  18. LesJones

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    Jan 8, 2017
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    This sounds very much like a strobe timing light for setting the ignition timing on a petrol engine. Am I correct ?

    Les.
     
  19. AnalogKid

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    That is the definition of a strobe.

    Also. 40,000/min is 667/sec, or one trigger every 1.5 ms. That leaves 0.5 ms for the LED power source to recharge if it is a capacitor-dump type of circuit. But at 1.2 A, that circuit is out. See? It took only 17 posts for you to give up enough detail to rule out most of the suggestions so far.

    You keep saying one of the switch contacts is connected to ground. ***What is the other one connected to?*** It's kinda important. It can't be the 12 V supply, because that would be a dead short. SO....??????????????????????????????????????????????????????????????????????????

    And - if the switch duty cycle is 50% (still a secret) exactly what kind of mechanical switch can open and close cleanly in less than 0.75 ms?

    ak
     
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  20. Alec_t

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    A (timing or reservoir) capacitor will inevitably be involved. Since the cap has to charge and discharge, frequency is a factor.
     
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