Help newbie with flashing led project please

Discussion in 'General Electronics Chat' started by Eldarnoe, Feb 19, 2017.

  1. Eldarnoe

    Thread Starter New Member

    Feb 18, 2017
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    Hi folks,

    I am trying to accomplish the following. I want to have an led flash for 20 seconds after I press a button. Flash rate can be 10th of a second. Could anyone please help me out? I can (marginally) read circuit diagrams and know what resistors, transistors, and capacitors are. I can also learn fast, but any help would help. Feel free to talk down to my level :)

    Thanks,

    Eldar
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    http://www.williamson-labs.com/555-AN170.htm
    The above gives lots of details on the '555 timer (the '556 is the dual version). What you want can be achieved by using the circuit shown in figure 14 on page 9 of the above.
     
  3. dl324

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    Mar 30, 2015
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    Welcome to AAC!

    Use a 20 second one shot to gate a 10Hz astable.
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    Are you one of those who requests that posters provide a schematic? Provide a schematic for the poster. :)
     
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  5. dl324

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    I only work for free when I feel like it.
     
  6. hp1729

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    Nov 23, 2015
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    Design 1113 flasher.PNG
    A starting point.
    If a slower flashing rate is suitable a self flashing LED is an option that gets rid of the second LM555. They flash at about 80 times per minute.
    Resistor R5 value depends on what LED you use.
     
  7. hp1729

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    :)
     
  8. hp1729

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    For flashing LEDs go to Electronic Goldmine and search for LEDs.
    http://www.goldmine-elec-products.com/
    They come in different colors or flashing all colors.
     
  9. Reloadron

    Distinguished Member

    Jan 15, 2015
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    You have several options to do what you are looking to do. A micro-controller type chip could be programmed to make it happen or a circuit built using discreet components like the as mentioned 555 Timer chip. One popular solution would be a 556 timer chip (two timers in a single chip) configured as a Two-Stage timer. There are a number of relatively uncomplicated ways to get where you want to go with this but each route will take some doing and minor skill. Pretty much up[ to you.

    Ron
     
  10. djsfantasi

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  11. ian field

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    Oct 27, 2012
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    There's many ways to skin a cat - likewise, there's dozens of ways to get the result you ask for.

    There's various circuit archives you can browse online and perhaps identify the type of circuitry you would feel most comfortable with - then the people here could guide you more directly to an end result.

    When I design a circuit sub section, I browse various appnotes and other resources for inspiration before I settle on a design philosophy.
     
  12. hp1729

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    Nov 23, 2015
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    Design 1113 single LM555 flasher.PNG Single LM555 with self flashing LED.
     
  13. ian field

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    That would leave me stumped - I don't have any flashing LEDs.

    It reminds me that the single transistor blocking oscillator can be rigged to flash a LED. It needs to have isolated base and collector windings so you can re arrange the bias and add a relaxation period capacitor. It produces narrow intense flashes and can be made to work from a single 1.5V cell - its a very simple circuit, but probably not what the TS is looking for.
     
  14. AnalogKid

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    Aug 1, 2013
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    Another approach is to use an oscillator and counter to generate the LED clock and decode the 20 second period. Fortunately, it is easier that it sounds. U1 has an oscillator and a 14-bit counter/divider. If you get he oscillator frequency right, it takes 20 seconds before the Q14 output goes high. When it does, it inhibits the oscillator by screwing up the timing circuit. Resetting the counter starts the cycle again.

    The ratio of 10 Hz to 0.025 Hz (the inverse of a 20 second flashing interval) is not a standard binary number. This is why the flash rate is 6.5 Hz - it is 256 times the operating cycle time of 40 seconds, the first 20 seconds of which are the operating time.

    Unlike the 555, the 4060 has a relatively weak CMOS output stage that can source only 4 mA. For more LED current and brightness, a small MOSFET (2N7000/7002) can be added as an LED driver.

    C2 is the power decoupling capacitor, and has nothing to do with the reset function. It should be connect as close as possible to pins 8 and 16.

    ak
    LED-Flasher-4060-1-c.gif
     
    Last edited: Feb 19, 2017
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  15. djsfantasi

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    Once again, AnalogKid has an elegant solution, Below is a schematic of a more conventional solution,

    The 555 circuit on the left is a 555 monostable circuit, with an output of a twenty second high signal. Once twenty seconds has elapsed, the signal goes low until it is triggered again.

    The 555 circuit on the right is a 555 astable circuit, that runs a little over 10Hz with a 52% duty cycle. This is what flashes the LED and is triggered by the 555 monostable circuit.

    Besides a 556, only a handful of passive components - resistors and capacitors - are needed.

    Driving Oscillator with monostable.PNG
     
  16. Eldarnoe

    Thread Starter New Member

    Feb 18, 2017
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    Folks,

    I am touched by how many useful responses I have received. And thank you so much for the diagrams and explanations. This definitely sets me up in a place from where I can build the circuit! I will keep you all posted on progress.
     
  17. ian field

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    You can get "ultra efficient" LEDs that give useable brightness with as little as 2mA.

    Net earth shattering - but good enough for some applications.
     
  18. Eldarnoe

    Thread Starter New Member

    Feb 18, 2017
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    Hi There,

    So I finally got all the components listed in your diagram, and I was about to start breadboarding when I realized that I actually don't understand what connects to what! When two lines cross, how do I know whether they are connected or whether one just crosses over the other? Thanks, Eldar
     
  19. AlbertHall

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    Jun 4, 2014
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    In that diagram, wherever two wires cross each other they are not connected with just one exception, the ground connection to the left hand '555. That exception, circled in red below, is connected.
    Driving Oscillator with monostable.jpg
     
  20. Sinus23

    Active Member

    Sep 7, 2013
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    When they have a "dot" at the intersection they are connected, otherwise they aren't
     
    Last edited: Feb 25, 2017
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