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    This is a simple flashing led circuit with 2 leds and 2 NPN transistors.
    It illustrates the behavior of transistors and capacitors and if you use an oscilloscope it will be very easy to determine what happens in this astable multivibrator circuit. It’s state is constantly changing and this change affect the flow of current and voltage and the effect will be visible with the two leds.

    The speed of the led flasher may be adjusted with potentiometer P1. Being an astable multivibrator, the circuit has no stable state but oscillates continuously between the two states back and forth. The two transistors T1 and T2 turn and lock each other by turn. The smaller the capacitor value is and the smaller the resistance, the appropriate LED goes out faster, for the benefit of other, who then immediately turns on.

    • The activation time of T2 is t, a = 0.7 x R1 x C1, the switch-off t off = 0.7 x R2 x C2.
    • The switch from T1 is t, a = 0.7 x R2 x C2, the off t off = 0.7 x R1 x C1.

    The transistors do not necessarily have to be BC547B, you may use BC238 or similar small-signal transistors. It is recommended to always use the equivalent transistors. If one of the transistors is defective, wrong or have a malfunction, so does this to the full functionality of this circuit. One LED lights up and the other is dimmed.

    The “two flashing led” circuit is designed for 9 Volts but it works at lower voltages too. In this design we used red leds but by changing the series resistors R1 and R4 you can also use different LED colors.

    Two flashing LED circuit diagram

    2 led flashing circuit schematic

    Components values
    R1 - 470 Ohm
    R2 - 470 Ohm
    R3 - 3.9 kOhm
    R4 – 3.9 kOhm
    P1 - 50 kOhm
    C1 – 47 μF / 16V
    C2 - 47 μF / 16V
    T1 - BC 547 B
    T2 - BC 547 B
    D1 - LED Standard , 5 mm , red
    D2 - LED Standard , 5 mm , red

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    14 Responses to "Flashing LED Circuit"

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    1. Multivibrator flasher circuit is nice.

      Want to learn more about electronics?

      The GuruSantiago can help. Checkout his videos here:

      http://www.youtube.com/user/ElectronicsIsFun
      And follow him on twitter @ElectronicsFun

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    2. am having trouble,,,I built this and other designs trying to get an oscillation of my LED’s,, all weekend ! I am using 2n3904′s and 10uf’s then 100uf’s have tried rebuilding and using various,In pairs of course R’s except for the LED’s 100k, I even found that the 3904′s by radioshack had weaker base sig then the Fairchilds so I swapped them out and started over,,any Ideas? am using a (9vdc battery) but have even tried using a 7805 ps reg in place What can I chk

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      • Try less voltage. I started with 1.5 volts and increased twice till I hit 4.5 volts. It then worked great. I recently bought a $9 adjustable power supply at Fry’s and it has made these type projects a lot easier.

        My question, why do the LED’s oscillate slower when I go from 3 to 4.5 volts? I would have thought the higher voltage would increase the current to the capacitors and decrease the charging time (thereby increasing the cycle speed.) Note: I only had 60V 220mF capacitors. Am I on the back side of some charging curve with such small currents?

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      • The LED’s blink more slow because the LED is already maxed out of current so the resistors are thus taking more of the current. When the voltage is lower the LED is using more of the current then the resistors and the opposite when the voltage goes up. That is why the LED slows when the voltage is increased.

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    3. Can you explain how the transistors works

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    4. can you explain in terms of how this circuit works ?

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    5. Will a +12V supply be too high??

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    6. https://picasaweb.google.com/aLAcrity.Boomerangs

      looking for the LEd circuit on the boomerang in the link, hopefully it opens.

      clifton

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    7. thanks for any help looking for the circuit. when you look close i see r1 c1 ,single led . bubble button.

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    8. Dan Meam says: on August 30, 2012 at 4:48 pm

      by the way… the “L1″ inductor its just an switch i made.. like one 0Volt Relay that make the camera take the picture everytime the led flash’s.. i think this can be used in many ways..

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    9. why delete my 1o comment.. now the second make no sense xD

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    10. Could’nt collect a potentiometer….What can i do ? Can i use two constant resistors ?

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    11. OH8EFI OH8EFI says: on February 2, 2013 at 9:03 pm
        member

      Heh, trying this with Crocodile Clips (Version 3.5)… First two flashes takes place and then it lags and goes near to crashing :D But hey, cool and simple design!

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