Led flasher that reverses Led voltage

Discussion in 'General Electronics Chat' started by Damien, Mar 28, 2008.

  1. Damien

    Thread Starter New Member

    Jun 6, 2007
    I purchased a bunch of 3mm Led's Bi-colour red/blue but turned out to have only 2 legs, not three as i thought, the polarity must be reversed to light second colour. I need a circuit that will alternate the Led voltage at around 2 Hz and operating on 12v Vf is 3.2-3.6v
    The circuit shown elsewhere using 2N3906 transistors works well for the 3 leg Bi Led's but not for these.
    Any help with a circuit or where to find one will be greatly appreciated.
    Regards Damien.
  2. gotumal

    Active Member

    Mar 24, 2008
    You may use a square wave generator with H bridge with transistors or transistor package IC. At this moment, I just could think of this circuit. Cheaper way should be available.
  3. Søren

    Senior Member

    Sep 2, 2006

    Piece of cake with CMOS4k gates, but if you want a specific turn key solution, ask yourself if it would help to post the circuit for a 3-pin LED.
  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    See the attached (but I didn't complete the 2Hz 555 timer):

    When Q2 is on, Q3 is on, and Q1 is off. D2 will light through R2 and Q2.

    When Q2 is off, Q3 is off, Q1 is on. D1 will light thru R1 and Q1.

    This circuit provides the versatility to choose Q1 and Q2, and size them per the number of parallel LEDs and amount of current drawn. Also, R1 & R2 can be adjusted accordingly in value and wattage.
  5. Wendy


    Mar 24, 2008
    Here's the way I'd do it...


    The LED resistors are calculated the normal way for the currents through the LEDs, as if it were in series with the LED. The 555 can source up to 100ma.

    So each LED resistor would be approx. 820 ohms for 10 ma drive.

    You didn't say how many LEDs you were planning on driving. If the requirments exceed the drive of a single 555 then more 555s can be added as drivers. One 555 can drive several LEDs in series, and even more in parallel, although the math gets a little more complex for the LED resistors as the LED dropping voltage increases.
  6. SgtWookie


    Jul 17, 2007
    See the attached circuit.
    NOTE: all references to the 4049 have been changed to 40106, which refers to the CD40106B/HEF40106B, which is a hex inverting Schmitt trigger. Refer to the datasheet for the pin connections.
    The 40106 is a hex buffer, that is an IC with six inverter circuits.
    R1 and C1 make an RC circuit that causes U1A to oscillate at roughly 2Hz.
    U1D and U1F are paired to double the standard maximum 12mA output current to 24mA.
    U1B inverts the input to U1C and U1E, which are also paired to double up on the output current.

    R2 should not be less than 500 Ohms for 12v operation with R2 as the only load. In any case, the current output from each 40106 inverter should be limited to 12mA maximum. As it is, the 570 Ohm resistor will allow approximately 20mA through D3 and D2 one way if D2 is a direct short (D3 drops about 0.65V) and 17.6mA the other way through D1, R3 and R2 if D1 is a direct short.

    R3 and D3 are optional. They are there in case you need to supply a different amount of current for one side of the dual diode.

    In order to calculate the values needed for R2 and R3, you'll need to know your current rating and Vf for each side of your dual LED. If they weren't specified, assume 20mA.

    You can make a constant current circuit using an LM317 and a resistor connected between the output and adjust terminals. A 60 Ohm resistor will give you ~20mA output from the adjust terminal. Connect +6v to +30v to the input terminal. Connect your LED up between the adjust terminal and your power supply return (ground), and measure/record the forward voltage (Vf), then reverse the LED connections, and again measure/record the Vf.

    To calculate R2:
    Take the largest Vf reading from the previous step, and calculate:
    R2 = (Vcc - (Vf(LED)+Vf(D3=0.65)) / ILED(20mA or specified, whichever is less)
    Vcc = 12V
    Vf(LED) = 3.3V
    Vf(D3) = 0.65V
    ILED = 20mA
    R2 = (12V - (3.3+0.65)/20mA
    R2 = (12 - 3.95) / 0.02A
    R2 = 8.05 / 0.02
    R2 = 402.5 Ohms
    Use the next standard value resistor that is equal to or larger than 402.5. For now, let's go with 430 Ohms as R2.

    To calculate R3: (the lower Vf side) - let's say the lower Vf was 2.8V
    R3 = ((Vcc - VfLED) / ILED ) -R2
    For example:
    R3 = ((12V - 2.8V) / 20mA ) - 430
    R3 = (9.2 / 0.02 ) - 430
    R3 = 460 - 430
    R3 = 30 Ohms
    Nearest standard value is 33 Ohms.

    Bill of Materials for:
    Code ( (Unknown Language)):
    2. Item    Count   Label-Value Attributes      Designation
    3. 1   1   1uF     RAD0.2          C1
    4. 2   2   LED2        SIP2            D1,D2
    5. 3   1   1N4148      DIODE0.4        D3
    6. 4   1   360k        AXIAL0.4        R1
    7. 5   1   570     AXIAL0.4        R2
    8. 6   1   110     AXIAL0.4        R3
    9. 7   1   40106       DIP16           U1
  7. Wendy


    Mar 24, 2008
    Every electronic geek in the world loves these kind of circuits. I used to build do nothing stuff like this when I was in college for my friends. :D

    I like the H output drive in the above design, something similar can be done with 555s also.
  8. Audioguru

    New Member

    Dec 20, 2007
    Hi Sgt. Woopie,
    Your 4049 oscillator won't work because it has the circuit of a Schmitt-trigger inverter oscillator.
    You need to make an oscillator with two inverters when the inverters are not Schmitt-trigger.
    The CD4584 and 74C14 Cmos ICs are Schmitt-trigger inverters and will work in your circuit.
    The CD4069, CD4049 and 74C04 are ordinary Cmos inverters.
  9. Damien

    Thread Starter New Member

    Jun 6, 2007
    Thanks for the great response guy's, A little more info the Leds will be used in Emergency vehicles on a model railway layout.
    Mr Marsden Sir I do appreciate geeks my son is one and until last year counted myself as a member of the fold. However radiation treatment of a tumor in my head and a series of mini strokes have severely affected my technical thinking abilities. I still have a reasonable parts stock from my now defunct elec repair business so sourcing bits is not a problem.
    Given my current abilities i think simple is better, so 555 cct looking real good, one or two led's per 555 cct will suffice.
    Thanks again Damien.
  10. Wendy


    Mar 24, 2008
    Would the 74C14 work in the same application on Wookies design? Your right, I had missed it. The 555 circuit I drew is such an inverting schmitt oscillator.
  11. Wendy


    Mar 24, 2008
    You're still there brother, not everyone likes to sling solder. That is the important thing.
  12. SgtWookie


    Jul 17, 2007
    You went to college for your friends? Wow, what a pal! :D

    It's a fun exercise. ;) Using a 555/556/etc timer can be really handy, but usually it's just the "heartbeat" for the rest of the circuit. Lots of folks ignore the venerable 4k series CMOS - but you can do lots of really neat stuff with them even outside of the digital realm.

    Had to re-do my original post; realized I didn't describe it correctly, and left out the calculations.
  13. Wendy


    Mar 24, 2008
    Being male and single, I am oblivious to sarcasm. :D

    I like the 4000 series. I memorized the 7400 series though, as that was state of the art in 76. You can't even get a lot of those chips anymore.

    The 555 is one handy chip. I have a LOT of applications for it, including simple FM modulation, demodulation, and what not.
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
    According to FairFrammis Semiconductor (p.3), the two-inverter oscillator may not work They say that you need 3 gates to guarantee oscillation.
  15. SgtWookie


    Jul 17, 2007
    Actually, I meant to change it to the CD40106B/HEF40106B/MC140106B, which IS a hex inverting Schmitt trigger. Unfortunately, that IC isn't in my library, and I can't add it. It is not pin for pin compatible, but does contain the same number of inverters, and it will work just fine for this app - and it only needs a single inverter for the oscillator.
  16. SgtWookie


    Jul 17, 2007
    Being married the 2nd time around, so am I. :rolleyes: :D

    Tell me about it! I still have TI's 2nd Edition of the TTL Data Book (1976) - and a number of them hard-to-get'ems. ;)

    Yes, it certainly is - works well for FSK stuff, too. You can get some mighty interesting signals out of it if you play with it long enough ...

    But, we're hijacking the dickens out of this thread. I need to go back and edit my previous post and put in the IC change.
  17. Wendy


    Mar 24, 2008