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555 timer how-to

Discussion in 'Electronic Basics' started by The Green Potato, Feb 16, 2009.

  1. I have wired 27 10mm leds in series and I'm trying to get them to blink
    using a 555 timer with 2 capacitors and 2 resistors but it won't work!

    Could someone please lead me to some helpful information? I've successfully
    used the 555 with a 4011 to create a sequencer, but using the 555 alone is
    giving me fits!

    Thanks!

    TGP
     
  2. supply votage varies from 3-4.5vdc depending on desired brightness. haven't
    scoped the voltage drop...
     
  3. the specs say 2-18v. i suspect this is the problem? should i switch to a PIC
    for so many leds?
     

  4. Electrical-Optical Characteristic (Ta=25oC)

    Item
    Symbol
    Condtitions
    Min.
    Typ.
    Max.
    Unit

    Forward Voltage
    VF
    If=20mA
    /
    3.2
    3.6
    V

    Reverse Current
    IR
    VR=5V
    /
    /
    10
    uA

    Dominant Wavelength
    /
    IF=20mA
    465
    /
    470
    nm

    Luminous Intensity
    IV
    IF=20mA
    /
    40,000
    /
    mcd

    50% Power Angle
    /
    IF=20mA
    /
    15
    /
    degree


    Absolute Maximum Ratings (Ta = 25oC)

    Item
    Symbol
    Absolute Max. Rating
    Unit

    Power Dissipation
    PD
    100
    mW

    Forward Current (DC)
    IF
    30
    mA

    Peak Forward Current
    IFP
    100
    mA

    Reverse Voltage
    VR
    5
    V

    Operation Temperature
    Topr
    -40~85oC
    /

    Storage Temperature
    Tstg
    -10~100oC
    /

    Lead Soldering Temperature
    Tsol
    Max. 260oC for 5 sec.

    (3mm from the base of the epoxy bulb)
     
  5. thanks! this looks simple but i do not understand parts of it... the leds
    are driven by pin 3, and pin 1 is ground and 8 is voltage in -- correct?

    what am i missing?
     
  6. Guest


    Use the 555 to power the gate of an N-channel MOSFET, which is rated
    to withstand the higher (54V?) voltage necessary to drive all those
    LEDs in series.

    Or use a relay. Up to you

    Michael
     
  7. yes... i didn't consider the 555 couldn't handle that much voltage -- or
    even needed to. i thought 3v from the source translated into 3v total on the
    board minus the drops...

    electronics for dummies
     
  8. Jamie

    Jamie Guest

    I think it's safe to say that wiring 27 LED's in series will require
    some voltage beyond what the timer can deliver.

    http://webpages.charter.net/jamie_5"
     
  9. Nobody

    Nobody Guest

    If you connected them in parallel rather than in series, you would only
    need 3.2-3.6V, but then you'd need to switch 27 times the current (e.g.
    540mA, assuming 20mA each).

    You could get by without an external transistor if you used e.g. 9 strings
    of 3 LEDs each. 3 LEDs in series @ 3.6V = 10.8V, 9 strings in parallel @
    20mA = 180mA.
     
  10. Guest

    you need a resister between each led and 27 leds has quite a voltage
    drop with resisters inbetween each one if the max v+ on your 555 is 18
    volts use leds with milli amp ratings at less than one volt per led
    becuase your cant have more one volt leds than your original b+supply
    with any real brightness you will have to supply a higher B+ of
    greater than the number of combined led lights in your circuit
     
  11. i have reconfigured the 27 leds in parallel. 3v makes them bright, 4.5 makes
    them very bright.

    is there now an option i have [or didn't explore] that i didn't with the
    leds in series?

    blinking lights are more complicated than first thought!
     
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