555 timer 50% duty cycle

Discussion in 'Electronic Basics' started by panfilero, Dec 27, 2006.

  1. panfilero

    panfilero Guest

    Hi, does anyone know how I could get a 555 timer to operate in astable
    mode at a 50% duty cycle? All the circuits I've seen are always for
    greater than 50%. thanks.
    panfilero, Dec 27, 2006
    #1
  2. panfilero

    James Beck Guest

    In article <>,
    says...
    > Hi, does anyone know how I could get a 555 timer to operate in astable
    > mode at a 50% duty cycle? All the circuits I've seen are always for
    > greater than 50%. thanks.
    >
    >

    I just googled 555 50% PWM and got :
    http://www.dprg.org/tutorials/2005-11a/index.html

    Look at what is done with the diodes D1 and D2

    Jim
    James Beck, Dec 27, 2006
    #2
  3. panfilero

    Ian Malcolm Guest

    James Beck wrote:

    > In article <>,
    > says...
    >
    >>Hi, does anyone know how I could get a 555 timer to operate in astable
    >>mode at a 50% duty cycle? All the circuits I've seen are always for
    >>greater than 50%. thanks.
    >>
    >>

    >
    > I just googled 555 50% PWM and got :
    > http://www.dprg.org/tutorials/2005-11a/index.html
    >
    > Look at what is done with the diodes D1 and D2
    >
    > Jim
    >

    Also notice that the timing resistor is fed from the Output and the Load
    fed from the Discharge pin (pullup resistor required or directly drive
    a load with low side switching). If you dont need a variable duty
    cycle, leave out the diodes and try a single timing resistor from the
    Output to the junction of Threshold and Trigger. It will probably be
    very close to 50% but might be off by a little. The circuit Jim gave is
    trimmable.



    --
    Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
    ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- &
    [dot]=.
    *Warning* SPAM TRAP set in header, Use email address in sig. if you must.
    Ian Malcolm, Dec 27, 2006
    #3
  4. panfilero

    John Fields Guest

    On 27 Dec 2006 10:59:18 -0800, "panfilero" <>
    wrote:

    >Hi, does anyone know how I could get a 555 timer to operate in astable
    >mode at a 50% duty cycle? All the circuits I've seen are always for
    >greater than 50%. thanks.


    ---
    An easy way is to use a 7555 and let the output feed the RC:

    View in Courier

    .. +-------------+
    .. | |
    .. [Rt] +V |
    .. | |8 |
    .. | 6+---+---+3 |
    .. +--|TH OUT|--+-->OUT
    .. | 2|___ _|4
    .. +-O|TR R|O--+V
    .. | +---+---+
    .. [Ct] 1| 7555
    .. | |
    .. GND GND

    Another way is to use the circuit you have and run the output
    through a divide-by-two circuit:


    +-----------+
    | +-----+ |
    +--|D Q|--|--->OUT
    555OUT>-----|> _| |
    | Q|--+
    +-----+
    HC74
    4013





    --
    JF
    John Fields, Dec 27, 2006
    #4
  5. panfilero

    Dorian Guest

    >> mode at a 50% duty cycle? All the circuits I've seen are always for
    >> greater than 50%. thanks.
    >>
    >>

    > I just googled 555 50% PWM and got :
    > http://www.dprg.org/tutorials/2005-11a/index.html
    >
    > Look at what is done with the diodes D1 and D2
    >
    > Jim


    The CMOS version of the 555 (e.g. 7555) is far superior to the Bipolar
    version (e.g. NE555) for a variety of reasons. Reason 1 is reduced power
    consumption. Reason 2 is the absence of the quirky short circuit spike (400
    ma) during an output transition. Reason 3 is that output levels of the CMOS
    version approach the supply rails (ground or common and VCC) as they should.
    Also I believe the voltage divider resistors in the bipolar version are 1K
    while they are 100K or more in the CMOS version. This is very useful if
    you're modulating the pin 5 voltage level.

    Hooking the output of the 555 (pin 3) to the RC network will give you a 50%
    duty cycle as John mentioned and this always works best with the CMOS
    version.

    Dorian
    Dorian, Dec 27, 2006
    #5
  6. panfilero

    kell Guest

    panfilero wrote:
    > Hi, does anyone know how I could get a 555 timer to operate in astable
    > mode at a 50% duty cycle? All the circuits I've seen are always for
    > greater than 50%. thanks.


    Connect a signal diode like 1N4148 in parallel with the resistor
    between pins 6 and 7, with the cathode (stripe) oriented toward pin 6.
    With this arrangement you can get any duty cycle you want. You can
    even get a fixed frequency, variable duty cycle oscillator if you
    replace the fixed resistors with potentiometer. Connect the ends of
    the pot to pins 6 and 8, the wiper to pin 7, and the diode from pin 7
    to pin 6.
    kell, Dec 28, 2006
    #6
  7. "James Beck" <> wrote in message
    news:...
    > In article <>,
    > says...
    > > Hi, does anyone know how I could get a 555 timer to operate in astable
    > > mode at a 50% duty cycle? All the circuits I've seen are always for
    > > greater than 50%. thanks.
    > >
    > >

    > I just googled 555 50% PWM and got :
    > http://www.dprg.org/tutorials/2005-11a/index.html
    >
    > Look at what is done with the diodes D1 and D2
    >
    > Jim
    >

    If you have the space, feed the output of the 555 into the clock input of an
    edge-triggered J-K FF. Tie J and K high to create a toggle and the output
    will be a nice 50% duty cylce at 1/2 the input frequency.

    Richard
    Richard Seriani, Sr., Dec 28, 2006
    #7
  8. panfilero

    jasen Guest

    On 2006-12-27, panfilero <> wrote:
    > Hi, does anyone know how I could get a 555 timer to operate in astable
    > mode at a 50% duty cycle? All the circuits I've seen are always for
    > greater than 50%. thanks.


    change the voltage on pin 5 or use a different circuit, how much precision
    do you need?

    Bye.
    Jasen
    jasen, Dec 28, 2006
    #8

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