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555 in astable mode

Discussion in 'General Electronics Discussion' started by Shandy, Dec 30, 2015.

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  1. Shandy

    Shandy

    2
    0
    Dec 30, 2015
    Hi all,

    I am trying to create a astable circuit to generate a 56khz carrier frequency for infrared but first i want to build the astable circuit using a 555 timer i have used the information online using this website here

    http://www.ohmslawcalculator.com/555-astable-calculator

    the values i have used are C - 0.1nf, R1 18k, R2 120k to generate 55.930 which is the closest i could get to 56khz with a duty cycle of 53.49.

    it is to my belief that a lower duty cycle is better? (as i understand this means i can use more current)

    i will be using a tsop 4856 to receive and a tsal6100 to send i have check the power supply on the oscillator using
    a arduino 5v and a dedicated power supply.

    My problem i am having is on my oscilloscope i am getting 43.72khz on the frequency with these awful surges at each rise in peak, i have attached a picture below if anyone can take a look it would be appreciated.

    P.S i am also using 1% tolerance resistors and ceramic capacitors with a NE555N timer.

    Thanks in advance, Chris.

    [​IMG]
     
  2. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    You attachment doesn't open.
    Try loading again.
    A pic of your actual build, and of the scope screen
     
  3. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    To get an accurate and stable 56KHz, your best bet is to use a crystal oscillator + divider. A 555 will drift with temperature and aging of components.

    Bob
     
    Anon_LG likes this.
  4. Shandy

    Shandy

    2
    0
    Dec 30, 2015
    [​IMG]
    here is the scope picture sorry i incorrectly uploaded it, i will get a picture of project for you in about an hour or so due to having a meal to go out to.
    thanks in advance, Chris.
     
  5. Anon_LG

    Anon_LG

    453
    117
    Jun 24, 2014
    Take bob's advice, if you are going to be building waveform correcting circuitry to improve the rise time and voltage spike, you might as well go with a crystal oscillator circuit. The 555 circuit is not the best to use in a an application such as yours, as Bob said, the components involved will encounter thermal ageing, rendering the circuit potentially useless within a matter of months (this is heavily dependant on use and tolerances required for the carrier frequency). You could look for prebuilt oscillators instead that come in integrated packages, and divide these down using binary dividers.

    If however you wish to use a 555 astable, use R1 as 20kΩ, R2 as 250kΩ and two 0.1nf capacitors in series, making 0.05nf for C1. This gives duty cycle of 51.92% (50% is absolute minimum on 555, feed a high duty cycle through an inverter to get a mirrored low duty cycle, this will not make your waveform any more attractive though!) and a frequency of 55.5khz.

    I hope this helps,
     
  6. AnalogKid

    AnalogKid

    2,393
    665
    Jun 10, 2015
    Your scope shot still does not open, and without that it is dificult to speculate on the spike. What is your acceptable frequency error tolerance? A 555 with quality external components can meet +/-5% or better, but for 2% or less you need a crystal or resonator.

    ak
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    0.5 nf is a low capacitance, and stray capacitance could easily affect the circuit. Stray capacitance will lower the frequency which is what you're seeing.

    A good frequency crystal is 4.9152 MHz because it divides nicely to frequencies used for RS232 and 56000 is close to one of those. (divide by 88).

    7MHz (or anything else with a factor of 7) would allow you to create an exact frequency.

    However, for your purpose, anything within about 10% should be fine. You could change one of the resistors to a trilogy and tune the 555 oscillator until it's close.
     
    Anon_LG likes this.
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