555 timer VCO

Discussion in 'The Projects Forum' started by suzuki, Nov 25, 2011.

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  1. suzuki

    Thread Starter Member

    Aug 10, 2011
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    hi,

    i am trying to use a 555 timer to build a voltage controlled oscillator. I am finding that by increasing the control voltage i send to the 555, the lower the frequency, so i do know how to generate for example, 50kHz or 500kHz. this seems to be verified after doing a couple google searches.

    My problem is how can i increase the range of frequencies that the 555 can output? for example, if i can generate 50kHz, the range of frequencies is from 50kHz to 100kHz, or if i generate 500kHz, the range is from 500k to 550k. I have seen the formula

    f = 1.44/(C*R)

    which i believe gives the center frequency, but i dont really know what parameters i can change to increase the range of possible frequencies. e.g. i want my 555 timer to be able to output between 50kHz and 500kHz, which i suppose means increasing the bandwidth of my vco?.

    thx
     
    Last edited: Nov 25, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You won't be able to get that wide of a bandwidth. You might increase or decrease the frequency by 100%, but not 1000%.

    That's because pin 5, CTRL only gives access to the threshold of the internal voltage divider, not the trigger.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Try an XR2209.
     
  4. suzuki

    Thread Starter Member

    Aug 10, 2011
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    Hmm ok, could you give me more details about this?. I would still be interested to know how i could get 100% of the frequency. I.e. if my center was 200kHz then how I would be able to get +/- 200kHz or 0 to 400k.

    I was finding that i could only get about 20% of bandwidth ie for 100k about 20kHz of possible range.

    Thanks for all replies
     
  5. Yako

    New Member

    Nov 24, 2011
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    Yeah 2/3 of VCC.

    The duty cycle for an NE555-based VCO is terrible too.

    Very, very narrow. Try interfacing it to some other parts.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Actually the duty cycle problem can be solved by using pin 5 as a VCO and the 555 Hysteretic Oscillator, though it would work better with a CMOS 555.
     
  7. Yako

    New Member

    Nov 24, 2011
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    I am going to take an educated guess and say that you've read most of Forrest M. Mims III 's Radio Shack books.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Yep, then gave them away while I was in college to a high school student. He visits this site occasionally.

    Forrest M. Mims III
     
  9. suzuki

    Thread Starter Member

    Aug 10, 2011
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    does this mean that the CTRL voltage can only go up to 2/3 of Vcc? im not sure if this is a problem, you would just get a broader step between frequencies. i.e instead of each volt going up 10kHz, each control voltage step goes up 50kHz...so less resolution, but a more broad range is still achievable, i think.

    still awaiting sgtwookie's response here
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    You can pull pin 5 nearly to Vcc, but as you do so, the time will be very susceptible to triggering by noise on the power rails.

    You can also pull pin 5 down to ground. [eta] When pin 5 is at ground, the ON duty cycle % will be the lowest, but the output frequency starts decreasing again once you get below a certain point; somewhere around 1/10th Vcc, but it depends on your Vcc and the particular timer as well.

    Whatever voltage pin 5 is set to (threshold), the trigger will be 1/2 of that voltage.

    The duty cycle will change very significantly as the CTRL input is varied from nearly Vcc down to ground.

    In a bjt 555, there are three 5k resistors in series from Vcc to GND. The threshold and pin 5 are connected to the upper junction. The trigger is connected to the lower junction; and there is no external access to this junction.

    If you want a frequency of zero Hz, turn the power off, short pins 2 & 6 to ground, short pins 2 & 6 to Vcc, or short pin 4 to ground, as those are about the only ways you'll get there.
     
    Last edited: Nov 25, 2011
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The 4046, phase -loched loop, contains a wideband,self contained VCO, requiring 2 ext Rs & one C.
     
  12. HF94

    New Member

    Feb 10, 2014
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    I really hope there's a complete theory with right formula to control the frequency range and duty cycle for this 555 VCO :(
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Necroposting and Hijacking are not allowed on AAC.

    Necroposting, posting on a long dead thread, where the TS has moved on and no body care.

    Hijacking, trying to take over someone elses thread. A thread belongs to the one who started it, so feel free to start your own.
     
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