555 as a VCO. problem

Discussion in 'The Projects Forum' started by Quintilis_Telescope, Dec 5, 2009.

  1. Quintilis_Telescope

    Thread Starter New Member

    Oct 13, 2009
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    Hello.
    I'm using the 555 as a VCO by using the 'control' input. The circuit to the left is a RC circuit. When switch J2 is closed, the capacitor slowly charges to 5V, and the frequency from the 555 should be 0Hz. The circuit works as I want it to in the simulation software, but when I build it, it seems the capacitor is discharging through the 555 control input and it never reaches 5V. What is a simple way to get around this? Thankyou.

    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    A few things to keep in mind:
    1) A standard (bjt) 555 timer has a minimum Vcc of 6v. It won't work properly below that.

    2) The internal divider that the CONTROL input connects to consists of three (nominally) 5k resistors in the bjt version (15k total), and three nominally 100k resistors in the CMOS version (300k total). It connects to the junction of the upper two resistors.

    3) Decrease your R1 to zero Ohms.

    4) Unless you put a large value resistor between the R2/C1 junction and ground, C1 will eventually charge high enough to trip the threshold with a CMOS 555, even if your R1 is zero Ohms.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
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    Shorting the 555 DIS pin to VCC with a zero ohm R1 would turn the 555 into a one-shot. :)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    You are correct! That is the only way to get a 555 timer to 0 Hz. :)
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I've noticed different manufacturers have different specs, and the datasheet I have on file lists 4.5VDC as the minimum voltage for a standard 555 (max 16VDC). I believe this is pretty much universal, but would not be surprised to find a datasheet that says differently.

    I suspect this was to insure maximum compatibility with TTL, which was the premier logic family when the 555 was invented.

    I plan on doing a 555 as VCO experiment in my The 555 Projects. You can use a variable resistor, or a voltage input. The voltage can not exceed 2/3 of the Vcc, and needs to be a bit above ground.
     
  6. Quintilis_Telescope

    Thread Starter New Member

    Oct 13, 2009
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    Let me clarify. I have the 555 to initially output a ~2kHZ signal. This frequency needs to be at the output when there is 0V across the capacitor. When J2 is closed, the capacitor slowly charges and the 555 output frequency slowly goes down. Once the capacitor reaches 5V there is no more oscillation (0.0000Hz). I need to have R1 because otherwise, the capacitor is instantly going to be equal to the supply voltage once J2 closes.

    Edit: I noticed there are two R1's. sorry
     
    Last edited: Dec 6, 2009
  7. Wendy

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    Mar 24, 2008
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    You will have to add circuitry to adjust the voltage ranges to match the frequency response you need. With op amps this is no big deal, but I don't see a way around it.

    So on a sliding scale...

    Vcc
    .
    ¾.....Freq 1
    .
    ½.....Freq 2
    .
    ¼,..,,Freq 3
    .
    GND..2Khz


    So what are the numbers you want again?
     
  8. Quintilis_Telescope

    Thread Starter New Member

    Oct 13, 2009
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    At GND the frequency is 2Khz and it gradually decreases to 0Hz when its = Vcc. I don't start learning op-amps until next semester so I am clueless at that.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    What kind of change in frequency did you expect before it stops oscillating altogether?

    It's not going to be a smooth decline to 0Hz. It's going to screech to a halt, like slamming into a brick wall.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Like I said, you will have to tweak the range a lot. The basic 555 in VCO mode wants to go slower as the voltage goes down to faster as the voltage goes up, and you will need to limit the swing since you don't want to go out side these ranges. I had a similar project where the OP wanted to generate a pulsating tone, the problem was pretty similar.

    I'll get back with you if someone doesn't beat me to it.

    Fair warning though, you'll need to build this on something like a protoboard, and it will be several chips.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Take another look at your simulation.

    Make sure you run it for 100mS or so.

    When it starts off, your C1 will have 0v on it, but that will eventually build up to 2/3 Vcc, or 3.333...V - providing you let the simulation run long enough.

    When you turn J2 on, you place a 600 Ohm resistor in parallel with the top 5k resistor, and in series with the lower 2/3 of the 15k internal voltage divider.

    You tell me what the voltage on C1 will eventually be.
     
  12. Wendy

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    Mar 24, 2008
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    0Hz is a form of infinity, if you think about it. That makes this a fair question.

    To the OP: why is this like infinity? Most frequency charts are logarithmic. Try showing 0Hz on a log chart. Or calculate the period, which is 1/frequency.
     
  13. Quintilis_Telescope

    Thread Starter New Member

    Oct 13, 2009
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    This is for a digital roulette wheel with LEDs as outputs. So:

    2Khz @ Vcc
    200Hz @ 4/5 Vcc
    100Hz @ 3/5 Vcc
    50Hz @ 2/5 Vcc
    10Hz @ 1/5 Vcc
    0Hz @ 0/5 Vcc

    The reason I have it starting out at 2Khz is so its somewhat random once you close J2. If the starting frequency too low, people could rig the wheel. I understand this is not a practical way of randomizing but its no big deal-Its just a little toy for my digital class.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    OK, then your 555 circuit isn't going to work as you want it to.

    It will shift frequency as the CONTROL input is varied, but not over the range you wish.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Try this, I don't know how well it will work, but it might get you in the ballpark. I'm thinking of a LM358 for the op amp.

    [​IMG]
     
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