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555 woes (monostable oscillator)

Discussion in 'Electronic Design' started by [email protected], May 26, 2007.

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  1. Guest

    I'm trying to do a simple 555 circuit and can't seem to get a square
    wave on my output.

    I'm using the circuit found at
    and am using 1.5K for R1, 5K for R2 and a 47 uF for C (although my
    radio shack meter says its a 59 uF).

    The output I'm trying to read on a 20 MHz oscope shows a slight ripple
    in the millivolt range, but no square wave.

    I know its an open collector output, so I put a 10K pullup resistor to
    the 5 volt rail, but still no square wave.

    Its possible that I'm not using my o-scope correctly because I just
    got it on ebay. It does seem to read correctly for the 5 volt supply
    as well as another 24 volt supply, but thats just DC volts, no wave

    Other points to note,

    * The 1.5K on R1 has a 15 turn pot
    * The 5 K is actually 2 10K's in parallel
    * This chip is a 556 (dual timer) but I have made sure to remap the
    * I'm running Vcc of 5 volts.

    Any suggestions would be greatly appreciated.

  2. MooseFET

    MooseFET Guest

    On May 25, 6:21 pm, wrote:
    [... about LM555 ...]
    Google on LM555 and datasheet, you are likely to find better
    information about the chip.

    What voltage (DC), do you see on pin 2, pin 6 and pin 3?

    If pin 2 is less than 1/3 Vcc, the output should go high
    If pin 6 is more than 2/3 Vcc, the output should go low.

    Which isn't it doing?

    No, the pin 7 is th eopen collector. Pine 3 is push pull.
  3. Jamie

    Jamie Guest

    to my knowledge the 555 does not have an open collector on
    the output..
    Try putting the input of your scope on DC and use the
    10:1 setting on the probe.
  4. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
    Check, recheck and have someone certify your corrections and findings.
    Advice: get/buy/steal single 555 and start from there.

  5. Bruce Varley

    Bruce Varley Guest

    I haven't checked your reference, but from the description I assume you want
    a square wave generator rather than a triggered monostable. If you're not
    locked into a 555 and can accept a duty cycle that's not exactly (but close
    to) 50%, then you can make an extremely simple squarewave generator out of a
    CMOS schmitt inverter, such as the 74HC14. Take one of the gates, R from
    output to input and C from input to 0V. That's all. Many datasheets provide
    the equation for calculating R and C for a required period, or maybe someone
    here could oblige....? If you don't need the other 5 gates, don't forget to
    tie the unused inputs to one or other rail. Or you can run several of them
    in parallel as a buffer to increase the drive capacity.
  6. You *do* realize that for a *monostable* pulse generator you must keep
    clicking the switch to get a square wave? The pulse will be just 250 mSec
    wide. If you have the astable circuit (which I see is #3), then it should

    Make sure you have a good bypass capacitor on the 5 VDC supply. It is not
    shown in the schematic but is critical for reliable operation.

    Good luck,

  7. D from BC

    D from BC Guest

    Getting beat up by a 555...omg... :p

    If R1 is turned to 0 ohms then the cap cannot discharge.
    (Assuming R1 connects between the discharge pin and V+).
    The astable cct will not oscillate.
    Does the 555 get hot?
    Make sure R1 is turned to something the 555 can tolerate.
    D from BC
  8. On 25 May 2007 18:21:08 -0700, the renowned

    Is it a CMOS version (eg. LMC555, TLC555 7555) rather than
    LM555/NE555? If so, you *must* tie 4 to Vcc. Otherwise, it's just a
    really good idea.

    You're only expecting to get a single pulse when the switch is
    pressed, right?

    These questions really belong in sci.electronics.basics, BTW.

    Best regards,
    Spehro Pefhany
  9. The 555 has been obsolete for two decades now.
    Use a PIC instead.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  10. Yes, the reset pin 4(4,10 for 556) is not shown as connected in the
    schematic, and is almost always connected directly to pin 8(14 on 556). The
    equivalent schematic for the LM556 does not show an internal pullup on this

    Also, the control voltage pin 5(3,11 on 556) should be bypassed with a
    capacitor. Makes it more stable if the power supply is noisy. Tell us what
    voltages you read at each of the pins on your 556. Might as well fix this
    here rather than moving to SEB.

  11. D from BC

    D from BC Guest

    Oh come on...
    Has mixed mode electronics gotten that PICy?

    D from BC
  12. Fred Bloggs

    Fred Bloggs Guest

    Your component values yield an oscillation frequency of about 4Hz, so
    you will want something like 100ms per division time scale to view the
    pulse train.
  13. Fred Bloggs

    Fred Bloggs Guest

    Is there a CMOS 556? Sounds rare...
  14. MooseFET

    MooseFET Guest

    With the right RC values, a PIC makes an ok oscillator.
  15. Fred Bloggs

    Fred Bloggs Guest

    I see, and minimum astable frequency of 1.2MHz at 5V too...looks like
    someone is using a lot of these "hobby" chips.
  16. Not rare. TLC556; about 30,000 in stock at Digikey. Probably others.

    Best regards,
    Spehro Pefhany
  17. D from BC

    D from BC Guest

    555's are for kids..
    Now PIC's..that the Arrfff! Arrrfff! muscle machine of choice... :)

    I don't care if 555's have a newbieish reputation, sometimes they fit
    the problem and I include them in designs.
    D from BC
  18. Rich Grise

    Rich Grise Guest

    There's a new rule here in the s.e.* hierarchy. Nobody's allowed to say
    "Use a Pic" without providing a circuit and a program. ;-)

  19. D from BC

    D from BC Guest

    For example:

    Use a PIC!
    Put 2 resistors on an I/O pin..
    Here's the code:

    INCF FSR, 0x1
    INCF INDF, 0x1
    MOVLW 0x22
    GOTO detdone
    CALL rndgen
    GOTO chgmode

    Have a nice day! :)
    D from BC
  20. You don't need code (or even program the PIC*) to make an oscillator. Just
    use the clock pins and add whatever RC network you like for the frequency
    on GP4. The PIC12F629 has the RC mode as default, and the CLKOUT is fOsc/4
    on GP5. You can eliminate the R and C if you set the clock mode to
    internal, but then you only have a 4 MHz clock with 1 MHz output.

    *(Maybe you have to set the GP5 pin as output)

    Of course it won't have the drive capability of the 555 :)

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