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Basic 555 circuit, capacitor not needed

Discussion in 'General Electronics Chat' started by joseche, Sep 9, 2011.

  1. joseche

    joseche New Member

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    Hi there,

    Why is that the capacitor on pin 5 is not required ?, why is it there then ?

    [​IMG]

    I tried the circuit and it works with and without it
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Its purpose is to filter any noise that may be on the +Vs power. Noise can cause erratic operation of the 555 otherwise. If you have no noise on the +Vs line, then you don't need the capacitor, but typically it's good insurance to add it.
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    With the CMOS series of 555's, due to their much lower internal switching currents, the datasheet advises that they should not be required.
     

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  4. dave

    Dave New Member

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  5. joseche

    joseche New Member

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    1) By noise should I understand current flow from +Vs to the IC ?, somehow pin 5 gets voltage ?

    2) when is this capacitor discharged ?, it means pin 5 has voltage from time to time and the cap is loaded, but then sometime the cap needs to be discharged
     
  6. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Pin 5 is control--you can control the operation of the timer from an outside source. The Control is often used for audio modulation, but for a simple square wave generator it is usually not used. However, if you leave the pin unconnected, it can pick up electromagnetic waves that can cause a tiny bit of erratic behavior, which is often undesired. If you're only using the chip to flash an LED, for example, you probably don't need the capacitor. You probably won't see the difference. The .01uF cap is only there to eliminate the unwanted signals.
    Der Strom
     
  7. crutschow

    crutschow Well-Known Member Most Helpful Member

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    1) If you look at the schematic of the 555 in the data sheet (scary I know) you will see that pin 5 goes to a resistive voltage divider connected between +Vs and ground. This voltage is a reference that determines the trip points the circuit. Thus any electrical noise on +Vs will also affect the trip points and the operation of the circuit.

    2) The capacitor is never discharged. It's purpose is just to filter the DC reference voltage as determined by the resistive voltage divider.
     
  8. colin55

    colin55 Well-Known Member

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    Pin 5 is low impedance (less than 5k) and a capacitor is really not needed.
     
  9. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    In some cases that's true, but if you need a precise output, it is best to have a capacitor, just in case. I don't often use them on breadboards, but I would definitely put one in on a PCB.
     
  10. colin55

    colin55 Well-Known Member

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    Changing the voltage on pin 5 only alters the ouput a very small amount. Who want's a precise frequency from a 555? It's only a junk chip.
     
  11. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    555s are often used to test circuits and equipment, and if the output isn't perfect (well, nearly perfect) then the results will be skewed and my give false information. My point is that it can only get better by putting on a capacitor, so why not put one in?
     
    Last edited: Sep 11, 2011
  12. crutschow

    crutschow Well-Known Member Most Helpful Member

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    What's a "junk" chip?

    The 555 is a well designed, versatile timer chip and is very widely used. I may not want a precise frequency from a 555 but I would want it to be stable and repeatable.
     
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi
    Some 'junk' chip, wish I could get that lucky.!

    http://spectrum.ieee.org/semiconductors/processors/25-microchips-that-shook-the-world/0

    Read the Section for the 555
    extract:
    When the 555 hit the market in 1971, it was a sensation. In 1975 Signetics was absorbed by Philips Semiconductors, now NXP, which says that many billions have been sold. Engineers still use the 555 to create useful electronic modules as well as less useful things like ”Knight Rider”–style lights for car grilles.
     
    Last edited: Sep 12, 2011
  14. colin55

    colin55 Well-Known Member

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    The 555 is the worst design you could possibly imagine.
    It takes 10mA when doing nothing.
    It puts a "crowbar load" on the power rail and the output doesn't get anywhere near rail voltage.
    They tried to improve it with CMOS versions.
    It is simply not an "Engineers chip."
     
  15. carbonzit

    carbonzit Active Member

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    Heh. While I don't think I can fully endorse your dim view of the 555, it is true that it is something of a mongrel--er, "mixed signal" chip.

    [​IMG]

    It is quite a mish-mosh of devices. Who would have thought of marrying a couple comparators to a flip-flop, an inverter and a lone transistor? Having said that, it does seem to have hit the spot for hundreds of thousands of peoples' needs. (Of course, to respond to Eric, I'd have to say that just because something is popular doesn't necessarily make it great. However, the 555 has certainly proven its usefulness over the decades.)

    So Colin, if you were to redesign this chip, what would you do to improve it? What makes its quiescent current so high?
     
  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    cz,
    Comparators, D type Latches and Inverters are common usage, many A to D IC's are made this way.

    I wouldn't call the 555 circuit a 'mish-mash', the earlier TTL version of the 555 has been been redesigned in low power CMOS.

    IMO most practising Qualified Engineers would be confident in using a the CMOS 555 versions in their design.

    The practising Engineer statement is not a side swipe at you CZ, its to cover the self acclaimed web based folk lore engineers.
     
    Last edited: Sep 13, 2011
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi Colin,

    I have to say that you are the first person i have ever heard say that about the 555.
    It is designed to maintain the timing even with power supply voltage variation. That's why it is called a "precision timer" chip.

    "They tried to improve it with CMOS versions."
    What makes you say they "tried", how did they fail, and what is it that you find bad about the 555 design?
     
  18. ghostman11

    ghostman11 Well-Known Member

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    when i first started building incubators or more precisely when i first wanted to make my own controllers, i came on here with an idea from a kit i had seen from velleman. alot of people explained things to me and my interest was ignited in electronics. it was advised i went down the MCU route wich i did, but first to get used to schematics and soldering and actualy making things, i purchased a load of 555 timers and counters to build stuff like simple led flashers etc. i guess for me the 555 gave me the confidence to give electronics a go.
    actualy at present i am 'knocking' up a hot foam cutter and using a 555 for the heat control! yes i could use a micro but decided it was overkill for what i wanted so i have gone with a 555.
    i wonder how many 1000's of people got started in electronics with simple junk 555?
    i am really looking forward to seeing colin's redesigned chip! if you do it right colin it will make you a fortune!! do it wrong or not at all and it will make you look a fool ;) dangerous things are sweeping statements. whatever the 555 stays in my must have collection :D:D although i dont think i have the cmos version
     
  19. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Au contraire. It could be much worse. Look at the design for the old 709.

    If concerned about quiescent current, use the CMOS version of the 555.

    It has a TTL type output which suffers from the same two problems. The spikes due to the momentary "crowbar load" during switching can be suppressed with a filter cap (normally used to decouple all ICs anyway) from the power pin to the common pin.

    It was improved. It reduced the operating current, the switching spike, and the output voltage drop. Why do you use the negative verb "tried" which makes is sound like they didn't succeed.

    Many engineers, including myself, would disagree with that opinion.
     

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