The wonderful, and old, LM555.

Discussion in 'General Electronics Chat' started by hp1729, Nov 29, 2015.

  1. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    LM555

    A wonderfully creative circuit. Basically it is an SR latch with a voltage comparator on the inputs. Between the two voltage comparator inputs is a voltage divider between +V and ground with one tap at 2/3 +V and another tap at 1/3 +V. Taking a look at the pinout with this in mind we have …

    1 – Ground

    2 – Trigger input. An input to a voltage comparator. When this input goes lower than 1/3 of +V the latch sets and the output goes high. The Discharge output is released.

    3 – Output. Q output of the latch. On the LM555 this can sink or source 200 mA.

    4 – Reset input. A low clears the latch.

    5 – Compensation. “Control input”. It is tied to the higher tap in the voltage divider. Normally it just has a small cap to ground. Creatively it can be used as an input to modify the timing cycle.

    6 – Threshold input. This goes to the other voltage comparator input. When the voltage on this pin goes higher than 2/3 +V the latch clears and the Discharge pin goes low.

    7 – Discharge output. An open collector output that goes low when the latch clears.

    8 – The +V line. Specs are guaranteed between 4.5 V and 16 V but this doesn’t mean it can’t operate a bit outside these limits. Typical current drain is about 3 mA at 4.5 V to 15 mA at 15 V.


    As a single shot (monostable multivibrator) …

    Threshold and Discharge are tied together. A resistor is tied from here to +V and a capacitor to ground. When Trigger goes lower than 1/3 +V the latch sets, Discharge is released, the output goes high and the capacitor starts charging. When the voltage on the capacitor reaches 2/3 +V the latch clears, the output goes low and the Discharge output goes low discharging the capacitor.


    As an oscillator (astable multivibrator) …

    One resistor is tied to Discharge and +V. Threshold is tied to Trigger and a resistor is tied between these two and Discharge. A capacitor is tied between Threshold / Trigger and ground. On initial power up the capacitor is discharged, Trigger is less than 1/3 +CV so the latch sets and the output goes high. Discharge is released. The capacitor is charged through both resistors. Eventually the capacitor charges to 2/3 +V. Threshold is met. The latch clears. The output goes low and Discharge goes low, discharging the capacitor through the lower resistor. When the voltage on the capacitor goes below 1/3 +V the latch sets again and the process repeats itself.


    Design 491, Creative use of an LM555.

    In theory this would improve the efficiency of a power supply. Normally the load current is drained off of the output capacitor. Proper design has it that this capacitor can hold a charge long enough to last between pulses from the AC line. Here we monitor the charge on the capacitor. When the cap drains down to a certain point the LM555 latch sets and the voltage regulator turns on charging the capacitor. When the cap is sufficiently charged the LM555 latch clears and the voltage regulator turns off, saving power. On light loads the regulator is off most of the time saving power (in theory).
     
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  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Yeh we already know about it, did you know Kennedy was killed?

    So what's your problem?
     
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Awe come now! -- I find descriptive treatment of venerable (yet contemporary) components in the 'historical perspective' both interesting and refreshing!:)

    Best regards
    HP

    PS
    Yebut - Legend holds that was down to a Carcano -- not an LM555?:confused:;););)
     
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  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Just passing through.
     
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  5. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Curious! The OP is hp1729; HP commented on how much s/he appreciated the post? :)
     
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  6. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    :)
    Of course us old people know about it. These old classics are still great for teaching basic theory to those just learning. The principles still apply to other circuits.
    The ancient LM723 is great for teaching voltage regulators. The 555 and 723 are very cheap for those learning on a budget. Thee designs are older than many of the readers who come here but they are still being made.
     
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  7. Hypatia's Protege

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    Mar 1, 2015
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    :rolleyes::rolleyes::rolleyes::D -- Based upon his apparent interest in vintage digital electronics/computers -- My guess is that the OP's username derives from 'Hewlett Packard'? --- Am I right @hp1729???:)

    Best regards
    Hypatia's Protégé
     
    Last edited: Nov 29, 2015
  8. AnalogKid

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    Aug 1, 2013
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    It is good to see the next gen discover history and recognize beauty.

    OTOH - the circuit in post #1 is a hysteretic control loop wrapped around a switcher acting as a pass transistor. Hysteresis was one of the earlier switching topologies, but fell out of favor as more complex control chips made better performing switching circuits more manageable and lower cost. I ran into one in a couple of Data General Nova 1220s. The 5V power supply was a linear power transformer with a 20 A secondary, a big bridge and a bigger filter cap, and single 20 A power transistor as the pass/switching element in a hysteretic regulator running at 20 kHz, a gutsy move at that time.

    Recently a couple of the big power supply chip houses have introduced hysteretic controllers. The output has more ripple than a forward converter, but it needs almost zero compensation.

    Using a 555 for this is a bit of a push; U1, Q1, and U4 can be replaced with a single comparator.

    ak
     
  9. Hypatia's Protege

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    Mar 1, 2015
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    Do you have any background info on 322 timers? -- I purchased about a dozen of them at a swapfest several years back (packaged with/as Radio Shack 'assorted timer ICs') --- 'Wise old Google' verifies their erstwhile existence - but that's about it:(

    Best regards
    HP
     
    Last edited: Nov 29, 2015
  10. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    No, Herschel Peeler, but "hp" was already in use. "1729" was the famous taxi number if you are a (were an old) math major. It is the sum of two cubed numbers, figured two different ways. The smallest number to have that feature. Yes, I am ancient also. But basics are still basics. It seems teachers still like the old 7476. Those are getting hard to find, especially at a decent price.
     
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  11. Hypatia's Protege

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    Would that be the 'Hardy' something product? I 'did' most of my post secondary/graduate schooling in the early 90's thus I seem to have missed the proverbial boat on that one:(:rolleyes:

    Best regards
    HP:)
     
  12. dl324

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    Mar 30, 2015
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  13. Wendy

    Moderator

    Mar 24, 2008
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    I wrote the chapter on 555's in the vol 6 textbook, literally. I still find uses for the old chip.

    As for power consumption, I use the CMOS versions to good effect. Just published a project in the Completed Projects area using one as a matter of fact. With a current draw somewhere around 10ua @ 3VDC, I would call that low power.
     
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  14. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    That's it. Dang, you are the first to even get that. :)
     
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  15. Sensacell

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    Jun 19, 2012
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    I am allergic to the 555 timer, have not used one since the 80's.

    The same way I am allergic to the 741 opamp, it's just a way to dampen a newbie's joy of electronics, wrestling with that ancient pathetic part.
     
  16. ISB123

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    May 21, 2014
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    I hate NE555 since my school days.
     
  17. Lestraveled

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    May 19, 2014
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    Oh come on..... I can see choosing a better op-amp over the 741 because we have a universe of better op-amps to choose from. But the 555 is still a viable part. There is a reason that it is still being made and in all the modern packages, it does what it does very well.
     
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  18. Hypatia's Protege

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    While the 555 is, indeed, a tad 'long in the tooth' - it does provide 'nubes' with the experience (limited though it may be) of design as opposed to mere programming -- IMO - Reason enough for it's continued existence!:)

    As to the 741? Agreed! 'tis long past time it 'rested in peace'!!!:rolleyes::rolleyes::rolleyes:

    Best regards
    HP
     
  19. dl324

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    I have a stack of vintage Tektronix scopes that use the 741, so I'll be keeping a number in my parts bin. I'll take any destined for landfill...
    I resemble that remark:rolleyes:
     
  20. hp1729

    Thread Starter Well-Known Member

    Nov 23, 2015
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    Once one gets to know the LM555 the CMOS version is no surprise, neither does any other timer. Once you know how the LM723 works you can make sense of any other voltage regulator. Does the LM741 work any different than most other op amps?
    No argument at all on "better". But for learning it is still useful.
     
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