The wonderful, and old, LM555.

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
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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Hypatia's Protege

Joined Mar 1, 2015
3,223
Yeh we already know about it, did you know Kennedy was killed?

So what's your problem?
Awe come now! -- I find descriptive treatment of venerable (yet contemporary) components in the 'historical perspective' both interesting and refreshing!:)

Best regards
HP

PS
did you know Kennedy was killed?
Yebut - Legend holds that was down to a Carcano -- not an LM555?:confused:;););)
 

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
Yeh we already know about it, did you know Kennedy was killed?

So what's your problem?
:)
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.
 

AnalogKid

Joined Aug 1, 2013
8,489
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
 

Hypatia's Protege

Joined Mar 1, 2015
3,223
:)
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.
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
 
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Thread Starter

hp1729

Joined Nov 23, 2015
2,304
:rolleyes::rolleyes::rolleyes::D -- Based upon his apparent interest in vintage digital electronics/computers -- My guess is that the OPs username derives from 'Hewlett Packard'? --- Am I right @hp1729???:)

Best regards
Hypatia's Protégé
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.
 
"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.
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:)
 

Wendy

Joined Mar 24, 2008
22,146
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.
 

Sensacell

Joined Jun 19, 2012
2,568
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.
 

Lestraveled

Joined May 19, 2014
1,946
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.
 
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.
I hate NE555 since my school days.
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
 

dl324

Joined Mar 30, 2015
10,985
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
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...
ancient (but not yet) pathetic
I resemble that remark:rolleyes:
 

Thread Starter

hp1729

Joined Nov 23, 2015
2,304
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.
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?
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.
No argument at all on "better". But for learning it is still useful.
 
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