Clean square waves using a 555 timer?

[Arouse1973]

To rule out the scope probe could we try and measure the calibration output square wave of the oscilloscope? Using the blue wire in your picture which the scope probe is currently connected to. Remove it from the circuit and connect it to the calibration output of the scope.
Adam

 

[CDRIVE]

Talk about shoveling sh!t against the tide! When dealing with anything other than DC I've lost count of how many times the pitfalls of solderless breadboards, lead length, ground planes, decoupling, etc have been reiterated throughout this thread.

I think this severely beaten horse died somewhere at the beginning of this thread and washed out to sea when the tide changed.

Chris

 

[Xeno Xenox]

Talk about shoveling sh!t against the tide! When dealing with anything other than DC I've lost count of how many times the pitfalls of solderless breadboards, lead length, ground planes, decoupling, etc have been reiterated throughout this thread.

I think this severely beaten horse died somewhere at the beginning of this thread and washed out to sea when the tide changed.

Chris

I hope you'll forgive me, but I can't tell if you support the proposition that solderless breadboards are useless for anything but DC or support it.

Am I the one beating a dead horse?

 

[Xeno Xenox]

To rule out the scope probe could we try and measure the calibration output square wave of the oscilloscope? Using the blue wire in your picture which the scope probe is currently connected to. Remove it from the circuit and connect it to the calibration output of the scope.
Adam

Here's the signal with the probe connected directly to the calibration terminal:



Here's the signal with the probe connected through the blue wire to the calibration terminal:



I couldn't find a meaningful difference at any time scale.

And here it is connected through the blue wire, through the full distance of a 50-tap power bus on the solderless breadboard, through another blue wire, back through another 50-tap power bus and through yet another blue wire:



The wires and breadboard seem to be holding up fine in your test.

 

[Xeno Xenox]

I successfully got the circuit to generate a 50% duty cycle by replacing R1 (in the first schematic in this thread) with a variable resistor and then tuning it. R1 acts as a pull-up resistor to help the capacitor charge faster and discharge slower. By making it tunable, I can tweak the duty ratio of the circuit:



Thanks to @dorke for explaining that 8V is not an unusual output voltage from a 555 under load with a Vcc of 9V. This led me to look for ways to compensate for the reduced voltage rather than eliminate it.

I still think the transient has an unusual shape and is much more pronounced than in other square wave circuits I've made on the same breadboard. However, there are strong opinions against breadboards in this thread, so I'll withdraw the question on the transient unless and until I build it without a breadboard.

 

[hevans1944]

So, I guess @Colin Mitchell is "vindicated" in his opinion that the 555 is a "very nasty chip". I've heard it called worse than that. Personally, I like it, whether in the low-power CMOS version or its original flavor, perhaps with some chocolate sprinkles. It clearly states on at least one datasheet that at least a one volt drop between Vcc and the output is "normal" at any load current when the output is high, so presumably you have to live with it. The overshoot is probably a result of parasitic capacitance charging up. I suppose you could zap the overshoot by running the 555 output through a Schmitt trigger... maybe follow that with a D-flop to get your 50% duty cycle. No adjustments necessary.

 

[dorke]

I successfully got the circuit to generate a 50% duty cycle by replacing R1 (in the first schematic in this thread) with a variable resistor and then tuning it. R1 acts as a pull-up resistor to help the capacitor charge faster and discharge slower. By making it tunable, I can tweak the duty ratio of the circuit:

View attachment 26402

Thanks to @dorke for explaining that 8V is not an unusual output voltage from a 555 under load with a Vcc of 9V. This led me to look for ways to compensate for the reduced voltage rather than eliminate it.

I still think the transient has an unusual shape and is much more pronounced than in other square wave circuits I've made on the same breadboard. However, there are strong opinions against breadboards in this thread, so I'll withdraw the question on the transient unless and until I build it without a breadboard.

You can achieve better results even on the "nasty" breadboard,by following these simple steps .

1. From post #38

"In the case of the non-working (circuit) there is a load(on pin 3) an R-C one, R being 1k.
What you can do to get the output a bit higher is use a higher value for R say 33K and adjust the value of C .
Try it and see the effect."


2. From post #36

"The capacitor is connected in the "failing circuit ",but isn't connected properly.
It should be like this:
Disconnect the black wire as shown .
Connect the capacitor with the shortest possible leads directly between pins 5 and 1."

3. From post #34

"The Electrolytic cap isn't enough, as I said before use a 10uF tantalum +a 0.1uF ceramic.
Again connect directly between pins 1 and 8(shortest wires possible,better use only the capacitor leads)."

 

[dorke]

So, I guess @Colin Mitchell is "vindicated" in his opinion that the 555 is a "very nasty chip". I've heard it called worse than that. Personally, I like it, whether in the low-power CMOS version or its original flavor, perhaps with some chocolate sprinkles. It clearly states on at least one datasheet that at least a one volt drop between Vcc and the output is "normal" at any load current when the output is high, so presumably you have to live with it. The overshoot is probably a result of parasitic capacitance charging up. I suppose you could zap the overshoot by running the 555 output through a Schmitt trigger... maybe follow that with a D-flop to get your 50% duty cycle. No adjustments necessary.

"vindicated"? you are too quick to let him off the hook

Calling the best selling IC(ever?) "nasty",
Is ....well, you fill in the dots.

This wonderful IC is truly ubiquitous (more so than the 32.768kHz watch crystal?).
It came out in 1971(45 years ago!) and is still manufactured by about 20 world leading companies.
More than 1 billion parts sell yearly,that is nasty indeed,isn't it?

 

[Xeno Xenox]

You can achieve better results even on the "nasty" breadboard,by following these simple steps .

1. From post #38

"In the case of the non-working (circuit) there is a load(on pin 3) an R-C one, R being 1k.
What you can do to get the output a bit higher is use a higher value for R say 33K and adjust the value of C .
Try it and see the effect."


2. From post #36

"The capacitor is connected in the "failing circuit ",but isn't connected properly.
It should be like this:
Disconnect the black wire as shown .
Connect the capacitor with the shortest possible leads directly between pins 5 and 1."

3. From post #34

"The Electrolytic cap isn't enough, as I said before use a 10uF tantalum +a 0.1uF ceramic.
Again connect directly between pins 1 and 8(shortest wires possible,better use only the capacitor leads)."

1. I understand your theory behind point 1, but it doesn't have much impact in practice. I replaced the 1K with a 33K and see no change in the <8V output level. I then tried 330K and see a very slight rise to around 8V exactly. I pushed it even further to 33M and the output still doesn't surpass 8V.

2. I understand why this is best practice. That said, connecting the pin 5 bypass capacitor directly between pin 1 and pin 5 with short leads has no effect on the transient or rest of the waveform. In fact, disconnecting this capacitor entirely has only a tiny effect on the frequency of the waveform and no impact on the transient.

3. Unfortunately, I don't have a 10uF tantalum on hand. I tried a 10uF electrolytic + 0.1uF ceramic connected directly between pins 1 and 8 with short leads. The higher frequencies change a bit, but the large lower frequency hump is still there.

Let me show you what I mean. Here's the falling edge of the square wave:



The transient at the bottom of this edge looks like typical ringing.

Now here's the rising edge with the same scope settings:



That hump is lower frequency than the ringing. All of the good hygiene tips I've received are helping to smooth the high frequencies off the hump, but nothing so far has affected the hump itself.

Furthermore, the top of the hump is 9V, which is Vcc. That's why I think the hump may be a feature of the 555 itself. It looks as though it's trying to drive the ouput to 9V and momentarily gets there, but then falls back and stabilizes at 8V.

 

[CDRIVE]

I hope you'll forgive me, but I can't tell if you support the proposition that solderless breadboards are useless for anything but DC or support it.

Am I the one beating a dead horse?

I'm saying that they're a wonderful invention for rapid prototyping and general experimentation but they're limitations (when used in the realm of fast rise and fall times) should always be factored in when analyzing a circuit. If you ever get the itch to play in the RF sandbox the solderless breadboard gets moved from the "Use with caution" category to the "Absolutely Verboten!" category.

That said I don't think it's the breadboard itself that's causing that much overshoot. I'm more concerned about those excessively long jumpers but I don't think they're causing the problem either.

I'd also like to add that my opinion of the xx55x series chip family echos Dork's opinions exactly. So much so that it had me musing over the term "ubiquitous" because no other word aptly describes it.

While typing this reply I went back and re-read the last page in an attempt to compose a "Try this next" reply. Glad I did because Dork explained everything in his last 2 posts. He did an excellent job!

Oops! I almost missed Hop's entry into this discussion. Who can not enjoy reading our resident word meister's posts!

Chris

 

[CDRIVE]

1. I understand your theory behind point 1, but it doesn't have much impact in practice. I replaced the 1K with a 33K and see no change in the <8V output level. I then tried 330K and see a very slight rise to around 8V exactly. I pushed it even further to 33M and the output still doesn't surpass 8V.

2. I understand why this is best practice. That said, connecting the pin 5 bypass capacitor directly between pin 1 and pin 5 with short leads has no effect on the transient or rest of the waveform. In fact, disconnecting this capacitor entirely has only a tiny effect on the frequency of the waveform and no impact on the transient.

3. Unfortunately, I don't have a 10uF tantalum on hand. I tried a 10uF electrolytic + 0.1uF ceramic connected directly between pins 1 and 8 with short leads. The higher frequencies change a bit, but the large lower frequency hump is still there.

Let me show you what I mean. Here's the falling edge of the square wave:

View attachment 26420

The transient at the bottom of this edge looks like typical ringing.

Now here's the rising edge with the same scope settings:

View attachment 26421

That hump is lower frequency than the ringing. All of the good hygiene tips I've received are helping to smooth the high frequencies off the hump, but nothing so far has affected the hump itself.

Furthermore, the top of the hump is 9V, which is Vcc. That's why I think the hump may be a feature of the 555 itself. It looks as though it's trying to drive the ouput to 9V and momentarily gets there, but then falls back and stabilizes at 8V.

For some unknown reason your images aren't visible to me. That said your tests in this quote has me really scratching my head. Possibly because I've lost track of things. Do I understand correctly that the 555 + Diode configuration doesn't reproduce the same overshoot and 8V (level off) issue?

Chris

 

[Xeno Xenox]

@CDRIVE:

I'm not sure what's going on with the images. It is affecting posts #44 and #49. The images sometimes show up and other times don't. It appears to be an issue with the Electronics Point forum itself.

You can see the scope traces for the working circuit in post #27 of this thread. It looks to me like the "hump" is still present, but it's less pronounced and the output voltage stabilizes above 8V. In that circuit, there's no load on the 555 other than the scope probe (which was at 1X in those pictures).

 

[Arouse1973]

Here's the signal with the probe connected directly to the calibration terminal:

View attachment 26397

Here's the signal with the probe connected through the blue wire to the calibration terminal:

View attachment 26398

I couldn't find a meaningful difference at any time scale.

And here it is connected through the blue wire, through the full distance of a 50-tap power bus on the solderless breadboard, through another blue wire, back through another 50-tap power bus and through yet another blue wire:

View attachment 26401

The wires and breadboard seem to be holding up fine in your test.

Ok another test if you have time. Remove the 555 timer and where the output pin would have been take the scope calibration and connect it there. then measure again with the blue wire connected as it would have been in the circuit.
Adam

 

[hevans1944]

Ok another test if you have time. Remove the 555 timer and where the output pin would have been take the scope calibration and connect it there. then measure again with the blue wire connected as it would have been in the circuit.
Adam

This is a good idea. If the bread-board fixture is introducing problems you should see some sort of difference in the calibration square wave when the probe, with the blue wire attached, is connected or disconnected from the bread-board. Not sure if this will help determine how to eliminate the problem, but it may shed some light on where the problem is coming from.

"vindicated"? you are too quick to let him off the hook

Calling the best selling IC(ever?) "nasty",
Is ....well, you fill in the dots.

This wonderful IC is truly ubiquitous (more so than the 32.768kHz watch crystal?).
It came out in 1971(45 years ago!) and is still manufactured by about 20 world leading companies.
More than 1 billion parts sell yearly,that is nasty indeed,isn't it?

I am not letting Colin "off the hook" but I do appreciate where he is coming from. It is a difficult IC to master because the current generation generally appears to know diddly about analog circuit design. Some folks do believe the 555 is mostly forgotten except by hobbyists. See this article on the Spark Fun website. Be sure to scroll down and read the comments!

This could be a new thread on Electronics Point with a similar polarization of opinion as that on the discussion of BJT theory of operation! A new thread probably wouldn't win any converts or persuade anyone to give up their 555 for a PIC, so I won't start one. I do keep my options open and will go either way depending on the design objective... or maybe the phase of the moon.

I remember when the Signetics NE555 first became widely available in the 1970s. I was still a technician then but some of us were doing engineering design. We were using TI mono-stable multi-vibrators for timing and the 555 appeared (at least to me) to be a more simple solution at that time. The Spark Fun article says the totem-pole output was not present in the original bi-polar 555, only an open-collector output. I don't remember that, but I do seem to recall that we used 1 kΩ pull-up resistors to +5 V on the output. All the latest versions, both bi-polar and CMOS, have the totem-pole output allowing 200 mA of current sourcing OR current sinking... at least with the bi-polar version; the CMOS versions are ±100 mA which is still quite decent.

And, according to this recently edited (16 April 2016) Wikipedia article, about one billion 555 parts were manufactured and sold in 2003. Has the volume drastically decreased over the past thirteen years? I agree that volume of sales was quite impressive for a "nasty" and, what some now consider, "obsolete" part! Perhaps the only thing as ubiquitous as the 555 is the NPN/PNP small-signal transistor pairs 2N3904/2N3906. Or am I showing my age there, too?

Hop

 

[CDRIVE]

I agree that volume of sales was quite impressive for a "nasty" and, what some now consider, "obsolete" part! Perhaps the only thing as ubiquitous as the 555 is the NPN/PNP small-signal transistor pairs 2N3904/2N3906. Or am I showing my age there, too?

Hop

Yes, you ancient, decrepit, old fart but I'll drink to that!

BTW, I love Adam's last suggestion regarding the scope cal output tied through the breadboard. It may only be yet another straw but no point in not grabbing at them now!

Xeno Xenox, has this been asked yet? Where did you get this 555? Have you tried others? If yes and you bought them from the same supplier they may all have the same issue.

Chris

 

[dorke]

Well this must be Collin's view of that "Bad to the Bone" IC.

"Wicked" ,isn't it'?

 

[Xeno Xenox]

Xeno Xenox, has this been asked yet? Where did you get this 555? Have you tried others? If yes and you bought them from the same supplier they may all have the same issue.

Chris

I used 555 timers from two batches. The first batch I've had in my collection for years and the second batch I ordered through Amazon (from China) a month or so ago. All of them have exhibited the same behavior.

 

[Xeno Xenox]

Ok another test if you have time. Remove the 555 timer and where the output pin would have been take the scope calibration and connect it there. then measure again with the blue wire connected as it would have been in the circuit.
Adam

The scope is generating a 3V peak-to-peak signal on its probe compensation output and to the best of my knowledge, that output is only designed for probe compensation. Connecting it to an active circuit driven by a 9V supply seems like a bad idea.

Instead, I used my function generator (a Siglent SDG1025) to generate an 8V waveform and drove it with that.

So, the circuit was identical, 9V power supply and all -- but the 555 was removed and instead I fed a 0-8V square wave of the same frequency (30kHz) directly to the breadboard hole where pin 3 of the 555 used to be.

I then measured the output using the same blue wire connected in the same way to the circuit. Here's the result:



Looks pretty clean to me!

I tried a 0-9V square wave as well with the same clean results.

 

[dorke]

Here is another simple test for you:
In the "working " circuit , use the same R-C from the" non-working" one as a load between pin3 and gnd.
remeasure the signal of pin3 ,what do you get?

 

[Xeno Xenox]

Here is another simple test for you:
In the "working " circuit , use the same R-C from the" non-working" one as a load between pin3 and gnd.
remeasure the signal of pin3 ,what do you get?
View attachment 26428

Here's the "working circuit" with no load:





And here's the same circuit with the RC load from the "non working circuit":





My interpretation is that the added load draws the voltage down below 8V and it gradually recovers to 8V as the capacitor charges and the current demand drops.

Do you see anything illuminating?