Duty cycle detection

earckens · Oct 11, 2017

ronsimpson said:
(...)
Next I did a analog design that is just like a 555 with out the three resistors. Two pots sets the two voltage levels. If I could get at the three resistors and force the levels to 50% and x% with a POT or two, then I would feel better.
(duty cycle dependent)
Could use a dual CMOS 555 and use 1/2 as a buffer.

Can you design something with 2 pots to set the 2 voltage levels?
I do have dual 555 available.
It would be great! Thank you for all your efforts!!

ronsimpson · Oct 11, 2017

earckens said:
Can you design something with 2 pots to set the 2 voltage levels?

With a 555 you can not get to the resistors. You will have to make a "555" out of a FF and two comparitors.

earckens · Oct 11, 2017

ronsimpson said:
(...)
I do not like using the RC filter method from "TTL". Because TTL does not pull up well. You should buffer the signal with a CMOS buffer so the pull up is good and predictable. CD40xx or 74Cxx

May I use an open collector transistor instead of a buffer IC?

earckens · Oct 11, 2017

Hi Ronsimpson, after doing u number of simulations with this I discovered that if you wait long enough eventually the output will always trigger. The time needed will of course depend on the actual duty cycle.
But this is not exactly what I need: no matter how long the time lapsed, when the duty cycle remains above a certain value the output should never be triggered; but as soon as the duty cycle drops below a certain value, then and only then should the output be triggered. It does not matter how long it takes for this output to trigger.
Example: suppose the duty cycle should never drop below 60% (60 on and 40 off), then as long as the duty cycle remains above that value nothin should happen. However, when the duty cycle drops to below 60%, then I need the trigger. When the duty cycle recovers to above 60%+hysteresis then trigger should be back off again.

earckens · Oct 11, 2017

The issue with this design is that the duty cycle needs to drop lower than about 52% for this to work.
I need detection from below 60%, or even better from 80% or lower.
Can someone help with a schematic based in the design idea as Ronsimpson proposed using a flipflop and 2 comparators?

alec_t · Oct 12, 2017

Here's a duty cycle threshold detector using a comparator, with one pot to set the median of two reference levels and another pot to set the hysteresis between those levels.

earckens · Oct 12, 2017

Thank you alec_t. I guess R1 and C1 are the low pass filter components?
I had been thinking of using a 2nd order active LP filter: would there be merit in this case?

cowboybob · Oct 12, 2017

ronsimpson said:
A 555 can any job! I am using one as a door stop. ...

Off topic, but coffee came out my nose on that one, ron... .

ronsimpson · Oct 12, 2017

cowboybob said:
but coffee came out my nose on that one

and a 555 can make coffee come out........
will add to my list.

ronsimpson · Oct 12, 2017

earckens said:
May I use an open collector transistor instead of a buffer IC?

No. The output of the RC filter, is a function of low voltage, high voltage, and duty cycle. A open collector transistor has the same problem as a TTL IC. It can not pull up hard and predictable.

earckens · Oct 12, 2017

ronsimpson said:
No. The output of the RC filter, is a function of low voltage, high voltage, and duty cycle. A open collector transistor has the same problem as a TTL IC. It can not pull up hard and predictable.

ok; what buffer IC do you suggest? Or would any 74xx inverter do?

ronsimpson · Oct 12, 2017

Altec_t #26:
Will work.
Try simulation that. If the 555 did not work right then this will have the same problem. Trying to understand your test.

ronsimpson · Oct 12, 2017

earckens said:
what buffer IC do you suggest? Or would any 74xx inverter do?

Do not use: 74LS??, 74L??, 74??,
Can use 74C01 is a 2 input NAND gate. Any CMOS logic.
74HC??, 74HCT, CD40?? (any 74CMOS or CD4thsound MC14thsound.

earckens · Oct 12, 2017

ronsimpson said:
Altec_t #26:
Will work.
Try simulation that. If the 555 did not work right then this will have the same problem. Trying to understand your test.

Well, maybe my simulation is wrong? Anyway, what I saw was that the output would not trigger unless the duty cycle dropped to less then 50%. And I need detection at less than 80% . So whatever gives me a signal when the duty cycle drops below 80% is fine for me.
Note: the duty cycle in question may not change for hours or days on end, and when it changes it would take hours to drop by just 10%.

ronsimpson · Oct 12, 2017

Both circuits (#26 and the 555 with 1K resistor) function on the same idea.
1) filter the AC signal to a almost DC. This voltage is a function of duty cycle.
2) Output is set by:
If input is below some level,
If the input is above other level.
If the input is in between the two levels leave the output alone. (latch)

edit-------
For circuit 555 + 1k resistor the trip points are 4.6 volts and 2.3 volts.

alec_t · Oct 12, 2017

earckens said:
I guess R1 and C1 are the low pass filter components?

Yes.
Can't see the need for an active filter for such a non-critical task.

alec_t · Oct 12, 2017

earckens said:
Well, maybe my simulation is wrong?

The sim I posted (#26) assumes input logic levels of 0.5V and 4V @ 2kHz and lets you set almost any % thresholds you like by varying the pot wiper positions. If hysteresis isn't great enough then reduce the value of the hysteresis pot from 100k to, say, 50k or less.

Edit: With Median and Hysteresis wipers at 0.6 and 0.2 respectively, my sim gives duty cycle switching points at 80% and 60%.

earckens · Oct 12, 2017

ronsimpson said:
Both circuits (#26 and the 555 with 1K resistor) function on the same idea.
1) filter the AC signal to a almost DC. This voltage is a function of duty cycle.
2) Output is set by:
If input is below some level,
If the input is above other level.
If the input is in between the two levels leave the output alone. (latch)

edit-------
For circuit 555 + 1k resistor the trip points are 4.6 volts and 2.3 volts.

Ah ok, indeed: below 2.3V (I thought less than 50%, 2.3V is a bit higher) it trips. How can I increase that trip point from 2.3V to say 4V?

ronsimpson · Oct 12, 2017

earckens said:
How can I increase that trip point from 2.3V to say 4V?

Thats the problem with the 555. You can not get to the points. You can look at the internal diagram of the 555 and build one from 2 comparitors and a SR flip flop.
I need to go work for a while. (for money) Back in 4 hours.

earckens · Oct 12, 2017

I try to open LTspice in Eagle after having made the schematic but it says "Default LTspice simulation library not found, please import first: C:/Program Files (x86)/Eagle-7_6_0/lbr/ltspice/lt-spice-simulation.lbr"

Both the library is present in the indicated map and I have imported this library

ronsimpson said:
Thats the problem with the 555. You can not get to the points. You can look at the internal diagram of the 555 and build one from 2 comparitors and a SR flip flop.
I need to go work for a while. (for money) Back in 4 hours.

Ok, thank you for the assistance.

If a 555 cannot do what I need, is it safe to assume that alec_t's proposal suits the bill?

alec_t said:
Here's a duty cycle threshold detector using a comparator, with one pot to set the median of two reference levels and another pot to set the hysteresis between those levels.
View attachment 108573

Hi Alec_t, I tried to import your drawing in LTspice but it cannot find the LM339: how do I import that and from where?

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