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4001 as Schmitt

After a longish lapse I'm in my shed/workshop, making a garden light controller for my son. Amongst other sections, it includes this dark detecting circuit:

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I've used the 4001 in this way for at least 3 decades, apparently successfully. But this time it didn't work as expected. When I took its output to the pin 3 clock input of a 4013 toggle FF, I didn't get reliable toggling when the darkness threshold was reached.

On examining the Schmitt's output it seems clear that this is because I'm getting unwanted pulses, typically around 1 us, just before the clean high swing. The number ranges from zero to maybe 3 or 4. Can someone tell me why please? Is it perhaps an inherent characteristic of this simple circuit that I've got away with before? Or something about the filtering I've added rather arbitrarily to the front end? Or maybe just a bad chip? (I have yet to try others; will do that this morning.)

Also, although I'd like to get to the bottom of this puzzle (especially as I still have large stocks of 20-year old 4001s ), I'd welcome any suggestions on better ways of achieving my aim: reliably toggle an output when it gets dark.

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Terry, West Sussex, UK
Reply to
Terry Pinnell
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Terry.. how much hysteresis do you reckon you have, say 1us or 10us after the output switches? Say it switches at 2.5V in. An estimate will do.

Split that 1M8 into a couple roughly equal pieces and put the 10uF in the middle. You may have to double it to get the effect you want.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Thanks Speff, I'll try that change to the filter.

I'm not sure I follow you about the hysteresis. As I recall, without the filter complication, with Rin = 1M8 and Rf = 10M, the hysteresis H = Vcc*Rin/Rf, so would be about 0.9V. It's a beautifully sunny hot afternoon here so I can't force myself back into the shed to measure it . But I'd expect it to respond to say a triangle wave by going high at say 3.0 V and low again at around 2.1 V.

Isn't H effectively a constant then for a given circuit configuration, as the above simplistic estimate obviously assumes? I'm not clear how I'd measure or estimate H at a certain *time* ...

Further questions are emerging over this little project, but I'll keep them in separate posts.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Terry, Spehro's right on the money in terms of a solution. Put simply, it looks like your problem is the fact that with the 10u cap on the input if the gate, the input voltage can only change slowly, so the gate spends significant time in the noise-prone region, and what you're seeing is noise-induced switching during that time. Hysteresis works by being fast.

Once you've solved that problem, the amount of hysteresis is unlikely to be of concern, your circuit should work fine.

Reply to
bruce varley

Shouldn't the feedback resistance also be much lower to speed-up switching? Even if he changes the PI to a T like Spehro said, 10Mohms times the input capacitance alone would produce a long switching interval like that observed.

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Reply to
Tom Del Rosso

Use...

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Eliminate U4A and R5. Change R4 to a short. Input via R1.

Then you get both noise integration, yet snap at the threshold.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Yup, nice design. Can the 74HC04 input withstand the 7.5V transient or should a resistor, say 510R be inserted between node V and U1A-1? What if a

74AC04 were used, is a 510R necessary?

Regards, Harry

Reply to
Harry Dellamano

I think R5 should be 1 or 2 5% values higher than R4.

Reply to
John Popelish

Bruce, Tom: Thanks both. Hope to change the filter to Speff's T-type this afternoon. I'm sure it will fix the problem by getting through that noisy stage faster. That would square with my never having problems like this before. Reckon I never used any long-interval smoothing in those. My thinking this time was to avoid switching the darkness detecting stage during, for example, heavy cloud cover in early evening.

Hadn't thought about that critical period - just assumed the deliberately high H would sort it.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Thanks Jim. I'll try adapting that circuit and maybe use it next time. Present circuit is already built on stripboard, with several other sections, so want to fix it with existing 4001/4011/4013.

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

And what part of "Eliminate" don't you understand ?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

ESD diodes catch it. Larkin would say that's OK ;-) but maybe a small R is called for to limit current.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

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