Looking for two-input comparator with hysteresis

Hi! Your 741 and I are friends since the 1970's. In those times it was DC-to very high audio amplifier so for the 60 or maybe 120Hz don't worry. But it uses split supply, in what you are doing is a headache. Try to concentrate on CMOS 3-16V supplies and dual NAND gate can be used as analog amplifier by using proper feedback and input resistors.

Good luck

Stanislaw Slack user from Ulladulla.

Thanks Spehro. I think I see what you're saying, though I still need to have a think to convince myself as to why a resistor is needed at both ends of the transformer winding. Perhaps when I've drawn a full schematic I'll post a copy for people to take a look at?

Best wishes,

Chris

Thanks for the suggestions, Ken. I'll look into those ideas tomorrow.

Best wishes,

Chris

I'm surprised you don't sell monster cables to go with the Piezo to get a crisper time. ;-)

LIkely, but it's not needed with anything this slow. Filter it with the 99.999999% of the free cycles left after counting to 60 a few times.

"Spehro Pefhany" a écrit dans le message de news: snipped-for-privacy@4ax.com...

He he... And in case you find the display flickers too much, you can still power your clock from a 3 phases mains.

On Tue, 23 May 2006 14:44:51 -0400, Keith wrote in Msg.

That's nuts. If your circuit is directly connected to the transformer's secondary, an optical isolator (OI) gains nothing in case of transformer isolation failure.

And if you suggest to use the OI to detect the signal directly off the line side (this is how I read your post), you're 1) encouraging the OP to build additional circuitry on the "live" AC mains side, and 2) you're adding a parallel path for isolation failure.

Wait -- you're arguing for the inclusion of a superflous part but at the same time encourage the replacement of something as cheap, simple and robert

On Wed, 24 May 2006 03:26:12 +0000, Christopher Tidy wrote in Msg.

Look, Chris, I don't know why people are suggesting all this complicated stuff -- software filtering, offset trim, optical insulators, diff amp, additional transformer windings.

Just look at the voltage of the "top" AC input node of the bridge rectifier with respect to logic ground. It varies between a small negative voltage (~ -0.7V) to roughly 17 V with line frequency (50 Hz).

So all you need is a large (100k) resistor from the rectifier AC input node to the input of a schmitt trigger, and a capacitor (22nF) from there to ground to filter spikes. The output of the schmitt trigger is used to drive your counter.

The voltage swing of -0.7 to 17 V will not damage the circuit because the current is limited by the 100k resistor to about .1 mA which can be safely handled by the input protection diodes of the schmitt trigger (check the data sheet).

Don't make the mistake of omitting the schmitt trigger (as I have, many years ago on an identical circuit) and feeding the signal directly into the counter. Synchronous counters need quick level transitions to work properly.

robert

Any old fool can tap into that transformer to pulse a Schmitt trigger or comparator or what have you, but tapping into the timing in such a way that the resulting reference clock is not cumulatively "pulled" by the DC loading of the circuit or mains variation is an entirely different story. What is the periodicity of the mechanicals, and what is the logic loading and its long term cycling profile? What kind of timing accuracy are you looking for over a 24 hour period? The answer to your problem may be much deeper than you expect.

If the schmitt is squaring up 60 Hz cycles, how can DC offsets or "mains variations" affect clock accuracy? If you count each line cycle exactly once (not zero times, and not twice) where's the error?

The 60 Hz line is sort of the definition of clock time.

John

See....

Newsgroups: alt.binaries.schematics.electronic Subject: SED Question: Looking for two-input comparator with hysteresis - SED2InputComparatorQuestion.pdf Message-ID:

...Jim Thompson

I doubt that the national power grid will be, at any instant, much more than a second ahead or behind, and longterm it's perfect.

John

Thanks for the very useful advice. I'm looking for something simple which just works reliably, and this seems to fit the bill. If I've done my calculations right a potential divider consisting of a 100k ohm resistor and 22 nF capacitor gives a maximum voltage of around 10 V between the Schmitt trigger input and logic ground (for a 50 Hz signal). Are the 100 k ohm and 22 nF standard values, or did you and John calculate them specially? Should I connect another resistor between logic ground and the "bottom" node of the bridge rectifier?

Many thanks,

Chris

The clock hands move once per minute. I would consider a 5 minute error in 6 months to be acceptable accuracy, and from what I know the UK power grid should be capable of providing this.

Best wishes,

Chris

I've thought about this a bit more. If you connect the "top" end of the transformer winding to the Schmitt trigger input through a 100k ohm resistor, then connect the Schmitt trigger input to logic ground through a 22 nF capacitor, you form a potential divider which also acts as a low pass filter. Presumably you are suggesting connecting the logic ground to the "bottom" end of the transformer winding through a 10k ohm resistor in order to remove the unknown which is the resistance of the rectifier diode, and place an upper limit on this resistance? But I can't see why you suggest connecting the "top" end of the transformer winding to logic ground through a 10k ohm resistor. Perhaps you could explain? I'm missing something here.

Best wishes,

Chris

And the problem with a CMOS schmidt trigger is...?

My complicated suggestion was because of what was requested.

This might work. I'd worry about the turn off spike though. I'd prefer to add a couple of real diodes of the 1M400X type.

Not much, really, except for crappy predictability.

And, in the OP's case, some risk of sending a signal that's over-rail, making the predictability problem even tighter.

...Jim Thompson

Exactly one square output cycle per sine input cycle, forever, sounds fairly predictable to me.

Some day I'll explain ESD diodes to you, if you ask real nice.

John

It's not nuts. Who knows what their skills are. Given that they have to ask...

NO, I'm saying that an isolation must be used (think series). It wasn't clear that he was tapping off an isolated secondary. Even so an isolator is a good idea and may not add to the parts court at all (given that he was looking at comparitors and other schmitt triggers). Optical isolators are cheap insurance.

Yes. He was suggesting a schmitt trigger or comparitor doing the filtering. In that case the isolator is *not* an additional component. He's building *ONE*. Yes, I am worried about his safety over the cost of an optical isolator. (yikes! you really can't be that dumb)