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# Schmitt trigger

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#### Mathieu3545

##### New Member
May anyone assist me in designing a schmitt trigger for an input that is between 0 to 1V or do i need to amplify itbefore it comes to the schmitt trigger in order to be able to set a proper window for the comparator?

You don't need to amplify it. You will need to decide at what input voltage the output will switch high and low though, then you'll find it easier to answer your question.

Hi Mathieu,

may be that circuit fits your needs. It can take any frequency from 1Hz to 1MHz without losses. At 10MHz the output voltage decreases.

The scope shows 10KHz at 1V input. (yellow=input, blue=output) The output swings to almost VCC (5V).

I'm using a modified circuit for zero crossing detection in an AC motor control circuit.

Boncuk

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You will need to decide at what input voltage the output will switch high and low though, then you'll find it easier to answer your question.

OK, I want the the output to switch high at an input of 100mV and low at 50mV.

Any suggestions?

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Hi Mathieu,

may be that circuit fits your needs. It can take any frequency from 1Hz to 1MHz without losses. At 10MHz the output voltage decreases.

The scope shows 10KHz at 1V input. (yellow=input, blue=output) The output swings to almost VCC (5V).

I'm using a modified circuit for zero crossing detection in an AC motor control circuit.

Boncuk

The thing is I'm doing a temperature controller project I'm using a schmitt trigger instead of comparator.
I'm sensing the temperature with a LM35 that gives 10 mV/degree celcius and now what i'm trying to design is a comparator with a tolerance of 1 degree C and by using milivolts I'm failing to get it right because milivolts are just too sensitive I sometimes end up with Bigger comparator window

OK, I want the the output to switch high at an input of 100mV and low at 50mV.

Any suggestions?

Why the big grin? I don't get it..

As for a schmitt meeting your spec, see attached cct.

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You don't need to amplify it. You will need to decide at what input voltage the output will switch high and low though, then you'll find it easier to answer your question.

The main concept of the project is: I am doing a temperature controller which i'm using LM35 as a sensing device and from the output of LM35 I fed it straight into LM358 comparator (with Vcc=12V) than what i'm failing to master is the design of a schmitt trigger

Like if I set it at tempeture of 20 degree C (200mV) that it must switch on the fan at 21 degree C (210mV) or switch on a heater at 19 degree C.

Mathieu (Mshasho3545)

Why the big grin? I don't get it..

As for a schmitt meeting your spec, see attached cct.

Sorry to say, but it doesn meet the specifications. They are: H at 100mV and L at 50mV input voltage.

It switches low as soon as the input voltage drops below the upper threshold voltage (100mV) while the specification says L at 50mV input voltage

Again: (double)

May be a window comparator would do the trick.

Boncuk

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Boncuk said:
Sorry to say, but it doesn meet the specifications. They are: H at 100mV and L at 50mV input voltage.

It switches low as soon as the input voltage drops below the upper threshold voltage (100mV) while the specification says L at 50mV input voltage
How did you work that out? I've attached the simulation; it's pretty clear that it switches low with 50mV input, and high with 100mV input. I fail to see where the confusion is (be it mine or yours).

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How did you work that out? I've attached the simulation; it's pretty clear that it switches low with 50mV input, and high with 100mV input. I fail to see where the confusion is (be it mine or yours).

You still didn't get the point.

That design switches H if the input voltage is higher than 100mV. It switches L if the input drops below 100mV. I want it to switch low when the input voltage has dropped to 50mV. (not some value between 50 and 100, but exactly 50 and 100, which I would call an estimating circuit).

If you get away from digital Schmitt-Triggers you'll see the difference.

The specifications I gave you should be understood clearly.

Boncuk

You still didn't get the point.

That design switches H if the input voltage is higher than 100mV. It switches L if the input drops below 100mV. I want it to switch low when the input voltage has dropped to 50mV. (not some value between 50 and 100, but exactly 50 and 100, which I would call an estimating circuit).

If you get away from digital Schmitt-Triggers you'll see the difference.

The specifications I gave you should be understood clearly.

Boncuk

This is reminiscent of when you told me (implicitly) your ignorance of how a 555 timer IC works. I take it you're not aware of positive feedback, or how to read the graph posted previously.

This is reminiscent of when you told me (implicitly) your ignorance of how a 555 timer IC works. I take it you're not aware of positive feedback, or how to read the graph posted previously.

The posted graph can't possibly be the one gained from your circuit which is a comparator.

The graph is from exactly the circuit I posted.

You would do well to educate yourself. Here's a link to a site that may be able to explain it for you.

The graph is from exactly the circuit I posted.

You would do well to educate yourself. Here's a link to a site that may be able to explain it for you.

Ary you seriously telling me to educate myself? I consider that post an insult!

There is absolutely no reason for you to be arrogant since your positive feedback doesn't seem to work properly.

My simulation shows exactly what I posted. The output goes high if 100mV input voltage are exceeded. It goes low if the input voltage drops below 100mV.

Otherwise please tell the engineers at Labcenter/UK to get the obviously better education in Australia!

Moreover the OP doesn't intend to use AC at the input, but variable DC. Did you consider that?

One more insult will make mad enough to report to the site owner.

Have I made myself clear enough?

The graph is from exactly the circuit I posted.

You would do well to educate yourself. Here's a link to a site that may be able to explain it for you.

hi dougy,
Ran a LTspice on your OPA comparator, I get 50mV and 100mV as the switching points.

The 10M and 1M are used to make the plot easier to read, but have minute effect on the thresholds.

Regards

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I consider that post an insult!
Rich, coming from someone who sent me a PM suggesting that I 'read the f*ing question'.

There is absolutely no reason for you to be arrogant since your positive feedback doesn't seem to work properly.
I am far from arrogant. That aside, the positive feedback works fine for me.

Moreover the OP doesn't intend to use AC at the input, but variable DC. Did you consider that?

One more insult will make mad enough to report to the site owner.

Have I made myself clear enough?
You started it.

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