Problem: Schmitt Trigger not giving ON/OFF output

[InterlinkKnight]

Hi. This should be easy question to answer, but I am a noob so... sorry, but I build a Non-symmetrical Schmitt Trigger using an op-amp UA741CP and 3 10K resistors.

Power supply is 5V. So, as far I found on the internet the output should be 0V when Vin is above the threshold, and the output should be 5V when Vin is below the other threshold.

Source: http://howtomechatronics.com/how-it-works/electrical-engineering/schmitt-trigger/

The threshold for turning ON is 1.6V and the threshold for turning OFF is 3.3V. That part is working fine. But the output is not really 0V for OFF, is actually 1.6V. And the output is not really 5V, is actually 3.3V.

This is what I did:

Why output is not ON/OFF (5V/0V)? I am using the wrong op-amp, or I did something wrong?

I really appreciate some help, because google didn't help me on this question.

 

[(*steve*)]

You have 2 problems.

The first is that the 741 is not really specced to operate from a power supply as low as you are using.

The second problem is that the 741 can't drive the outputs close to the power supply rails.

What you want to look for is an op-amp which has "rail to rail" outputs, and is specced to operate from a 5V single ended power supply. I'm not in a position to look one up for you right now.

 

[InterlinkKnight]

Thanks for the reply. I found strange the following website recommends that op-amp with those voltages: http://www.talkingelectronics.com/projects/OP-AMP/OP-AMP-2.html

I found on the internet this op-amp: LT1219L. Should that work?

The OFFSET N1 and OFFSET N2 on my 741 could fix anything? I don't know what those OFFSET pins do. Maybe I need to connect them to somewhere?

 

[Arouse1973]

Maybe @Colin Mitchell can answer that, it's his design. He is a member here.
Adam

 

[InterlinkKnight]

The datasheet of the 741 say should work fine with 5V:

But still, I don't know why some websites, like I mention, say output should be 0V when OFF and 5V when ON.

I think the problem might be related to the offset thing, that I don't know what really does.

 

[Alec_t]

I found on the internet this op-amp: LT1219L. Should that work?

Should be fine (albeit expensive).
As for offset-nulling on the 741, read the guidance in its datasheet.

But still, I don't know why some websites, like I mention, say output should be 0V when OFF and 5V when ON.

Some authors on some websites don't bother checking the datasheets .

 

[Arouse1973]

Look for the parameter "output voltage swing" what does it say?

 

[Arouse1973]

Have a look at Microchip, they do some opamps and they are much cheaper. Although I am a LT fan boy, their parts are expensive.

 

[InterlinkKnight]

I read the datasheet but I really don't understand the thing about offset. I am very noob.

The datasheet say this about voltage swing:

I am trying to get the easiest solution for a Non-symmetrical Schmitt Trigger. A microchip would complicate my life, I think. I can only do what internet tell me, in a very clear way, and all the websites say I should use an op-amp. That is why when a website tell me "do this, and you will get this result" and the result is not what they say, I get stuck (like now).

 

[Arouse1973]

No you don't understand. Microchip is the manufacturer, they sell more than just microcontrollers.

 

[InterlinkKnight]

Ah ok. I found on the microship website this ones: MCP6031, MCP6V11, MCP6441. Say is Rail-to-Rail, so should work?

Hope I can find them on my city since here is hard, sometimes, to get basic stuffs like resistors (true story).

 

[Colin Mitchell]

When we talk about HIGH and LOW output, we are actually saying "very close to rail voltage" and "very close to 0v."
In most cases, anything above 70% is HIGH as most stages will accept (read) this as a HIGH.

 

[InterlinkKnight]

I would not call 1.6V very close to 0V, nor 3.3V very close to 5V, but I get your point. Arduino so far has no problem with the current values, but still I need something else that do needs something closer to 0V.

I will try get the Rail-to-Rail op-amp, but if I don't find them I think I will try with a zener diode of 2V. That should get the 0V I need. The 3.3V for high is not what I was expecting but should be enough.

Thanks for the help everybody.

 

[(*steve*)]

Yeah, you'll find that the 741 has problems pulling the output within about 2 V of the supply rail. If you use a 5V rail, you get about a 1V swing on the output.

This is affected by load, and clearly you're getting a bit more than a 1V swing.

Rather than looking for an op amp with a rail-to-rail output and trying to source it locally, I suggest you determine what is available locally and see if any have the appropriate specs. (This works better when the local choice is limited)

 

[Bluejets]

Why not use a comparitor.
Some detail and comparison in the video here............. http://www.bristolwatch.com/ele/vc.htm

 

[InterlinkKnight]

I will explore the easiest options first, and if they don't work i will try the other options. So far I see the simplest is add a zener diode of 2V, and that is easy to find locally. That should get the 0V when LOW. The HIGH part is not that important to me that is 3.3V.

But thanks anyway.

 

[AnalogKid]

To answer some of your questions...

Post #3, the circuit is architecturally OK, but the 741 will not function as indicated under those circuit conditions The output voltage range is not close to 5 V overall with a single 5 V power supply.

Post #5, yes it says a 741 will operate with Vcc+ = +5 V. But it also says that Vcc- must be -5 V. Note that it doesn't say how well it will operate. That is in another section of the datasheet.

ak

 

[(*steve*)]

I'm surprised @Colin Mitchell hasn't noticed this thread.

 

[InterlinkKnight]

I'm surprised @Colin Mitchell hasn't noticed this thread.

He did reply.



I try with the zener diode to force a 0V LOW, but I found that is too slow. I am using this for high frequency, so the zener diode can't keep up.

Like I say, I am more interested on having a 0V LOW, than a 5V HIGH. Sadly I could not find a rail-to-rail op-amp so I am stuck with the op-amp I got.

So, I need to build something to do the job of a zener diode, but without a zener diode because is too slow. I thought maybe with a transistor, but so far I can't find something that could trigger the transistor at a fix voltage, and turn completely OFF when is lower than that voltage.

Any suggestions?

 

[(*steve*)]

yeah. Tell us who your suppliers are and we'll try to identify an appropriate op-amp.