Why does my Schmitt Trigger (Op Amp) NOT work as intended?

Thread Starter

777funk

Joined Apr 13, 2010
12
I wired this circuit up (10k for both R1 and R2) and where I'd like to see a 0V and a Vcc (approx 5V in this case), for some reason, I'm getting a 1.5V for the LOW.

3-s2.0-B9780750645829500111-f10-01-9780750645829.gif

What can I do to fix this?

I'm using a schmitt to buffer a push button input to a D-flip flop (74LS74). I have it latching and unlatching just fine, but noise is causing enough of a headache that I can see a schmitt is in order. This is the first time I've experimented with them.
 

Thread Starter

777funk

Joined Apr 13, 2010
12
I just realized... I didn't say what I was feeding the op amp. I'm feeding it with a Vcc and a GND signal to the in. I'd like to see a Vcc and GND out. I'm getting close to VCC out but it doesn't drop to a GND (LOW) output. It's VCC (approx) or 1.5V at the output. This isn't going to work for the next stage in the circuit (the D flip flop latch circuit).
 

Thread Starter

777funk

Joined Apr 13, 2010
12
Ahhh... I see and thanks! I just happened to have an RC4558 chip. I also had a 741. I saw somewhere that a 4558 is similar to a 741, but dual and without the offset null options. The 741 didn't work either and I had seen it suggested somewhere.

So rail to rail... does this mean I will have to use a negative voltage for V- or can I tie it to zero volts?

Forgive the ignorance.
 

bignobody

Joined Jan 21, 2020
63
So rail to rail... does this mean I will have to use a negative voltage for V- or can I tie it to zero volts?

Forgive the ignorance.
I'm also very new to this, so take this with a grain of salt. Most Op-Amps want you to use a dual rail, so yes, a negative voltage.

That being said, you can "cheat" it with a single rail - search for "create a virtual ground" to get the idea. The drawback with using a virtual ground is that it dramatically reduces the amount the signal can "swing".
 

MisterBill2

Joined Jan 23, 2018
6,025
No, you usually do not need a negative supply for an opamp. Many kinds are listed as "single supply" types. But for this application a comparator would be a better choice.
And the expression "rail to rail? means that the output can rise to the supply voltage and drop to the supply common voltage. Which usually, they just come fairly close.
 

Thread Starter

777funk

Joined Apr 13, 2010
12
Audio, in the first post I mentioned 0v and 5v for Vcc.

And yes, it is an antique opamp (introduced in the 70s maybe)... I just happened to have a bunch of them for an audio circuit design. They're still used today for certain (lofi) sounds. I was familiar with the chip enough to know it doesn't need a negative supply for the - pin, but I wasn't sure if maybe that's why I'm not getting a true 0V output. Also, the mention of a wide rail op amp... I was thinking this meant a bi polar supply.

I'm very unfamiliar with using an opamp for digital logic (never have done so before... only audio use). So this is new territory. Sounds like a dedicated schmitt trigger chip is a better choice? I was trying to use what I had here (unsuccessfully at that!! but knowing little about op amp capability, it's all new to me).
 
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