Op amp as a Schmitt trigger

Question

I am working on a project which needs a Schmitt trigger. I designed it using IC UA741. I am using a single power supply of 9V. I am grounding the V-.

enter image description here

Here is my design.

The circuit works perfectly in this software as well as other software.

When I do the same in hardware it is not working. I was even getting 2V at V- pin i.e 4 pin. Even though I had grounded it. And the output voltage is always 5.67 V at pin 6.

Can Anyone help me and highlight my fault?

Thank you

Edit... This is my new circuit. Is there any visible erors here? I haven't tried it on hardware yet. enter image description here

Thank you

If you have grounded pin 4, but measure 2 volts there, it is not grounded - check your wiring. The schematic shows the LED the wrong way up. — Peter Bennett, Feb 04 '20 at 02:44
The voltage levels you are using will not work with a 741 (or even an LM358). You need to pay attention to details such as the common mode range of the opamp. — Kevin White, Feb 04 '20 at 03:13
Look down the list of reasons not to use a 741 and you will see at least one reason right at the top. — Andy aka, Feb 04 '20 at 08:11
I even saw when I connected +9V at V+Pin in op amp the output led was turned on .(The led was connected properly in normal circuit).The led was grounded but the V- was not .Is it normal for Op amp or some trouble.Thank you — Newton Nadar, Feb 04 '20 at 11:35
@Newton Nadar, I want to ask you a simple question, "Do you understand how this circuit works?" Because this is the necessary condition to start making and exploring it... — Circuit fantasist, Feb 04 '20 at 12:35

score 1 · Answer 1 · answered Feb 04 '20 at 04:13

Few thoughts. Check your connection to ground as peter Bennett mentioned.

Second Issue is that your op-amp cant run close to the ground rail as Kevin White mentioned. Look up common mode input range. Your input is running at 1.8 V where as you can only go down to about 3V on the inputs according to the data sheet assuming a +-15v supply it can go +- 12 (VICR "Common mode input range"). It will behave similarly at 9 Volts. This effectively leaves your operating area on the inputs between 3V and 6V on this op-amp. Where as you are running the op-amp at 1.8 volts on the input.

If this is the problem, the solution would be to find an op-amp that can go down to 1.8 volts and lower on the inputs or to change your dividers. Where 8k and 2k resistors changed to 5k on the Vin-. And the 10K going to 1k and 5k going to 9k on Vin+. This would let the operating voltage of the input be 4.5 with an output of 5V.

Thanks I didn't knew about common input range.I will try it in hardware then. — Newton Nadar, Feb 04 '20 at 12:03
yes it is an annoying factor to consider. I would recommend getting a different op-amp that runs closer to the rails as you are using 9 volts which is much different from the +- 15 volts this was designed for. — nootnootmachine, Feb 04 '20 at 21:25

Op amp as a Schmitt trigger

1 Answers1