I have a LM311P which I want to use as a comparator in a different project. To test the working of the comparator I created a simple circuit as shown in the diagram below:

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As you can see in the diagram, I have used 5V on the non inverting terminal as a reference voltage which is fixed. The 9V source is connected through a potentiometer which allows me to control the voltage between about 1V and 9V. The VCC of the LM311 is connected to a 12V supply (I also used a 5V supply for this but that one didn't work either). Pin 1 and 4 are connected to the ground and the pin 7 (output) is connected to an indicator LED.

The problem I am having is there is no output. I varied the potentiometer all the way from both extremes but still the LED does not glow. I checked the LED to see if it is broken and it works fine when directly connected to a power source. I even changed the LM311 chip itself on suspicion of physical damage, but still the same result.

Why is this happening and how can I fix it?


1 Answer 1


Pin 7 is an open-collector output. It can sink current, but it cannot source current.

Reverse the LED and exchange the inputs.

  • 1
    \$\begingroup\$ And power the LED from +12, rather than return it to GND \$\endgroup\$ Commented Aug 8, 2023 at 12:22

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