Hello

I built this comparator using LM339 but it didn't work

When I add a resistor (R6 = 1K) between VCC and pin #2 (the output pin of the first comparator), it works good

My question is: What is the role played by that resistance?

thanks

 

The LM339 comparators have an "open collector" output. They require a pull up resistor (R6). Google "open collector" for more info.

 

Of course the LM339 data sheet will also show that the output is open collector (but who looks at a data sheet? ).

 

This comparator with an open collector. Additional resistor gives current sourcing. The scheme can be simplified. Make R6 = 10kOhm. Remove the resistor R4. The base of the transistor connect to the output of the comparator.

 

Of course the LM339 data sheet will also show that the output is open collector (but who looks at a data sheet? ).

At least he supplied a schematic, which made the answer immediately obvious.

 

The LM339 comparators have an "open collector" output. They require a pull up resistor (R6). Google "open collector" for more info.

Of course the LM339 data sheet will also show that the output is open collector (but who looks at a data sheet? ).

This comparator with an open collector. Additional resistor gives current sourcing. The scheme can be simplified. Make R6 = 10kOhm. Remove the resistor R4. The base of the transistor connect to the output of the comparator.
View attachment 91900

Thank you very much