I am working on a on going project and need to know if this will work as I intend it to. It's based loosely on a Bill Marsden circuit, just changed to suit my project.

The A and B are from the rest of the circuit. I want the led to light when either of the voltages feeding the LM339 go below a set point. What I don't know is can two circuit branches control one led?

As always any help is appreciated, thank you, cary

 

Yep that'll work.

 

Open collector is a common way to create a gate. In this case it is a type of NAND, since both outputs have to be high to turn off the LED. It is used a lot by TTL.

I agree, looks good.

 

The circuit will work backwards to what you want.
The LED lights when an input goes above the reference voltage, not below.

 

If the input (which is negative) goes above the positive input then the output of the comparitor goes low. A low on the base of the transistor will turn on the LED.

AG is right. Just reverse the inputs, easy fix.

 

Thanks Everybody!!

Yeah after I posted I saw my 339 inputs were back wards. Not thinking when I drew schematic.

I want to thank you all on this forum for all the help and sharing of knowledge that you do!

 

Nice schematic.

 

Have you looked at LTSpice? It is free and it will even let you simulate your circuits.

http://www.linear.com/designtools/software/