LM339 Help

Thread Starter

Razor Concepts

Joined Oct 7, 2008
214
Hi, I built this circuit here:

http://razorconcepts.net/photoresist.html


In breadboard form, it works fine. But I constructed it on perf board, exactly the same but instead of LED outputs the outputs are left hanging so that I can read it with the multimeter.

No matter what, the outputs are always low. I tried using a 4v supply and a 9v supply. I verified the voltages going in to the comparator as correct, but the voltage coming out of the output is always low. Any ideas?
 

Audioguru

Joined Dec 20, 2007
11,248
The outputs of an LM339 quad comparator and an LM393 dual comparator are the collector of a NPN transistor that pulls down but needs something to pull it up. The LED and its current-limiting resistor pulls it up. A resistor by itself (less than 6mA when low) will also pullup the output when it should be high.

These are low power comparators so their output low current is very low (only 6mA minimum).
 

Thread Starter

Razor Concepts

Joined Oct 7, 2008
214
Wow I completely forgot that it sinks current instead of sourcing it.

But I want to trigger a IRFZ44 MOSFET with this. I suppose I could just use a 3906 PNP transistor?
 

Audioguru

Joined Dec 20, 2007
11,248
The output of a comparator needs a pullup resistor as the collector resistor for its output transistor. This resistor can turn on an N-channel Mosfet.
 
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