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LM339 quad comparator and 1458

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bigt78130

New Member
Hey Everyone,
Im new to electronics and i have a project that i am working on using a quad voltage comparator and a 1458 dual op amp IC chip.

Well im basically trying to get a photocell circuit (lm339) to turn on and off the 1458 circuit to make LED's fade on and off.

Both circuits work perfectly when working by themselves but when i put them together taking the cathode and anode of the with positive and negative of the 1458 the LED blinks rapidly and im trying to figure out why it wont work.

Sry if i confused anybody:confused:

Photo Detector Circuits

and "Fading red eyes"

Led circuits page
 

audioguru

Well-Known Member
Most Helpful Member
The comparator IC has an output current that is too low to drive the LED fading circuit.
If the comparator circuit is re-designed to use a transistor at its output then the transistor can turn on and off the LED fading circuit.
 

bigt78130

New Member
Ok one problem with the transistor. so please correct me if im wrong but i have a 1k resistor on the output of the LM339 going to the base of the transistor. from there i have the emitter to ground and the collector to the 1458 project. what do i need to do now b/c the project dosnt work at all
 

audioguru

Well-Known Member
Most Helpful Member
i have a 1k resistor on the output of the LM339 going to the base of the transistor.
What is the part number of the transistor?
The output of the comparator goes low. The resistor will turn off the transistor. But you need something to turn on the transistor.

from there i have the emitter to ground
Then the transistor is probably an NPN type. If something goes positive into the 1k resistor to turn on the transistor then its collector will go to ground.

The collector to the 1458 project.
Where is the collector connected on the LED fading circuit?

What do i need to do now b/c the project dosnt work at all
You need to sketch and post EXACTLY how the transistor is connected to the two circuits so that we can show you what is wrong.
 

bigt78130

New Member
This is the multisim outline. The dual op amp is wrong on the schematic but i know its suppose to be the 1458. Idk if this helped you or not :( this is a 2N3904 transistor
 

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audioguru

Well-Known Member
Most Helpful Member
You have the new transistor not being turned on by anything. The output of the comparator goes low to turn off the transistor but does not go high and a resistor is needed to the positive supply to turn on the transistor.
I reversed the inputs of the comparator so that the fader works when it is dark.

You completely messed up the fader circuit and its transistor.

An LM393 is a dual comparator in an 8-pins case and an LM339 is a quad comparator in a 14 pins case.
 

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