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comparator using lm741?

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gnuga

New Member
Hi,

I've been assembling an opto-isolator sensor, and after noticing that the output voltage during the transition point is not digitally ON-OFF, I decided to add a comparator. After browsing, I've found a schematic using the LM741 in a single supply mode used as a comparator.

I hope to get a good result. However, the transition is not as I expected. I do have an ON-OFF state but the voltage swing is too thin. The high state is 4.2 V and low state is 2 V, which is totally USELESS to drive my ATMega8535. It still see it as a high state on both condition. I use 5V for the Vcc.


I tried to browse again and found out that LM741 is not recommended for comparator. Is there any mistake on my design? or should I just change the LM741.

The thing is that I already assemble them on the PCB. Is there any comparator IC which has similar pin configuration? at least the Vcc, Gnd, V+, V-, and Vout.

I admit it's an unnecessary mistake since I only look at the LED and do not measure the output voltage, when I tested it on my bread board. Damn.:mad:

Here's the schematic:
9034-optocouplercomparator.jpg
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi,

I've been assembling an opto-isolator sensor, and after noticing that the output voltage during the transition point is not digitally ON-OFF, I decided to add a comparator. After browsing, I've found a schematic using the LM741 in a single supply mode used as a comparator.

I hope to get a good result. However, the transition is not as I expected. I do have an ON-OFF state but the voltage swing is too thin. The high state is 4.2 V and low state is 2 V, which is totally USELESS to drive my ATMega8535. It still see it as a high state on both condition. I use 5V for the Vcc.


I tried to browse again and found out that LM741 is not recommended for comparator. Is there any mistake on my design? or should I just change the LM741.

The thing is that I already assemble them on the PCB. Is there any comparator IC which has similar pin configuration? at least the Vcc, Gnd, V+, V-, and Vout.

I admit it's an unnecessary mistake since I only look at the LED and do not measure the output voltage, when I tested it on my bread board. Damn.:mad:

Here's the schematic:


hi,
Its a common problem with the LM741.

You could use a direct 8 pin OPA replacement, like a CA3140, you need a OPA that has its output rated to switch close to 0V.
 
Last edited:

Mikebits

Well-Known Member
Maybe try removing D1 and adding pulldown resistor from op amp output to ground.
 

gnuga

New Member
hi,
Its a common problem with the LM741.

You could use a direct 8 pin OPA replacement, like a CA3140, you need a OPA that has its output rated to switch close to 0V.

Right, I'll try to find CA3140 here. I'm in Indonesia so I've limited options here. Thx Eric

Maybe try removing D1 and adding pulldown resistor from op amp output to ground.

How much do you think the R might be? is it a big one like 10K?
 

Mikebits

Well-Known Member
Not sure, you will have to take care not to screw up your high volt output, so maybe start with 4.7k and experiment with lower values, all the while monitor hi/low value. This is not an ideal fix but if your lucky it might work. Sort of a band-aid fix.
 

AllVol

New Member
A 741 used as a comparator does not need amplification as you have configured.
 
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