op-amp as a comparator

I think that 741 does not go to the negative supply on the output. You want something like an LM358.

Luhan

No you are not.....

You were sitting at home having not figured out how to download p*rn from the interwank and then you got seriously frustrated and thought you'd sign up to Google and try and have a wank over some sort of disgusting.....

Let's start from the beginning....

sc

The 741 only swings to within about 2 volts of V+ and about 2 volts of V-. Are you using it on a single 5-volt supply? It's not supposed to work at all with so little voltage.

You want a low-voltage rail-to-rail op-amp.

The 741 is not suited for this job. The old time opamps were designed to run off of bipolar supplies so most of the input and output range was centered about the mean of the supply voltages, which is 0V in the case of +/-Vsupply. When you go single supply on one of these, those ranges are now centered about Vsupply/2 or 2.5V in your case. Allow approximately 1.5V clearance headroom from the supply rails for proper operation and you're left with really no more than about 1.5V to 3.5V operating range. This will not work very well for logic circuits.

How about an LM339. Just about as old as the 741, it'll work on 5V, and it's actually made to be a comparator.

No, depending upon the level of input and such, an op amp may make a crappy comparator. He wants a 5v rail to rail comparator.

But why ?

Graham

Hi,

use a lm311 instead. I'm sure you will like it.

Marte

Or just use a PIC with an internal comparator (eg. 12F629). An LM339 has better specs though.

Best regards, Spehro Pefhany

yes, i like the LM311 also, it has enough drive output to operate an average PCB relay with plutty of gain. lowers the parts count greatly.

I know that of course.

Why a 741 ? Hardly a great choice.

Graham

With 5 volt supply, an LM324 will give you about 0 to 3.5 - is that good enough? Ed

One of Robert Widlar's fine designs. Rather expensive when it first came out, but now pretty cheap. NSC has a 1970 appnote he wrote on the part, AN-41.

One caution, although it can run on 5V, and has PNP input transistors, like the LM339, LM2901, LM3302, LM393, LM2903, etc., it can't work all the way down to zero volts. But, unlike those parts, it can deal with a +/-5 to 15V split supply with a grounded output load switch. It can also do high-side load switching. Pretty impressive for 1969.

In article , Winfield Hill wrote: [... LM311 ...]

They are a bit slower when the output section's emitter is used to drive the load.

Others may have suggested similar but why make life so difficult anyway? The pic10F's have a comparator ad fixed Vref on board to do what you want for cheaper than just the 509. The 629 & 675 do lots more (variable referencing etc) cheaper than a 509 with a 741.