If you had read post #2 you would have seen that other members agree with me. 741 cannot read low voltages. <1v is a low voltage.

I read all of the posts. The 741 datasheet (TI version) clearly states that guaranteed input voltage range is 3V from either supply rail. That clearly means that when operated from a single supply, support for voltages less than 3V are not guaranteed.

You were told to use appropriate power supplies or use a different opamp. That doesn't make LM741 a bad opamp; you just have to know it's limitations and how to deal with them.

 

I've added the comparator with a pot of 100k
But I'm getting a constant -9v coming out of the lm331 changing the pot values nothing changes...

 

duh I inverted a polarity...that was silly
Now I'm getting a constant +4/5v from the first opamp, and a constant 8.45 from the second

 

You need a spike-suppression diode across the relay coil or you will kill the opamp.
What is the relay spec (particularly its coil resistance or current draw)?

 

duh I inverted a polarity...that was silly
Now I'm getting a constant +4/5v from the first opamp, and a constant 8.45 from the second

Are we talking about the schematic in #39? A mid-range output from the op-amp is a good thing, right? It's not pegged to one rail or the other. You should see it move as you place your finger in the LM35 to warm it.
What is the reference voltage on the pin of the comparator? You'll need to adjust that in order to get the comparator to trip at the temperature you want

 

I've been all day trying to apply the schmitt trigger but I just cannot understand it.
I've managed to apply a comparator though. Problem is that the relay clicketyclacks furiously when i heat up the lm35, must be because of the noise. I can't do any better than this.

 

I suspect that the relay is drawing enough current to drop the battery voltage, which in turn drops the reference voltage at the 2nd op-amp. That turns the relay off, the load comes off the battery, the voltage rises again, and the cycle repeats.

A genuine voltage reference for the 2nd-op-amp would help solve that. Are you familiar with the LM317, or do you have any zener diodes? Maybe try your circuit as-is with a power supply at 9V.

Or, add a little hysteresis with a 1MΩ or so resistor from the op-amp output back to the input. Look up non-inverting comparator with hysteresis for the configuration.

What is that diode doing? If you meant it as a snubber, it should be in parallel (not series) with the really coil, reverse biased (non conducting in normal operation).

And finally, are you sure that op-amp can drive that relay directly? You would normally use a MOSFET switch in between to take the load off the op-amp.

 

Your comparator is wired wrong, the input goes to the inverting pin, the hysteresis resistor goes to the non-inverting pin...like i showed you earlier in post #30,,This will make the output high until the temperature is met, then the output will go low.

 

For the devices used, the design is near optimal.

If you could afford to use a PNP or p CH to drive the relay, you can reconfigure the comparator to eliminate the npn.

 

For the devices used, the design is near optimal.

We don't know the design criteria; temperature range (which becomes limited by the first op-amp), required accuracy or precision, power usage, cost, teacher's pet ideas, and so on. We're in no position to judge whether it's optimal. I agree that it will, at least, work.

 

What about LM358?

 

The simplest would be what I suggested. Earlier - introducing s common mode signal. That can be done via diodes, LEDs, vot references, regulators, etc.