Bump ... is my 741 damaged possibly? RS only had 1 in stock.

 

It's more likely that there's a missing/poor connection somewhere. Remove and re-insert all the wires on the breadboard.

 

Your plot looks ok. If a thermistor had a perfect first order exponential response, the line would be straight. But they don't, so it isn't. It would be nice to know the internal source resistor value as this would tell us how much current your circuit needs to overcome, but I think 2.5K is a safe starting point.

ak

 

I will check connections and try again. Is there a way to measure or calculate resistance by measuring across the 2 pins in the diagram possibly?

 

Yes, the classic output impedance measurement technique.

Measure the output voltage open circuit.
Connect a variable resistor.
Adjust the resistor until the output voltage drops by 50%.
Remove the resistor.
Measure the resistor.

ak

 

I was hoping creating a resistor ladder would do it, but I wasn't sure.

So I was able to halve the voltage for sense2 (5.12vDc) with a 2700Ω resistor and I was able to have the voltage for sense1 (4.80vDc) with a 2930Ω resistor.

Strangely enough, this 741 is really giving me trouble. I've checked all of the connections and breadboard points and I'm just not sure what is going on. At pin 6, I am seeing 1.74vDc when it is connected to pin 2 and ~Vcc when pin 6 is not connected to pin 2 and no voltage on pin 2 when it is not connected to pin 6. Should I throw this op amp in the trash, or is this somehow something I have done? I tried a pull down on pin 6, but that didn't seem to help?

 

Ok, I've replaced the op amp, same behavior, I've replaced every wire on the bread board and power supply wiring and moved the IC to a different location on the bread board, as well as jiggled every connection while watching my DMM. I have built much more complicated circuits than this, so I'm pretty confused. Maybe there is some sort of limitation where pin 6 (output) will only be as low as a % of Vcc? I've even tried a 10k pull down resistor with no affect? I had hoped to keep pin 6 at 0vDc with a pull down until V+ was supplied with some voltage. The only reaction I can get from the circuit at all is to rail V+ and then I will see Vout go almost to rail. If I put that 10k resistor in between pins 2 and 6, it sits at 1.74vDc just as if they were hard wired together. Vcc is 5.15vDc.

What am I missing?

 

Ok, the op amp Vout must not go below ~1.74 vDc for output pin 6. I've tried a few different voltages above that and the voltage follower will match V+ within a few hundredths of a volt. Is this maybe because a component inside of the LM741 is a 3.3v component?

Wow, that was a hard lesson to learn. Can anyone recommend a voltage follower op amp that will go down to 0.1 vDc? I know that the NE5534 was recommended earlier in this thread, so I will check into that op amp. Any other recommendations for such a low voltage?

I thought about trying to scale the output of the LM741, but I don't think that would work since I need it to match V+? Maybe I could scale it down and then use some sort of gain to bring it back up?

 

Wow, that was a hard lesson to learn

Yup. Now you know why you see so many 'I wouldn't use a 741 if I were you' posts in the forums

 

So what recommended op amp will fit my requirements?

 

I know that the NE5534 was recommended earlier in this thread

Did you check that out?

 

No, I'd have to mail order it and I'd rather order a few suggestions together so I can get this sorted quick

 

Check the opamps listed by your intended vendor. If you can, do a parametric search looking for those needing a single supply voltage which can be as low as 5V, with rail-to-rail input and output. The sensor draws only ~1mA so that shouldn't be a limitation on choice.

 

The radioshack website says they sell a LM324 and the inputs on that can can go from 0V to 1.7V less than the supply voltage.

 

Hi, thanks but that won't meet my needs as I need it to go to Vcc - Vcc or rail to rail, Vcc could be 5, 10 or 12v, but no less than 5v.

The radioshack website says they sell a LM324 and the inputs on that can can go from 0V to 1.7V less than the supply voltage.

 

What do you think of this? This looked like a winner until I started reading the datasheet and lines like the one below start to confuse me.

http://www.adafruit.com/products/808

Vicm Common mode input voltage range VCC- -0.2 to VCC+ +0.2 V

Check the opamps listed by your intended vendor. If you can, do a parametric search looking for those needing a single supply voltage which can be as low as 5V, with rail-to-rail input and output. The sensor draws only ~1mA so that shouldn't be a limitation on choice.

 

.... but the '324 output isn't rail-to-rail, so can't get much below ~1V.

Edit:

What do you think of this?

Looks fine. The Vicm figures mean the input (inverting or non-inverting) can go right to ground or V+ (in fact 0.2V below ground or above V+). Since you will be using this as a unity-gain voltage-follower the non-inverting input will always track the inverting input, so the difference between the two inputs is always less than the maximum allowed 1V (as per the datasheet). You will need to ground the inv and non-inv inputs of the unused second opamp in the package.

 

Ok, thanks for explaining that.

I just ordered a few, can't wait to try them when they arrive ~ Wednesday. I will post results then.

.... but the '324 output isn't rail-to-rail, so can't get much below ~1V.

Edit:
Looks fine. The Vicm figures mean the input (inverting or non-inverting) can go right to ground or V+ (in fact 0.2V below ground or above V+). Since you will be using this as a unity-gain voltage-follower the non-inverting input will always track the inverting input, so the difference between the two inputs is always less than the maximum allowed 1V (as per the datasheet). You will need to ground the inv and non-inv inputs of the unused second opamp in the package.

 

Well, total fail tonight. The 5534 must not be rail to rail and the TS922 came in a package other than DIP8, it's so small I couldn't even solder legs to it lol.

I reordered the TS922 DIP8 with overnight shipping, hoping to get it on Friday? I'll give it one more shot and if it doesn't work with the DIP8 TS922, I'll just fall on my sword and chalk this up as a Unicorn.

Thank you to everyone that has helped me in this thread.

 

Total success tonight, it tracks identically and is great. Thanks again to everyone who helped me sort through this.