Need help with my temp sensor circuit

grasshopper

I am designing this temp sensor circuit that amplifies 2-4V from an LM235AH. My current problem is that I had to make a voltage divider R3 – R4 to take the 2 – 4 volts “range of lm235AH” down to 1 – 2 volts before going into the amp.

My question; is there a better way to do this. I want to amplify the range of 0 – 5 volts but I can’t seem to get the 2 minimum from the lm235 volts down to 0 volts.

I would then run the voltage into a A/d and into the propeller for further control. Any help would be appreciated.

grasshopper

No one??? Can i post this some where else?

Bean

grasshopper said...
I am designing this temp sensor circuit that amplifies 2-4V from an LM235AH. My current problem is that I had to make a voltage divider R3 – R4 to take the 2 – 4 volts “range of lm235AH” down to 1 – 2 volts before going into the amp.

WHY do you need to reduce it to 1 - 2 volts ?

My question; is there a better way to do this. I want to amplify the range of 0 – 5 volts but I can’t seem to get the 2 minimum from the lm235 volts down to 0 volts.

WHY do you need to get the minimum from 2 down to zero volts ?

I would then run the voltage into a A/d and into the propeller for further control. Any help would be appreciated.


WHY do you not just run the voltage into the A/D ?

Bean.

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Sparks-R-Fun

I am not familiar with the sensor, but as a place to start you might try using 2K resistors for your voltage divider. That should result in 2V out at 1mA with a 4V input.

- Sparks

grasshopper

I wish to drop the voltage out of the LM235ah so that i can read the temperatures from -44c to 150c [noparse][[/noparse]2v - 4v] totaling close to 200 readings using only 5volts.

The idea, I thought, would be offsetting the voltage out of the lma23ah from [noparse][[/noparse]2 - 4] to [noparse][[/noparse]0 - 2]. Once the offset is accomplished with [noparse][[/noparse]R3 and R4 voltage divider] I could then run it through an op-amp that amplifies it from 0v - 5v thus resulting in a wider reading for the a/d converter [noparse][[/noparse]gaining more precise readings like 10.02c].

The a/d would then run into the propeller so that i can apply a PID closed loop program to control temperature with a heater.

It seems to work on paper but before i build the circuit i wanted more input from more experienced people.

After typing this i am wondering if there is a such thing as an offset amp circuit? time to google!!!!

RDL2004

I don't think you need to do what you are doing. Just run the 2 to 4 volts directly into an ADC0831. Set the ADC to span from 2 to 4.55 volts and that gives you 1 degree per count. If one degree is not good enough go to a 12 bit ADC.

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- Rick

grasshopper

Well i am considering using just an a/d but i don't think the resolution will be what i am looking for. A 12bit A/D would be a bit more expensive as well than say a 8bit A/D that i have amplified.

RDL2004

Why wouldn't the 1 degree resolution you would get from the ADC0831 not be good enough? The LM235 is only going to be accurate to about 1 degree in your circuit.

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- Rick