Temperature Sensor

VEX V5 General Discussion Technical Discussion
1

Hello,

I am trying to design a temperature sensor using a LM34.

(http://www.parallax.com/Store/Components/AllIntegratedCircuits/tabid/154/CategoryID/49/List/0/Level/a/ProductID/87/Default.aspx?SortField=ProductName%2CProductName)

The data sheet has examples of circuits, but I am wondering which would work best with the vex microcontroller. I don’t have much experience designing circuits and would really appreciate any help from more experienced builders.

Thanks,

-Brando:)

2

Hmm, I think I’d start by just hooking it directly to an analog input and see what the resulting values look like. At 10mv/deg that’ll give you a 1V reading (about 205) at 100 degrees (F) and should give you a resolution of about half a degree.

I don’t know how the series and pullup resistors that are built into the VEX Micro are going to effect the reading, though. Hopefully not too much.

One improvement may be to add an op-amp on the output of the temperature sensor to drive the vex analog pin. You could set it for a gain of 2 or 3 so that you’d be using more of the 0-5V range of the analog ports. That would theoretically give you better resolution (about a quarter of a degree) and might help overcome any problems caused by the built in resistors.

Let us know how it goes!

  • Dean
3

Dean,

I will try them out… I just ordered a few of the LM34’s. Any suggestions for a suitable op-amp? What criteria should I use when selecting one?

Thanks again,

-Brando

4

I wouldn’t worry too much about an op-amp until you know if you need one. Since you aren’t concerned about bandwidth in this case, I don’t think it matters too much what op-amp you pick. Just a vanilla TL082 or LM324 ought to do just fine.

Google around for a reference on op-amps (such as this one). If you build the “Noninverting Amplifier” circuit on that page, using 1K for RA and RB would give you a gain of 2 (an input voltage of 1V gives on output voltage of 2V). This should give you temperature range of about 0’F to 250’F, though I think I’d only trust reading that are not at the extreme ends of the scale (so, a useful range of around 20’F to 230’F)

Since the LM34 will never output a negative voltage, you can run the op-amp off a single-ended supply (which means the negative supply voltage and gnd are wired together; and the positive supply voltage is +5VDC supplied by the VEX Microcontroller)

I’ll end this post by saying that I am NOT an op-amp or analog electronics expert. I’ve pick up quite a bit over the years, but I have no formal training in this area. If anybody spots errors, please correct (& educate) me.

Thanks,

  • Dean