October Aero'Lectric Temp Sensor Project

[Paul Dow (Remove Caps in mail address)]

We're always looking to do some educational subject at our EAA chapter
meetings, so I think Jim's project in the October Kitplanes will be a
useful item to build. It looks like a small project that can be put
together in a couple of hours (although with our group, I better
allocate some overtime.)

I ordered a bunch of parts to make them, and today I got a volunteer
from the chapter to see if I could translate the schematic to a board,
and if they could follow my instructions. I used 1/2 of a Radio Shack
276-159 project board to assemble it on. It's the perfect size for the
circuit.

The good news...I didn't let the smoke out of the parts. The bad
news...I'm not getting close to the voltage output I'm supposed to.

With having the adjustment pots near the center, I get about 4.8 volts
in ice, 6.3 volts in boiling water. (The diode, not the whole circuit.)
That's about a 1.5 volt range which is close to the 1.8 volt range
desired. Therefore, it looks like the offset isn't right.

I did build another one on my own without looking at the first one, and
it puts out similar voltages.

I do get 5 volts at the first stage of the opamp, so I believe I'm OK at
that point. I get 1.2 volts (hot) .78 volts (cold) at the input to the
2nd stage.

I know it's tough to see my solder blobs to see where I screwed up, but
I was wondering if there's any other hints, or if someone could let me
know what voltages I should see at various points in the circuit.

Thanks,
Paul

[RST Engineering]

The left end of R8 connects to the output of U1A, not to the 9-12 volt
battery line. One comment from another reader noted that the adjustment of
R6 was pretty close to one end of the control. I did this a-purpose to get
maximum ease of adjust of R6, but if you want the adjustment closer to the
middle, make R7 a 22k.

I make 1 to 2 mistakes a year (no matter how closely I proofread before
sending) and I'm sorry you were the beneficiary of this year's oopsie.

Jim



"RST Engineering" > wrote in message
...
> There is a small but important error in the schematic that somebody
> pointed out to me that hasn't hit print yet. Let me look it up and I'll
> post it here.
>
> Jim
>
>
>
> "Paul Dow (Remove Caps in mail address)" > wrote in
> message ...
>> We're always looking to do some educational subject at our EAA chapter
>> meetings, so I think Jim's project in the October Kitplanes
>
>

[Paul Dow (Remove Caps in mail address)]

I downloaded a SPICE-type demo off the Texas Instruments web site.
Program is called TINA. I entered the circuit, and got output voltages
similar to what the physical device I made delivered. The demo version
only allows 2 active elements, so I removed the two voltage follower
opamp segments U1B and U1D.

I'm not an electronic engineer, and I don't even play one on TV, but I
can figure out the V=IR stuff. The basic circuit idea makes sense to me,
so I went in and did some trial and error changes on some of the
resistor values. I changed:
R8 from 180K to 270K
R7 from 18K to 10K
R4 from 560K to 510K
and R2 from 4.7K to 180K

The program shows some non-linearity from about 32 to 45 degrees, but
the output is close to the desired range now.

I'm sure I haven't chosen the optimal values for the resistors, but it
seems to work for me.

Paul

Paul Dow (Remove Caps in mail address) wrote:
> We're always looking to do some educational subject at our EAA chapter
> meetings, so I think Jim's project in the October Kitplanes will be a
> useful item to build. It looks like a small project that can be put
> together in a couple of hours (although with our group, I better
> allocate some overtime.)
>
> I ordered a bunch of parts to make them, and today I got a volunteer
> from the chapter to see if I could translate the schematic to a board,
> and if they could follow my instructions. I used 1/2 of a Radio Shack
> 276-159 project board to assemble it on. It's the perfect size for the
> circuit.
>
> The good news...I didn't let the smoke out of the parts. The bad
> news...I'm not getting close to the voltage output I'm supposed to.
>
> With having the adjustment pots near the center, I get about 4.8 volts
> in ice, 6.3 volts in boiling water. (The diode, not the whole circuit.)
> That's about a 1.5 volt range which is close to the 1.8 volt range
> desired. Therefore, it looks like the offset isn't right.
>
> I did build another one on my own without looking at the first one, and
> it puts out similar voltages.
>
> I do get 5 volts at the first stage of the opamp, so I believe I'm OK at
> that point. I get 1.2 volts (hot) .78 volts (cold) at the input to the
> 2nd stage.
>
> I know it's tough to see my solder blobs to see where I screwed up, but
> I was wondering if there's any other hints, or if someone could let me
> know what voltages I should see at various points in the circuit.
>
> Thanks,
> Paul