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October Aero'Lectric Temp Sensor Project
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 |
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October Aero'Lectric Temp Sensor Project
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 |
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