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What the World
Needs Now -- A Large Outdoor Thermometer
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Previous Issues |
| I got
an email from a guy in Death Valley California. He worked at a general store where
the outside temperature often exceeds 120 degrees F. He wanted a large analog
thermometer display with some advertising nearby, so patrons could snap photos in front of
the display, showing these hellish temperatures.
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| He
didn’t have any money to spend on a custom thermometer so I really couldn’t help him much
but I thought it would have been an interesting project. I imagine a big custom
temperature dial display, maybe something like the one below. It would use a classic
“servo” mechanism, where a potentiometer would be connected to a gear motor with two
shafts. One side would move the pointer and the other would move the servo pot.
An accurate solid-state temperature sensor, such as the AD590, would produce a voltage,
corresponding to the temperature. That signal would be fed to an op Amp. The
other input of the op Amp, would come from the potentiometer slider. The output of
the op Amp would be fed to a buffer circuit, which would drive one side of the motor.
The other side of the motor would be attached to another buffer, fixed at the half supply
point. Using this approach, the motor can be moved in either direction from a single
supply. |
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Some servo pots have active elements,
which approach 360 degrees but many others only go to about 270 degrees. So,
the correct servo pot would be important. Also, some servo pots have a
convenient dual shaft, so they can more easily be coupled between the speed
reduction gear output and the pointer. Those with a single shaft would need to
connect to the gear shaft with a 1:1 ratio gear. |
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To calibrate
the system, a couple accurate voltages, corresponding to the low and high
temperature calibration points on the dial, would be used to set the zero and span
of the dial. Since less than a full 360 degree revolution would be needed by
the pointer, the gear ratio of the motor speed reduction gear could be fairly high.
Perhaps a 40:1 ratio would work. |
| This simple
servo type circuit would move the pointer of the temperature dial so the readout
pointer matched the actual air temperature. The finished temperature display
could be several feet in diameter. |
| Using a
pulley system, a vertical bar graph display might also be fabricated. The same
servo system could be used. Instead of moving a pointer, a red painted bar
would be moved up and down on the display. This method could form a
temperature display 6 feet tall or more. Boasting as having the largest
thermometer in the world, another Death Valley outpost uses a digital display as
shown below. Now that is a big display. |
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