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What the World Needs Now -- A Large Outdoor Thermometer

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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.  
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.

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.

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.

May 2010     Issue 9

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What the World
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