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Instead of blinking LEDs like we did in
0:02
the previous project, this circuit
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senses temperature and makes a decision.
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Just like real electronics inside
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robots, computers, and machines, the 555
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temperature controlled fan controller,
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once finished, will be a circuit that
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cools when the temperature rises with
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the use of a 12vt DC fan and a
0:19
potentiometer. The DC fan turns on when
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the circuit gets warm and turns off when
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it cools down. The fan stays off when
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the circuit is cool until a certain
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threshold is reached when the circuit
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warms up. Then the fan turns on. The fan
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stays on when the circuit is warm until
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a certain threshold is reached when the
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circuit cools down. Then the fan turns
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off. A potentiometer lets you adjust the
0:43
temperature threshold. It uses a 555
0:46
timer as a Schmidt trigger comparator so
0:48
it doesn't chatter on and off around the
0:51
threshold. A MOSFET handles the fan
0:53
current safely. Instead of switching on
0:56
and off at the same point, a circuit
0:58
with hysteresus turns on at one level
1:01
and turns off at a different level. This
1:03
separation between switching points
1:05
makes the circuit stable and
1:07
predictable. For this project, we're
1:09
using a thermister as part of the sensor
1:12
node of the circuit. A thermister is a
1:15
special type of resistor whose
1:17
resistance changes with temperature.
1:19
We're going to use a 10K NTC thermister,
1:24
where NTC stands for negative
1:26
temperature coefficient, meaning the
1:28
resistance decreases as temperature
1:30
increases. A thermister by itself only
1:33
changes resistance, not voltage. To make
1:36
it useful, we place it in a voltage
1:38
divider with a regular resistor. As
1:41
temperature rises, the thermister's
1:43
resistance drops. This causes the
1:45
divider's output voltage to change. That
1:48
changing voltage represents the current
