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Circuits designed by David Johnson, P.E.
Last Updated on:
Thursday, March 22, 2012 06:29 AM
Master Category List
- Dave's Circuits
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Copyright © David A. Johnson, PE - ALL Rights Reserved.
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Temperature Sensors / Detectors Circuits |
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Freezing Temperature Alarm
This circuit turns on a beeper whenever the outside air temperature is less
than zero degrees Centigrade. The circuit draws only a few microamps
from a 9 volt battery. It uses a glass bead thermistor accurate to 1
degree C. (added 12/04)
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Freezer Over-Temperature Alarm
This circuit turns on a beeper whenever the inside temperature of a freezer is
greater then zero degrees Centigrade. The circuit draws only a few
microamps from a 9 volt battery. It uses a glass bead thermistor
accurate to 1 degree C. (added 12/04)
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- MICRO POWER OVER-TEMPERATURE
ALARM
The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm
when the temperature exceeds a certain point. With some minor changes the circuit could
also be configured for an under temperature (freeze) alarm. The circuit uses a cheap but
accurate thermistor as the temperature sensor.
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- TEMPERATURE SENSOR WITH 4 TO
20mA CURRENT LOOP
I designed a circuit similar to this one years ago to accurately measure the air
temperature inside a building 1000s of feet from a control room. The circuit uses
the very robust current loop method. It uses a highly accurate semiconductor
temperature sensor and an equally accurate voltage reference. The circuit includes a
diode bridge, so it is polarity independent. By using the component values
indicated, the circuit should not require calibration. It has a range from –40F to
+120F and an accuracy of plus or minus one degree F.
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