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Freezer
/ Frost Alarms
Last Updated on:
Friday, October 16, 2009 03:42 AM
| Circuits Designed by Dave Johnson,
P.E. : |
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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.
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Freezer Over-Temperature Alarm
This electronic hobby circuit below is designed to activate a beeper alarm,
whenever the outside air temperature is below 0 degrees C (32F). A 9v battery
powers the hobby circuit. The average 9ua current is so low that the battery
should last for many years....
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Links to electronic circuits, electronic
schematics, designs for engineers, hobbyists, students & inventors:
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Cold Activated Switch - The thermistor used has a resistance of 15k at 25 degrees and
45k at 0 degrees celsius. A suitable bead type thermistor is found in the Maplin
catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures.
A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents
relay chatter when temperature is near the switching threshold of this circuit. (added
2/06) |
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Freezer Over-Temperature Alarm - This electronic hobby circuit below is designed to
activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).
A 9v battery powers the hobby circuit. The average 9ua current is so low that the
battery should last for many years.... ….(designed by David A. Johnson) |
| 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. ….(designed by David A. Johnson) |
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Fridge door Alarm - Beeps if you leave open the door over 20 seconds. 3V battery
operation, simple circuitry. (circuit added 9/04) |
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Frost Alarm (Andy Collinson) - The thermistor used has a resistance of 15k at 25
degrees and 45k at 0 degrees celsius. A suitable bead type thermistor is found in the
Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of
temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor.
This prevents relay chatter when temperature is near the switching threshold of this
circuit. (added 2/06) |
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Open door alarm prevents accidental defrosts - 02/01/07 EDN-Design Ideas: Circuit
helps save a lab freezer or temperature chamber's contents.....(design idea added 9/08) |
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