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Freezer
/ Frost Alarms
Last Updated on:
Monday, October 03, 2011 06:07 PM
Circuits Designed by Dave
Johnson, P.E. :
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.
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....
Links to electronic circuits, electronic schematics, designs for engineers, hobbyists,
students & inventors:
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)
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..... [Hobby Corner Circuit by Dave 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..... [Hobby Corner
Circuit by Dave Johnson]
Fridge door Alarm - Beeps if you leave open the door over 20 seconds.
3V battery operation, simple circuitry. (circuit added 9/04)
Frost Alarm - 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....[Circuit design by Andy Collison]
Open-door alarm prevents accidental defrosts - 1-Feb-07 Issue of EDN
Circuit helps save a lab freezer or temperature chamber's contents..... [Design Idea
by Tom Lyons Fisher, Juniata College, Huntingdon, PA]
