Alarm Sounds At Freezing Temperatures  -  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 Circuit designed by David A. Johnson P.E.,  07/11/06]

Battery-Powered Freezer Alarm  -  A typical freezer can contain hundreds of dollars with of food.  If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled.  The hobby circuit below is a battery powered alarm system, which is attached . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/06/06]

Freeze Alarm  -  I chose an accurate thermistor from Keystone as the temperature sensor for this circuit.  This device has a very specific resistance at a particular temperature.  According to the data sheet for the thermistor, the device will have a resistance of 361K at zero degrees C, which is the freezing point of water.  The thermistor is wired into a simple bridge circuit, which uses 1% resistors. The bridge is connected to a very low power voltage comparator.  The circuit forms a switch circuit, which changes state, when the resistance of the thermistor reaches a certain value.  At zero degrees C the voltage at the non-inverting input of the comparator exceeds . . . [Circuit Solution by David A. Johnson P.E.,  05/07/06]

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. . . . [Circuit by David Johnson P.E.,  11/05/14]

Freezing Temperature Alarm  -  The 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 circuit.  The average 9ua current is so low that the battery should last for many years. . . . [Circuit by David A. Johnson P.E.,  11/05/14]

High Temperature Thermocouple activates Relay  -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Thermocouple Temperature Activates Relay  -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F . . . [Circuit by Dave Johnson P.E.,  07/09/06]