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Circuits designed by David Johnson, P.E.
Last Updated on: Monday, July 11, 2016 01:59 PM

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Temperature Sensors / Detectors Circuits
 

12v Cooling Fan Thermostat  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . [Circuit by Dave Johnson P.E.,  02/06/11]


4 to 20ma Current Loop Senses Temperature  -  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 a very robust current loop method.  It uses a highly accurate semiconductor temperature sensor . . . [Hobby Circuit designed by David Johnson P.E.,  07/04/00]


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]


Cooling Fan Thermostat (12v)  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/06/11]


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]


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 . . . [Circuit by David Johnson P.E.,  06/04/00]


Over-Temperature Micro-Power 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 . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/04/00]


Temperature Sensor MOSFET  -  Recently I was asked by a company to discover why one of their products failed to operate properly in cold temperatures.  Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in that state until another activation of the switch.  . . . [Circuit by David A. Johnson P.E.,  12/01/10]


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 a very robust current loop method.  It uses a highly accurate semiconductor temperature sensor . . . [Circuit by David A. Johnson P.E.,  07/04/00]


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]


Thermostat for 12v Cooling Fan  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . [Hobby Circuit designed by David Johnson P.E.,  02/06/11]

 

 

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