I need help for my temperature sensor

[deidei007]

anyone can help me about temperature sensor activated fan for 5v-12v only?if the temperature reach the (i.e. 25deg. to 40deg.) the fan will activate but if the temp. is below 25deg. the fan will stay off. hopefully this post may help me about my miniature project.

 

[Boncuk]

Hi deidei007,

you didn't mention the fan current requirement.

The circuit I suggest uses an LM35 for temperature sensing. Output transistor is a BD139 which should suffice even for fans up to 1.5A. (use a heat sink at currents > 0.5A.)

RV1 is the simulated temperature sensor. Omit R1 and connect V+ of the temperature sensor to the supply voltage (+).

U1:A works as non-inverting amplifier amplifying the input signal by the factor 11. (Vout=Vin*1+(R2/R3)=Vin*1+(47/4.7). U1:B works as comparator with hysteresis (R8=100K).

The trip point can be adjusted using RV2 (1K).

The fan will be off if the trip point voltage at the inverting input (pin6) is higher than U1:A output voltage. It will be on at any input value at the non inverting input (pin5) higher than the trip point.

D1 and R7 are optional. D1 lits when the fan is running. D2 is a MUST. It eliminates back EMF caused by the relay.

You might also connect the motor between the collector of Q1 and +12V. The diode should stay there as shown in the images.

Boncuk

 

[alec_t]

U1:B works as comparator with hysteresis (R8=100K).

R8 should feed back to the non-inverting input to provide hysteresis.

 

[Boncuk]

R8 should feed back to the non-inverting input to provide hysteresis.

Thanks a lot for the correction.

Boncuk

 

[deidei007]

thank you Boncuk,

any other ckt about temp. sensor you suggest? im confused about the ckt. it looks like difficult for me :|

 

[Boncuk]

There is no circuit for the LM35 as suggested for a temperature sensor.

The LM35 is an integrated temperature sensor to be connected just the same way as RV1 omitting R1.

Just download the data sheet and use the pin-out of the description to connect it the same way as RV1. V+ has to be connected to the positive rail of the power supply, Vout has to be connected the same way as the tap of the pot and V- has to be connected to circuit ground.

There is nothing mystic about the sensor.

Boncuk

 

[deidei007]

how about this ckt mr.Boncuk https://schematicwiring.com/wp-content/uploads/2010/04/fan-speed-control-circuit-schematic-diagram.PNG this is good ckt too you think?but I dont know if this working in Proteus,co'z I don't know how to operate Proteus. Lol

 

[Reloadron]

The circuit you linked to will work. It is attached.

Anytime we want to measure temperature for any number of reasons we start with a device to sense the temperature. Boncuk's circuit uses a LM35 temperature sensor which is a very good choice. The circuit you linked to uses a 'thermistor" as a temperature sensor and it will also work. However, the circuit has one drawback as drawn that I can see. Note Boncuk's circuit includes what we call Hysteresis and the latter circuit does not.

Hysteresis is important in circuits like this that turn a fan on and off based on temperature. Give this link a read. Pay attention to the section about Hysteresis.

As temperature rises we want the fan to turn on at a given point for cooling. When cooling begins the temperature will drop and the fan will turn off. This can result in the fan constantly turning on and off over and over again. So based on your 25 C temperature if the temperature exceeds 25 the fan is on and below the fan is off. This would work better if the fan came on above 25 and turned off at maybe 22 or 23 C. For your circuit a 100KΩ resistor placed between Pin 7 output and Pin 2 + Input of the comparator would give you some Hysteresis.

Read the link and try to gain understanding of how a comparator circuit works. Then you should understand how your circuit actually works.

Ron

 

[deidei007]

The circuit you linked to will work. It is attached.

Anytime we want to measure temperature for any number of reasons we start with a device to sense the temperature. Boncuk's circuit uses a LM35 temperature sensor which is a very good choice. The circuit you linked to uses a 'thermistor" as a temperature sensor and it will also work. However, the circuit has one drawback as drawn that I can see. Note Boncuk's circuit includes what we call Hysteresis and the latter circuit does not.

Hysteresis is important in circuits like this that turn a fan on and off based on temperature. Give this link a read. Pay attention to the section about Hysteresis.

As temperature rises we want the fan to turn on at a given point for cooling. When cooling begins the temperature will drop and the fan will turn off. This can result in the fan constantly turning on and off over and over again. So based on your 25 C temperature if the temperature exceeds 25 the fan is on and below the fan is off. This would work better if the fan came on above 25 and turned off at maybe 22 or 23 C. For your circuit a 100KΩ resistor placed between Pin 7 output and Pin 2 + Input of the comparator would give you some Hysteresis.

Read the link and try to gain understanding of how a comparator circuit works. Then you should understand how your circuit actually works.

Ron

Hi Ron,
so I will able to change the LM31 to LM35?but I guess its hard to find those IC's at the shop. by the way thank you for the info. ppl here are really nice.

 

[Reloadron]

The circuit that Boncuk posted used a LM35 as the temperature sensor. The LM35 is an integrated temperature sensor package. It outputs an analog voltage that is proportional to temperature.

The circuit that you posted uses a thermistor to sense the temperature. They are two completely different types of temperature sensors. Because they each sense temperature differently the circuits used with them are different designs.

Which type(s) of temperature sensors are available to you at your location?

Ron

 

[gogetax1]

check out the LM35 ic. its very easy to use. and needs about 3 to 6 volts of usage.

 

[Boncuk]

check out the LM35 ic. its very easy to use. and needs about 3 to 6 volts of usage.

Where did you pick those numbers?

According to the data sheet the safe operating voltage is 4 to 20V. Absolute maximum supply voltage is 32V.

Boncuk

 

[Boncuk]

Hi Ron,
so I will able to change the LM31 to LM35?but I guess its hard to find those IC's at the shop. by the way thank you for the info. ppl here are really nice.

No, you can't replace the LM311 with an LM35! The LM311 is a comparator circuit while the LM35 is a temperature sensor circuit.

You could use an LM35 to replace the NTC as temperature sensor. Replacing the NTC with an LM35 in that circuit requires some changes as input voltages won't change the same with increasing temperature.

The voltage drop across an NTC will decrease with increasing temperature while the voltage output of the LM35 increases by 10mV per degree Centigrade.

Boncuk

 

[aljamri]

The voltage drop across an NTC will decrease with increasing temperature while the voltage output of the LM35 increases by 10mV per degree Centigrade.

Hi Boncuk,
I think to solve this issue, he may interchange the temperarue sensor position with R1 (10k).

 

[deidei007]

I tested this ckt. in Proteus but isn't working, Im using Isis 7 Professional https://schematicwiring.com/simple-electronic-applications/circuit-schematic-diagram-of-fan-speed-control/