# Temperature sensor using lm35 and lm3914

#### 1N Power

Joined Nov 19, 2023
4
I have a complete analogue circuit and I want measure a temperature range from 30 degrees to 80 degrees and eventually output the temperature to lm3914 ic . Can some one guide me on how can I achieve this particular temperature range and eventually output it on the lm3914 ? TIA

#### liaifat85

Joined Sep 12, 2023
47
The LM35 electronic led thermometer does not require any external calibration or trimming to provide typical accuracies of ±1⁄4°C at room temperature and ±3⁄4°C over a full −55 to +150°C temperature range.
You can check this LED THERMOMETER USING LM3914 AND LM35.

#### 1N Power

Joined Nov 19, 2023
4
The LM35 electronic led thermometer does not require any external calibration or trimming to provide typical accuracies of ±1⁄4°C at room temperature and ±3⁄4°C over a full −55 to +150°C temperature range.
You can check this LED THERMOMETER USING LM3914 AND LM35.

Thank you for your reply but I was thinking about how I can achieve the resistor values in order to output a temperature range of 30 - 80 degrees.

#### liaifat85

Joined Sep 12, 2023
47
Thank you for your reply but I was thinking about how I can achieve the resistor values in order to output a temperature range of 30 - 80 degrees.
LM35 does not output a temperature directly. It outputs a voltage depending on the atmospheric temperature. By reading the voltage, you have to calculate the temperature. It is not possible to manipulate the output in any other way. For more details you can see here:
https://www.theengineeringprojects.com/2019/01/introduction-to-lm35.html

#### wayneh

Joined Sep 9, 2010
17,493
It is not possible to manipulate the output in any other way.
You cannot manipulate what the LM35 outputs, but you can use simple op-amp circuits to provide any slope and offset you want to apply to that output.

So the challenge is to match up the output behavior of the LM35 to the desired behavior of the LM3914. Once you can define what you want, you can design a relatively simple circuit to accomplish it.

#### 1N Power

Joined Nov 19, 2023
4
You cannot manipulate what the LM35 outputs, but you can use simple op-amp circuits to provide any slope and offset you want to apply to that output.

So the challenge is to match up the output behavior of the LM35 to the desired behavior of the LM3914. Once you can define what you want, you can design a relatively simple circuit to accomplish it.

I am viewing the scemathic above which was taken from the following link:

Would it be possible to explain whu certain resistance values where taken to account ?

TIA

#### crutschow

Joined Mar 14, 2008
33,962
Read the data sheet for the LM3914, as that explains how to scale the input.
If you have questions after reading that, get back with us.

#### 1N Power

Joined Nov 19, 2023
4
Read the data sheet for the LM3914, as that explains how to scale the input.
If you have questions after reading that, get back with us.
I haven't yet understood why certain resistance values we taken into account.
Can you please guide me on this cause ?

TIA

#### crutschow

Joined Mar 14, 2008
33,962
I haven't yet understood why certain resistance values we taken into account.
How can that very general question be answered?
What resistance values?

#### wayneh

Joined Sep 9, 2010
17,493
Would it be possible to explain whu certain resistance values where taken to account ?
I'm assuming you're asking about the op-amp circuit, R1 and R2. The op-amp will adjust its output pin to make the two input pins have the same voltage. So with the values of R1 and R2 chosen, V2 = V3 = Vout • R2/(R1+R2) . Rearranged, Vout = V3/R2•(R1+R2) = V3/1000•(3300+1000) = 4.3.

So you can see that for any LM35 voltage within the operating range of the op-amp, the output voltage will be 4.3X higher.