13/09/13 Add a thermometer to your digital multimeter :Embedded Lab

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Add a thermometer to your digital multimeter

R-B

May 3rd, 2012; 10,954 views

Curtir 31 pessoas curtiram isso. Seja o primeiro entre

seus amigos.

A digital multimeter

is a very useful

instrument that

combines several

measurement functions

in one unit. A typical

multimeter includes

features of a variable-

range ohmmeter,

voltmeter, and

ammeter. Some of

them also include

capabilities of testing

diodes and transistors.

In this article, I am

going to talk about a

technique of adding

thermometer feature to a regular digital multimeter. The technique is very simple and uses one temperature sensor along

with two resistors and a DPDT slide switch.

► LCD Voltmeter ► Probe Thermometer ► Thermometer ► Analog Multimeter

► Fluke Digital Multimeter

► Temperature Thermometer

► Thermometer Calibration

13/09/13 Add a thermometer to your digital multimeter :Embedded Lab

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Multimeter showing the surrounding temperature in degree Celsius

Theory

In my previous article (Testing analog temperature sensors with a multimeter), I described a method of using a

multimeter to test analog temperature sensors like LM34, LM35, MCP9701, TMP35, etc. These sensors provide an

analog output voltage that is linearly proportional to the temperature, and therefore, by measuring the output voltage with amultimeter, we can verify if the sensor is working or not. Now we are going to embed one such sensor inside a digital

multimeter and use the voltmeter feature of the meter to display temperature on the LCD. The sensor will acquire power

supply from the multimeter circuit itself. Although this sounds pretty simple, there are few issues that must be taken care of

to make it work.

Let’s look at the setup shown in the picture below. Here, the LM35 sensor is powered from an external 9V battery source

and its output is measured with a digital multimeter setup as a voltmeter. The LM35 output is linearly proportional to

the Celsius temperature with a scaling factor of +10mV/°C, which means if the temperature is 24.5°C, the multimeter will

measure the sensor output as 245 mV. Now the question is will the multimeter output be the same if the LM35 sensor isplaced inside the multimeter and powered from the same battery (usually 9V or 12V) that is powering the multimeter?

Actually,it is not. Let’s see why.

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LM35 output voltage is proportional to centigrade temperature

Most of the inexpensive digital multimeters available in the market are based on ICL7106 chip, which is a low power A/D

converter with a built-in 3 1/2 digit LCD display driver. The maximum voltage that can be applied between its power

supply pins, V+ and V-, is +15V. However, the multimeters based on this chip are usually seen to be powered by either a

9V PP3 or a 12V A23 battery. Now if the LM35 sensor uses the same battery to power itself, then it will use the negative

terminal of the battery as the reference point (ground) to generate the output voltage, which is proportional to thetemperature. But the ICL7106 A/D converter does not measure the input voltage with reference to the negative terminal of

the battery. Instead, it uses a separate reference voltage (known as common terminal, or COM point) which is derived

from the supply voltage and is set somewhere between V+ and V-. The multimeter has two leads: red and black. The

black lead goes to the COM terminal (which is the reference point) and the red lead goes to the IN+ terminal of the

multimeter circuit. The voltage at IN+ is appropriately scaled (based on the selected range) through an on-board circuit

before it is fed to the input of the ICL7106 A/D converter, which then measure it with reference to the COM voltage.The easiest way to find out the voltage of the COM terminal is to set the multimeter as voltmeter and connect the IN+

terminal (red lead) to the positive terminal of the battery as shown below.

e

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COM and IN+ terminals inside the multimeter

The multimeter will show this voltage around 3V, which means the COM terminal is set to 3V lower than the battery’spositive voltage. I have tested this with a few other digital multimeters and they all show that the COM terminal is 3V lowerthan the battery’s positive terminal.

Potential difference between the positive terminal of the battery and the COM terminal is 3.0 V

Imagine what would happen if you connect the ground pin of the temperature sensor to the COM lead, instead of the

negative terminal of the battery. The sensor will now provide output with reference to the COM terminal and the multimeterwill be able to measure the sensor output correctly. But, unfortunately, the LM35 sensor does not operate at 3V. Therecommended supply voltage range for LM35 is 4-30 V. But there are other similar sensors that operate at 3V, such as

TMP35, which is functionally compatible with the LM35 sensor and operates at a single-supply voltage from 2.7 V to 5.5V. It also does not require any external calibration to provide typical accuracies of ±1°C at +25°C and has an output scale

factor of 10 mV/°C. However, the TMP35 reads temperature only from 10°C to 125°C.

Now we have resolved the reference voltage problem, and found a temperature sensor that operates at 3V. The next thing

required is a divide-by-ten circuit. There is a factor of 10 in the sensor’s output voltage (mV) which must be taken out toget the actual temperature (°C). The divide-by-ten circuit can be simply constructed using two resistors (1.8K and 200Ω)connected in series as a voltage divider. The voltage across the 200Ω resistor is 1/1o th of the total voltage applied across

the series combination of the two. The complete circuit diagram of this setup is shown below. A DPDT slide switch isincluded in the circuit to turn the thermometer on and off. When the switch is turned on, two things happen:

1. The TMP35 gets power supply.

2. The divided sensor output is connected to the IN+ terminal of the battery.

If you set the multimeter to measure voltage ranging from 0-200 mV, the temperature in °C will be displayed on the LCDscreen.

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Circuit diagram

In the circuit above, when the switch is turned off, the sensor’s power supply is cutoff, which is important because wedon’t want the sensor to draw current from the battery all the time, and the multimeter’s IN+ terminal is disconnected fromthe sensor so that the multimeter can resume its normal operation.

Here’s my TMP35 sensor with the divide-by-ten network. I didn’t have a 200Ω resistor so I cascaded two 100Ωresistors.

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Sensor and the resistive voltage divider network

Circuit placed inside the multimeter

I fixed the slide switch next to the battery compartment as shown below.

DPDT slide switch

After the circuit is placed inside the multimeter, put its cover back. Turn the multimeter knob to measure DC voltagebetween 0-200 mV, and push the slide switch to ON position. The sensor gets powered and its output is connected to theIN+ terminal of the multimeter. The meter then displays the temperature (°C) on the LCD screen.

13/09/13 Add a thermometer to your digital multimeter :Embedded Lab

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Displaying room temperature in ° C

The following picture shows the temperature when the meter is placed inside refrigerator. It would probably go furtherdown but I took it out after a couple minutes as the TMP35 can read temperature only above 10°C.

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Meter placed inside the refrigerator

Now my multimeter has an added feature of displaying the ambient temperature on its LCD. How about yours?

I hope you enjoyed reading this!

Like 31

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16 Responses to this post

1. Ludovic on May 4th, 2012 2:09 pm

Very smart! Nice idea and well done!I wanted to make a thermometer and a battery monitoring device for my motorbike, now I got the idea that with acheap multimeter and a few additional components I could make it!

A small PIC10F could simply switch the voltage range and the sensor every 1 sec or so…I’ll think a bit more about that.

2. Upgrading a digital multimeter to tell the temperature - Hack a Day on May 4th, 2012 7:04 pm

[...] tipped us off to this really slick hack he’s done to allow his multimeter to tell the ambient temperature. He’sbasically measuring the output of an LM35 temp sensor that he has mounted in the case. [...]

3. Upgrading a digital multimeter to tell the temperature | Share Blog on May 4th, 2012 7:11 pm

[...] tipped us off to this really slick hack he’s done to allow his multimeter to tell the ambient temperature. He’sbasically measuring the output of an LM35 temp sensor that he has mounted in the case. [...]

4. Upgrading a digital multimeter to tell the temperature » Geko Geek on May 4th, 2012 7:30 pm

[...] tipped us off to this really slick hack he’s done to allow his multimeter to tell the ambient temperature. He’sbasically measuring the output of an LM35 temp sensor that he has mounted in the case. The [...]

5. Upgrading a digital multimeter to tell the temperature « Hackaday « Cool Internet Projects on May 4th, 2012 8:06pm

[...] tipped us off to this really slick hack he’s done to allow his multimeter to tell the ambient temperature. He’sbasically measuring the output of an LM35 temp sensor that he has mounted in the case. The [...]

6. elfrek on May 5th, 2012 6:41 am

Nice. Good article

7. Tim on May 5th, 2012 10:58 am

My meter shows ambient temperature out of the box…but then it’s a _real_ multimeter.

8. MAKE | Add a Thermometer to your Multimeter on May 7th, 2012 2:02 pm

13/09/13 Add a thermometer to your digital multimeter :Embedded Lab

embedded-lab.com/blog/?p=4951 10/13

[...] hacker Raj wanted to expand the functionality of his multimeter, so he hardwired a temperature sensor to it. [...]

9. Add a Thermometer to Your Multimeter | House of Mods on May 7th, 2012 6:36 pm

[...] hacker Raj wanted to expand the functionality of his multimeter, so he hardwired a temperature sensor to it. [...]

10. Add a Thermometer to Your Multimeter on May 8th, 2012 1:41 am

[...] hacker Raj wanted to expand the functionality of his multimeter, so he hardwired a temperature sensor to it. [...]

11. Add a Thermometer to Your Multimeter | Indoor Digital Billboards on May 15th, 2012 5:08 am

[...] hacker Raj wanted to expand the functionality of his multimeter, so he hardwired a temperature sensor to it. [...]

12. Multimeter Temperature Mod « Mint Electronics on July 1st, 2012 11:39 pm

[...] by Embedded Lab, so I take no credit for the idea of this build. Here is the link to the project: http://embedded-

lab.com/blog/?p=4951. I was thinking about doing a tutorial, but it’s already on Embedded Lab and I don’t [...]

13. Mint Electronics on July 2nd, 2012 12:06 am

Hello!I just did a small post on my blog based on this project, I hope that you do not mind.

-Cheers.

14. R-B on July 2nd, 2012 12:43 am

That’s alright!

15. Mile on June 10th, 2013 4:31 am

Hi.Can anyone tell me if this works the same with 12v hooked up to a screen that is just a voltmeter?I want to use one of these dual volt meters to display the temprature of my camping fridge in my 4wd, (the tempsensor can just be on a cable that i shut in the fridge and resisters behind the meter where i mount it)

Thanks,Mile

16. Sallam on June 19th, 2013 5:20 pm

Thanks for everything,

But it didn’t work with me, i connect all like the picture with LM35 but I don’t have anything.

please can anybody help me.

Thanks for advance.

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