Make a Digital Thermometer




Introduction: Make a Digital Thermometer

In this instructable, you will learn how to make a simple digital thermometer for under £10 using a few simple components and 1 IC.

Finished project should look something like this:

Step 1: The Parts

Here is a list of all the parts that you will need:

-LM3914 Bargraph Display Driver (you should be able to get one of these from your local electronics shop, and if not you can find them online)
-10 segment LED bargraph display (alternitively you could use 10 individual LEDs)
-150k resistor (you could stick together smaller value resistors)
-2.2k resistor (you could stick together smaller value resistors)
-4.7k variable resistor (potentiometer)
-470k variable resistor (potentiometer)
-10µf electroltic capacitor (ceramic and polymer ones probably work too)
-18-pin DIL socket (you can just use  20-pin one like I did if they dont have any 18-pins)
-20-pin DIL socket (these aren't necasary, but protect the IC and bargraph display from the heat of soldering)
-5k thermistor (these are a bit tricky to find, I had to settle for a 4.7k one, but you can get them of the internet)
-PCB (if you can make one, I would highly recomend it but you can do without)
-enclosure (again, this is not necassary, but it make the finished project look a lot nicer. Make sure it is the right size, I had to buy a bigger one)
-PP3 battery clip (these are easy enough to find, but I suppose you could just solder wires directly onto the battery)
-9v battery (to power it, everywhere sells them)
-2 switches (any kind will do, as long as they are locking and on/off. Also, these aren't neccasary, one is for switching the display from bar/dot and one is to turn the whole thing off and on. I only eneded up using one for power)

Tool you will need:
-Soldering iron
-Side cutters (or anything to cut off the remaining legs of the capacitor and resistors)
-Wire strippers (or you can just use the side cutters, or your teeth)
-Drill (only required if you are making the enclosure too, pillar drill recomended)
-Files (to neaten up the drill holes, if you dont have any you can use sandpaper or just drill it some more)
-Solder sucker (if you are likely to make a mistake)
-some form of adhesive (I use hot glue just to secure the circuit board, switches, and thermistor in place)
-Screwdriver (to adjust the potentiometers, unless you got the ones with knobs like mine, and to close the enclosure)

Step 2: The PCB

Well first off, your going to need a schematic to build it from. The second ones is the layout I used, the red wiggly lines are wires. You can etch your own PCB or just conect it up using loads of wires (like me). I wouldnt recomend stripboard since the whole circuit would be huge and take a lot of editing (althoguh I did use stripboard to hold the DIL sockets). Everything in the second and third schematic are already the right way around, so you can turn it straight into a circuit board (if you want to make a double sided one, make the red wires the second side, blue wire in the third one).

Step 3: Soldering the Components

well this step is pretty straight foward- solder the components together. I would start with the resistors and the capacitor, then the thermistor and battery clip, then the switched and potentiometers and the IC and bargraph display last. Remember if you used DIL sockets to put the actual IC and bargraph display in last, and if you didn't use them, be very careful when soldering the  and wait for it to cool down before soldering the next pin, since they are very sensitive to heat and it can destroy the thin wires inside the IC, so dont leave the soldering iron on a pin for more than about 4 seconds, and wait about 5 seconds before soldering the next pin. I would also recomend having the switches and thermistor on wires so that they can be mounted on the enclosure, and for the bargraph display, you can just cut a hole and stick the whole PCB onto the top of the enclosure, unless you really want to so 11 wires.

Step 4: Making the Enclosure

This step is optional since it isnt required for it to operate, but it looks a lot nicer in a plastic enclosure than as a loose circuit board. I would recomend mounting the switches on the outside, drilling a hole for the bargraph dislay and sticking the whole circuit board to the top with the display going through the hole, and mounting the thermistor on the inside, with some small holes to allow the heat to get to the thermistor easier.

EDIT: I just got a new enclosure, and it looks pretty good now. I have attached a picture below.

Step 5: Testing

Now, you can attach a 9v battery, close the enclosure, and turn it on. You should be able to see several bars light up. If not, you may have to open it up and play with the potentiometers until it does. You can use the other switch to swap between solid bar (lights up current temperature and all the way to the bottom, like a traditional thermometer would, or just show 1 line at the curent temperature) only if you added the second switch. If it shows only 1 line and you want a bar, you need to add a wire from MO to positive (last pin of the IC), or if it shows a solid bar and you want 1 bar, you can cut the wire between MO and positive.

If it still dosent work, try the bargraph display the other way around. Since LEDs are diodes, they only let electricity pass through one way, and unlike individual LEDs, the bargraph display dosent have a very clear identification of possitive and negative.  If you are still having problems, check the scematic and make sure everything is in the right place, there are no breaks in the tracks, or solder bridges (especially check between the IC pins since they a so close together) and that the resistors and capacitor are the right values, and that the battery is new. You can also message me with a picture of your circuit and I can try and help you find the problem. If your circuit works, then congratulations! Please take some pictures of your finished project and post them in the comments section. Here is the final thermometer working (it didnt fit inside the enclosure properly)

To calibrate it, the 470k potentiometer adjusts the temperature range, and the 4.7k adjusts the accuracy (how much change in temperatue it takes for it to go up a bar) I would suggest getting another thermometer and keep adjusting it when it comes in contact with hot and cold stuff until it is fairly acurate. Mine goes from 10-30 degrees (C) and is accurate to about +-2 degrees. The scale goes up in 2 degrees for each bar.



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    16 Discussions

    This is a cool project.

    Hello sir,
    Your project was very useful and worked for me.
    I wanted to know if this works for a broader temperature range with the interconnection of 2/more above circuits. Could you please help me with the connection and the accuracy.
    Thank you.

    Thanks Andy70707! The Circuit worked on me. I only used 10 individual LED since LED Bargraph is not available here in our area. but I have a problem here. As I've seen in your video provided in this site... Once you touched the Thermistor, the Bargraph instantly lighted up. but mine doesn't. My thermistor must be place beside a fire or a soldering iron to lightened up all the LED. What could be the problem in my circuit? Is my thermistor not that sensitive?? I hope you will help me with regards to this. Thanks for the project :D

    1 reply

    Firstly, make sure your thermistor is the correct resistance. You should use a 4.7k or 5k NTC (negative-temperature coefficient) thermistor. This is the resistance at 25c which is around room temperature so you can check the resistance of your thermistor with a multimeter.

    Second thing is that it must be calibrated, use the potentiometers to adjust the sensitivity and range and you should be able to get it within a tight enough range with enough sensitivity to detect heat from touch.

    It's not an amazingly accurate circuit for being able to accurately tell the temperature but you should be able to get it to show more or less LEDs dependant on the temperature.

    I have managed to link to ICs together and get it working in bar and dot mode, the only problem i am getting is the LEDs are very dim, did you experiance this problem of have any idea what is limiting the voltage? cheers

    1 reply

    The 3914 chips are rather unusual in that, instead of drawing the power through the chip, they instead sink into it, and are controlled by the allowance of current flow through the chip. If you have 2 bargraphs (effectively 20 individual LEDs), and assuming they're all 25mA and 3v, 500mA@3v = 1.5W 9V/1.5W=166mA. I'm not certain what the current draw for the rest of the circuit is, but assuming your 9V battery is over 250mA, as most of them should be, that should be plenty of power.

    The only problems I can think of would be if you are using an old 9V battery that has lost a lot of it's power, or if the way you've linked the two 3914s is limiting the current sinking in. You could test this by wiring both LED displays up directly to the battery on some breadboard (remember a 330ohm resistor).

    would this be possible using a 20 segment LED bargraph display to give a more accurate reading by simply changng the display over?

    1 reply

    Yes-but not in the current form. The LM3914 only allows control of 10 LEDs, but you could modify the circuit to use two 3914s and then connect the outputs to a 20 segment display.

    hey Andy awsome work ;)
    but i need to make a small digital thermometer for a coffee cup. so is it possible if i follow the above instructions??
    and what about the temperatures and all??
    Please help :)
    Thanks alot :)

    1 reply

    This would work for your application, I assume the thermistor is going to be immersed in the cup for this. You can get water proof ones for immersion, or alternatively coat it in a thin layer of food grade silicone or something to avoid the wires shorting out.

    The temperature range you calibrate it to is also going to be quite wide, so you might have 5-10 degrees difference between each bar. This still isn't a very accurate thermometer and probably shouldn't be relied on if you actually want an accurate reading. I am currently working on a digital thermometer with a 7-segment readout based on the LM35 or similar, and also a thermoelectric cooling unit with temperature control adjusted by a feedback loop once I get the thermometer done.

    Hello andy.Very nice work man. I'm interested to tell the operating principle of this digital thermometer and if you can explain me the role of individual circuits.thanks!

    nice one bro. could i use that as a outdoor thermometer? how could i transmit the data to a display inside my room

    i want to ask you that how much resistance should i set so that my thermometer shows 20 to 40 degree centigrade ...
    i have made this in my engineering 1st semester, got the highest marks...

    1 reply

    Congratulations on your project! I really can't advise on the resistor settings, as it depends on the tolerance and type you are using, and they are supposed to be left variable so you can calibrate it, as this is an analogue circuit. If I could give definite values, I wouldn't bother with the variable resistors and simply use fixed ones.

    To calibrate it, find somewhere hot and somewhere cold, and place the thermistor in those locations along with another thermometer, and adjust the resistors on your one to match the reading of the per-calibrated one. I can't remember which order it is, but one of them does range, and the other does accuracy.

    Hi andy do you know the range of this??
    im interested betwen 10 and 150 celsius
    If you can please let me know at
    Thanks and keep working :)

    1 reply

    I have mine set up to do 10 to 30 degrees, but by changing the pots, you can change the range. You will have to check the datasheet of the thermistor to make sure it can go that high. This is also an old instructable, I am preparing to make a new (digital) one with a 7-segment display readout. I got some LM35 linear thermometers, it's a 3-pin TO220 package, and the middle pin gives you 0Mv/c, so if I can get an ADC working, I will have the circuit running. I was trying to do it only using CMOS chips, the 7107 looks promising, but I don't like the large footprint of the chip and the external circuitry, so I may go for a picaxe 08m instead. Anyway, keep checking back, an instructable should be up in a few months.