Instructables

Make a digital thermometer

Picture of 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:
 
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Step 1: The parts

Picture of 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
-Solder
-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)

garethcoda9 months ago
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
andy70707 (author)  garethcoda9 months ago
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).
garethcoda10 months ago
would this be possible using a 20 segment LED bargraph display to give a more accurate reading by simply changng the display over?
andy70707 (author)  garethcoda10 months ago
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.
Shabber_Shiraz11 months ago
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 :)
andy70707 (author)  Shabber_Shiraz11 months ago
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!
itge131 year ago
nice one bro. could i use that as a outdoor thermometer? how could i transmit the data to a display inside my room
saud881 year ago
hye...
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...
thanks.....
andy70707 (author)  saud881 year ago
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.
fernandohdz2 years ago
Hi andy do you know the range of this??
im interested betwen 10 and 150 celsius
If you can please let me know at fernandohdz@yahoo.com
Thanks and keep working :)
andy70707 (author)  fernandohdz2 years ago
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.
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