Make a Digital Thermometer
Intro: Make a Digital Thermometer
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
-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)
STEP 2: The PCB
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
EDIT: I just got a new enclosure, and it looks pretty good now. I have attached a picture below.
STEP 5: Testing
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.
17 Comments
seijunlee1 2 years ago
metalmarci 6 years ago
This is a cool project.
NamithaD 6 years ago
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.
francisbatayon04 9 years ago
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
andy70707 9 years ago
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.
garethcoda 10 years ago
andy70707 10 years ago
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).
garethcoda 10 years ago
andy70707 10 years ago
Shabber_Shiraz 11 years ago
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 11 years ago
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.
bourkarianos 11 years ago
itge13 11 years ago
saud88 11 years ago
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 11 years ago
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.
fernandohdz 12 years ago
im interested betwen 10 and 150 celsius
If you can please let me know at fernandohdz@yahoo.com
Thanks and keep working :)
andy70707 12 years ago