Make a Digital Thermometer

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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:

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)

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).

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.

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.

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.