We will use a Nokia 5110 Display, Digital Temperature module, and an Arduino Uno to make this. A 9V jack with wires, not a barrel, is also needed, along with a switch, and wires.

A soldering iron may be needed, but you can also just twist wires.

Step 1: Wiring

First, take the digital temperature module and the Arduino.

Connect the sensor in this fashion:

A0 to Arduino pin A0, G to ground, + to 5V, and D0 to Pin 3.

The sensor used is from the Elegoo Sensor V2 Kit, but I suppose these are quite common. A picture is attached.

Step 2: Sensor Calibration

Not all sensors are perfect at detection- you may need to fiddle with them a bit!

Please plug your Arduino in to your computer.

A .txt file is above containing the code for sensor calibration.

(Please not that this code is not my property but elegoo's. This code can be found here: https://www.elegoo.com/download/)

The .txt file is not malware. It is plain text and the text is copied into your Arduino IDE.

Run the code, and open the Serial Monitor.

Take a guide, this can be a commercial temperature monitor, a thermostat, or AC.

There is a fine-precision potentiometer on the sensor. Take a small screwdriver and adjust it to the temperature on the AC.

As shown in the second image, my sensor gave readings of 70 degrees C!

instructables .ino file for Calibration.txt
Download

Step 3: Connect the Display.

This is a wiring guide from Last Minute Engineers that I followed. However, instead of pin 3, I used pin 2 as 3 was already in use.

There are 8 pins.

Connect RST to pin 2, CE to 4, DC to 5, DIR to 6, CLK to 7. VCC doesn't need to be connected as BL is for backlight and also powers the display. However, if you are not looking to use the backlight, connect VCC to power.

Both should be connected to 3.3v. GND goes to ground.

Step 4: Submit the Code!

This time, the actual code!

A .txt file is again attached.

Note that there are no automatic updates, and you'll have to incorporate a button for resetting the Arduino.

Sorry, I'm still a newbie.

This code has C and F temperatures.

Now, use this!

Oh yeah, and also forgot to say...

It refreshes every 10 seconds or so but couldn't figure out how to clear it each refresh...

Sorry... press reset... and if you figure out a solution please let me know!

instructables .ino for actual project.txt
Download

Step 5: Battery!

To conserve space, we will use the VIN pin.

Also attach a switch to turn the Arduino on and off.

Apparently, twisting wires onto a tiny breadboard-unfriendly switch was too hard, so I soldered the wires on.

Connect a 9V battery to the clip, turn it on with the switch and it should work!

Note that there are 3 pins on a switch. Attach the a pin to the center and the other one to one of the sides.

Step 6: Case!

Of course, this jumble of wires isn't going to fit. Let's make a simple cardboard case to keep it neat.

Draw out a rectangle that is just enough to fit the arduino. You also add a compartment for the battery.

Please note that my case is really ugly.

Really really ugly.

I mean, It does look like a gameboy.

*shrugs*

I decided to make the 2 ports open for expansion and any other thing I may require with the screen...

Happy tinkering!

Note that for image 1, the device was still waiting for sensor result so nothing displayed

REMIXES:

I couldn't find my DHT11 sensor. Using that, you could have both temperature and humidity.

You could also do stuff like acceleration, light levels, UV levels, air quality, etc.

Using an Arduino Nano would be smaller, and you could use a LCD over I2C, but I2C didn't work for me for some reason(I think it's a problem with my board)

You could even try using rechargeable batteries.

Oh yeah, and if you use a pulse sensor that'd be a cheap way to check pulse.

:)