Introduction: DIY Miniature Thermometer

About: Youtube engineer that has a passion for understanding what's actually happening. I try my very best to make thorough tutorials that explain everything in-depth so you leave with a completed understanding, rath…

For just a couple of bucks and a few components you have laying around your work area you can build a min thermometer than fits in your pocket. The whole circuit sits nicely right on top of a 9v battery, and is connected to it with double sided tape. Changing batteries was never so easy!

I hope you enjoy the project and vote it for the contests!

Step 1: Mini Thermometer Demonstration

In case you were wondering why the LEDs are flashing multiple times on each flash it's because I'm using PWM to power the pins. 

Step 2: Ingredients:


  • Three bananas with hot sauce
  • Arduino (of course!)
  • Breadboard and jumper wires!
  • ATtiny85/45
  • 8 DIP socket
  • Temperature sensor (I'll be using the LM35)
  • 3 LEDs(Preferable different colors with appropriate resistors)
  • 9v battery and clip
  • Perf board
  • LM7805 5v regulator (Instead of using a 9v batt and a regulator you can use a 3v battery or 3 AAA)
  • 1 small pushbutton
  • 1 small on/off latching button
  • 10k ohm resistor
  • Multimeter
  • Dremel (or other tool to cut perf board)
  • Soldering gun and solder
  • Sharpie
  • Wire cutters & strippers
  • Double Sided Tape

Step 3: Schematic & Files

Here is the sketch I will be running. I will be talking about it a little later.

Step 4: The Circuit P.1

Before you make the circuit it is always a good idea to breaboard the circuit. I made this mistake and I hope I won't ever do it again. I was so frustrated thinking I had soldered something wrong when really it was something wrong with the code. I recommend testing the circuit with your Arduino first then trying with an ATtiny85. After than you can begin to work on the permanent circuit. 

First of all you need to cut a piece of perf board to the size of a 9v batter. Then get a 9v battery clip. Now I made one myself and you can find the instructions in step 2 of my i'ble.

  1. On either side of the board solder the 9v and GND connections.
  2. Push in the latching pushbutton. This will be the on/off of the whole circuit so you want it to stay in either the up position or the down position. Solder one lead to 9v on the board.
  3. The LM7805 is to cut down from 9v from the battery to a 5v that the ATtiny85 runs of off. Push it into the breadboard near the switch. Solder the input pin of the 7805 to the opposite side of the switch. Then solder the GND from the 7805 to the GND on your board. The reason I put the switch before the LM7805 is because I didn't want voltage constantly flowing through it.
    For your information the fourth picture contains the pinout for the LM7805.
  4. At this point you might want to test the circuit and make sure that you are getting 5v.

Step 5: The Circuit P.2

  1. I'm not really doing this in order but push in the 8 DIP socket and the pushbutton.
  2. Connect pin 8 of the DIP socket to 5v.
  3. Connect pin 4 to GND.
  4. Now I took my LED and chopped in in half- just to save some space. Solder the anode to 5v and the cathode to a resistor. Run the resistor to GND.
  5. Connect one side of the pushbutton to 5v. Connect the other side of the button to GND with a 10k ohm resistor. Also on the same side as the resistor, solder a wire from the button (BEFORE THE RESISTOR) to pin 3 on the socket.
  6. Check your pushbutton. This is a very important step because if you don't have your button soldered correctly, the whole circuit will not work. Time to pull out your multimeter! Attach one end to GND on the board and the other to pin 3 on the socket. Turn the circuit on (latching button, LED should come on) and press the button. If all goes well you should get 5v every time you press the button. If all doesn't go well recheck your connections and refer to the schematic.

Step 6: The Circuit P.3

  1. Trim down two more LEDs.
  2. Solder one of the LEDs to pin 0 and the other to pin 1. You can solder the cathodes together and run them to GND using 1 resistor or you could use 1 resistor per LED. I chose not to use one at all.
So this may all be a little confusing but at this point we should have properly connected the on/off button, LM7805, power LED, 2 indicator LEDs, 8 DIP socket, and the pushbutton.

Step 7: Last Bit of Soldering!

The last soldering you have to do is to connect the temperature sensor to the circuit. Now in my project I used the LM35 but you can use pretty much any temperature sensor you wish to use. Connect the sensor's Vcc and GND to 5v and GND of your board. Then connect Vout to pin 2 on the socket. You done with soldering!

The last thing to do is to connect the 9v battery clip to the batter, and using some double sided tape, attache the board to the battery. 

It's time to get to some programming!

Step 8: Programming!

You will need to upload this sketch to your ATtiny85. If you don't know how to do that click here for more instructions. I will be post a i'ble on how to do it soon as most of the instructions out there have old and non functioning files. Upload this sketch to your ATtiny85, push it into the socket, connect the circuit to a 9v battery, push the on/off button, then press the second button, and PRAY that it works! Lol you should be able to tell the temperature.

In the next step I will give you some details about the code. I also laced the code with comments so that should help out a little bit. 

Step 9: Code Preview

Make note

In my conversion from Celsius to Fahrenheit I added and extra 4. In other words, in order to convert from C˚ to F˚ you multiply the C˚ by 1.8, then you add by 32.

/* DIY Miniture Thermometer

This circuit is small thermometer that fits on the back of a
9v battery. Two LEDs flash out the temperature. For instance, if the
temp is 75, the first led will flash 7 times, and the second LED will
flash 5 times.

The circuit:
*Vout from the sensor is connected to A3
*Input from the button goes on pin 4.
Make note: This is not the same as physical pins.
Please see for more info.

created in 2013
by Daniel Nicholls
Edited by Enjoying Electronics

Additonal Notes:
The conversion scale you see that Ctemp is set to is just for the LM35 temperature sensor.
Look up your own temp sensor's conversion scale and change my number to whatever your number is.



int Ctemp; // Temp in C˚
int digit10s; // Temp 10s digit
int digit1s; // Temp 1s digit
float temp; // Temp in F˚
int tempPin = 3; // Raw input from the temp sensor

int LED10 = 1; // LED to flash the 10s digit
int LED1 = 0; // LED to flash the 1s digit

int button = 4; // Digital input from the pushbutton
int bPress; // Variable that names the sate of the button

int time = 250; // this variable "time" is the delay between the flashes of the LED.
// so if the LEDs are flasing too slow or too fast you can
// adjust it here and it will change the delay everywhere.

void setup()
{ // You should know what all this is :P
pinMode(LED10, OUTPUT);
pinMode(LED1, OUTPUT);
pinMode(button, INPUT);
pinMode(tempPin, INPUT);

void loop()

Ctemp = analogRead(tempPin); // Okaydokey we are setting "Ctemp" to the anlog Reading at tempPin. Remember what tempPin is?
Ctemp = Ctemp * 0.48828125; // Converting the analog voltage from the temp sensor to celcius. This is ONLY FOR THE LM35
// Note: This number may be a little off for the LM35. It seems to work OK though.

temp = (Ctemp * 9 / 5) + 36; // Now this is for converting the Celsius reading (Ctemp) to a farenheit temperature.
// We are setting "temp" to the new F˚ reading.

bPress = digitalRead(button); // Setting "bPress" to a digital reading at button. If you recall "button" is set to pin 4.

/******************************BEGINNING OF PLAIN COMPLICATED!*****************************/

if (bPress == HIGH) // The fist if statement. None of the code below will run unless this condition is met.
digit10s = int(temp / 10); // find the 10s digit of the temp. If your temp was 26.89 then it will become 2.689 (divide by 10) and the then int() instruction casts it to an integer removing everything after the decimal place then leaving you with digit10s = 2
digit1s = int(temp - (digit10s * 10)); //find the 1s digit. We use the 10sdigit from above and multiply it by ten, giving 20 and subtract that from our temp (26.89) leaving us with 6.89, then cast this to an int reducing it to 6.

while(digit10s != 0) //do a while loop while digit10s does not equal 0
analogWrite(LED10, 75);
analogWrite(LED10, 0);
digit10s--; //decrement digit10s by 1
} // end of the while statement


while(digit1s != 0) //do a while loop while digit1s does not equal 0
analogWrite(LED1, 75);
analogWrite(LED1, 0);
digit1s--; //decrement digit1s by 1
} // end of the while statement

} // button check

else // Now, do you remeber our IF statement? It was testing the button. So, if the button insn't pressed, turn all the LEDs off.
digitalWrite(LED10, LOW);
digitalWrite(LED1, LOW);

delay(10); // it's to stop the code running too often.

} // void loop

Step 10: How Does the Code Work?


The first thing that needs to happen is we need to get temperature into two separate digits. This is actually easier than you might think.

    digit10s = int(temp / 10);
    digit1s = int(temp - (digit10s * 10));
With this we can separate the two digits. Lets tackle the first one. digit10s is equal to the temperature divided by 10. Our example temp is 63, so 63 / 10 = 6. Now digit10s is equal to 6.

Now for the 1s digit. digit1s is equal to temp -  digit10s * 10.  So if our temp was 63, we subtract (digit10s * 10 = 60) from temp which gives us 3.

digit10s was 6, then we multiplied it by 10, which gives us 60, then we subtract 60 from temp (63). That then gives us the 3. We then set digit1s = to 3.

And that is how the digits are broken down!

Our next bit of code is just a bit more complicated.

This is our main block of code because it's what makes the LEDs flash the temperature.

    while(digit10s != 0) //do a while loop while digit10s does not equal 0
       analogWrite(LED10, 75);
       analogWrite(LED10, 0);
       digit10s--;       //decrement digit10s by 1
    } // end of the while statement


The first thing to notice is that we have a while loop and what this does is that as long as the condition is met, the code will run over and over.

Now what condition?

while(digit10s != 0) *** This is saying run the block below as long as digit10s is not equal to 0.

Lets look into digit10s. Digit10s is the 10's digit from the temperature. We dived the temperature by 10 to give us the 10's digit. So say it's 63 degrees outside. The 10's digit (digit10s) is equal to 6.

Now how does this flash the LEDs the proper amount of times? Well, it's saying, "digit10s is not equal 0, (remember, it's equal to 6 now) so run the block below. The first thing is it turns the LED on for 1 second then turns it off. Then we come to this

digit10s--; Here we use the '--' operation and what that does is it takes off 1 from the variable every time it gets passed. So that means if digit10s was equal to 6, it is now equal to 5. Now 5 is still not equal to 0, so we run the block again. Running the block again turns the LED on and off one more time and then we also come to digit10s--; again. That decrement digit10s by one again so now digit10s is equal to 4. This block is run 6 times (which flashes the LED 6 times). Why 6? Because digit10s was equal to 6, remember? Now, if we run the digit10s--; 6 times, that subtracts 6 from digit10s. That leaves digit10s equal to 0.

But what stops the block after it's gone through 6 times? Well if we look back we will see the operation being tested.

while(digit10s != 0) // while digit10s is NOT equal to 0, run the code below over and over.

It's saying run the block over and over while digit10s is NOT equal to zero. But now digit10s IS equal to 0! So that block is stopped and the Arduino moves onto the next block of code.

Step 11: Conclusion

I hope you enjoyed my project please PM me or comment if you have any questions, comments or suggestions. 
If you make one,  please let me know how it worked!


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