Thermal Coaster

A drink coaster which changes color based on a cups temperature

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The idea is simple: create a drink coaster which indicates what temperature a drink placed on it is. whether hot or cold, by turning red or blue. The ideal use for this is to be able to tell if your coffee has gone cold

I plan on using ATTiny85 do the work, since it should run on 3v 

CR2023 is 3v and should have enough mA to power it and some SK6812 / 5050 RGB LEDS
And a thermresistor will sense the temp, obviously.

The end game is to dip the board in resin to waterproof and protect the board making into an actual coaster.

The bottom side of the board will have the battery holder and pads to program the Attiny85.  I plan on leaving a recess in the epoxy to allow battery access.  I may make a recess for the pads for debugging,  but i plan to work out the code and kinks before putting it in resin

Refer the Git page for updated schematics and code 

This is intended to be an Entry for the Hackaday's Coin Cell Challenge

Keeping in with the rules of the challenge, i've added the board to OSHPark


Eagle Schematic

sch - 678.71 kB - 12/27/2017 at 00:16



Eagle Board

brd - 82.23 kB - 12/27/2017 at 00:16


  • 1 × ATTINY85-20SU Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × NCP21XV103J03RA Thermal Management and Protection / Thermistors
  • 1 × CR2023 Resistors (Fixed) / Thick Film Surface Mount
  • 4 × NeoPixel 5050 RGB LED SK6812 with Integrated Driver Chip
  • 1 × BAT-HLD-001 Battery Holders, Snaps and Contacts

View all 8 components

  • CAD Model aka Waiting for the Parts

    Ray Olsen01/08/2018 at 22:34 0 comments

    While I wait for the parts form Mouser & OSHPark and the Resin from Amazon, I decided to model the board for fun (and to make this page a bit better for the contest)

    Since I am mostly use the Autodesk family of products, I found they make this incredibly easy to model PCBs and Components, even though I still use EAGLE 7.7

    This setup requires, of course, Eagle and Fusion 360

    1. Create board in EAGLE
    2. Log on to with Autodesk credentials
    3. Import you eagle .brd file
      1. You end up with a 3D render of the board and some low poly shapes representing your components
      2. If accuracy matters, take some time correcting these shapes and chosing your components, lucky for me, they were all there in the database.
        1. I'm told it is easy enough to add your own but I have yet to need to
      3. You may have to edit some things here or in the .brd add the HEIGHT attribute to you components to get things working
    4. Go to File > Create an MCAD file. Choose STEP AP214 Assembly File
      1. I prefer this one as it will give you the board and components as individual parts in Fusion 360 that are linked together as one component.  This allows you to edit the individual components if you need to e.g. trimming the leads
    5. Grab a coffee, this process takes some time.  You'll get an email when completed
    6. Go to the project, and expand it.  You will see the file is ready.  You can send it directly to an open Fusion 360 on you computer or send it A360 which is Autodesk's version of Github.

    Normally I create a new project in Fusion 360 for everything to keep stuff separated and organized.  So i'll save this board in its own file, then import it into a new design for the coaster design. If I ever change anything in the board model it will be updated where ever it is used in Fusion 360

    Next is just simply making a shape around the board to my liking and setting the material to clear plastic

  • Formulation of an Idea

    Ray Olsen01/08/2018 at 22:04 0 comments

    I just want to jot down the design process for this build.

    Nobody likes cold coffee or tea, or a warm soda either.  I've been mulling over this idea for a long time now and the coin cell challenge gave me the final push to make a go at it.  

    I basically knew that a coin cell was a must. I remembered that the ATTiny85 also works on just 3v! So i began thinking up what else i needed

    I began searching for some low power RGB LEDs.  Of the ones i was able to dig up, they seemed a bit more complicated to utilize and I really wanted more color options and functionality that I am used to with the Neopixel / WS2812.  I found that they make a smaller version of that the SK6812 and hope that the current draw isnt too big on the CR2023

    I wanted to coat the whole thing in a clear resin to protect the board whilst giving a medium for the LEDs to shine through.  

    I whipped up a board in Eagle and sent it off to the lads at OSHPark

View all 2 project logs

  • 1
    Solder the components
  • 2
    Program the ATTINY

    Using your favorite method, flash the code to the attiny.  I've included pads on the underside of the board to assit with this

View all instructions

Enjoy this project?



Moldovanu Ionut wrote 02/16/2018 at 14:23 point


Did this one a few years ago :)

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Stephen Ludgate wrote 01/12/2018 at 14:19 point

Cool idea, will be good to see how it performs. 

I don't think you need the x4 capacitors between your power and ground though, only one for the LEDS and one for the battery Vcc and ground? And you shouldn't need resistors on all lines going to the LEDS, or is this for something important? Have you tested this layout in a breadboard, can't wait to see it work!!


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Solenoid wrote 01/02/2018 at 09:24 point

I had a similar project in mind, I'm happy you took on the challenge. I don't like my tea burning hot or lukewarm, yet I want to gulp it down in one go... this is usually a challenge. Having a temperature-indicating coaster would be ideal.

I was curious about the measurement accuracy and method, will measuring the bottom of the cup yield accurate overall liquid temperature measurements? I was thinking about an infra-red sensor to read the emitted IR (using the ATtiny's ADC), although I have no idea if that is feasible (maybe an op-amp needed?). Also how about calibrating it to a certain temperature without having to own the tools and skills to reprogram the ATtiny? Perhaps a potentiometer?

For how much time do you think the coin-cell will be able to power the coaster? The ATtiny85 will go from ~20mA on full power down to 100nA in the deepest sleep modes, but the fat LED's seem like they're going to pull the most current.

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Alastair Young wrote 12/28/2017 at 00:20 point

Coincidentally our girl scout team are working on a similar concept - but in a wrap-the -mug format and using a one-wire temp sensor.

Looking at the design, shouldn't the bypass caps be as close as possible to the devices they are supporting?

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siluxmedia wrote 12/27/2017 at 00:46 point

what about using an RGB led and hook a thermistor directly to it?

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Ray Olsen wrote 12/27/2017 at 01:42 point

I plan on throwing some sleep cycles into the code to increase the battery lifetime. Also the LEDS are data driven and still require input from an mcu

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