DIY Smart Coaster Keeps Your Office Lifeblood a Comfortable 46 Degrees

As a cream and sugar kind of guy—that's a "regular" to we Yanks in Boston—getting a cup of coffee that's consistently at the "right" temperature is a crapshoot at best. If there's not enough cream, I'll scald myself with a fresh cup. Other times there's not enough; I'll get sidetracked, and then go back to my cup for a disgusting gulp of cold nastiness. The cream throws me off. It ruins my calm, and plays havoc with my coffee temperature. I need something that keeps my cafe at a consistent temperature. Popsci, thankfully, has delivered this evening with a DIY "Smart Coaster" they've made using an old shoe polish tin. The work week starts tomorrow; this couldn't have come a moment too soon.

TIME: 3 hours


* LM324N (Mouser #512-LM324N)
* Thermistor (I used an obsolete thermistor from Electronic Goldmine #G1929 for my circuit.) Try this one from SparkFun Electronics #SEN-00250
* 1K resistor (Mouser #660-CFS1/4C102J)
* Red LED (I used a salvaged LED for my circuit.) Try this one from Mouser #696-SSLLX5093HD-5V
* 10K resistor (Mouser #660-CFS1/4CT52R103G)
* 10K potentiometer [I used a salvaged pot for my circuit.]Try this one from SparkFun Electronics #COM-00104
* 3.7V battery (SparkFun Electronics #PRT-08818)
* Scrap wire
* Round flat-top metal container (I used a salvaged Kiwi Brands shoe polish container — just to prove to some of you that I do know shinola.)


1. Clip off the unnecessary legs from the LM324N IC: keep pins 1, 2, 3, 4, and 11.

2. Attach the thermistor to the inside of the metal container's lid with some tape. Be sure to insulate both of the thermistor leads.

3. Drill a hole in the side of your metal container for holding the red LED. Insulate the bottom of the metal container with some scrap paper or 2mm hobby foam.

4. Follow our schematic for building the Smart Coaster circuit.

5. Fit the components inside the metal container, connect the 3.7V battery, and close the metal lid. You can easily test your completed project by turning the potentiometer until the LED just turns off (this is roughly equal to room temperature). Now hold one of your hot little fingers on the lid of the Smart Coaster. Within about 5-10 seconds, the LED should turn on. If not, then open up your metal container and carefully reexamine your wiring. If this test result is OK, you are now ready to calibrate your Smart Coaster.

Place your cup of fresh brew on the Smart Coaster and use the 10K potentiometer to adjust the sensitivity of the thermistor. Turn the potentiometer until the red LED glows and closely monitor the temperature of your beverage. When the cup's temps have fallen to your desired drinking level, turn the potentiometer until the LED goes out. Your Smart Coaster is now calibrated for monitoring your future drink temps. Remember, if you change cups or mugs you will have to recalibrate the Smart Coaster. Bottoms up.

See? Easy. And now the mornings will go by *slightly* faster than normal. [Popsci]

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