Aniomagic - smart coaster

Apparently I am a "nekojita," (Japanese for a person who can't handle hot food). Cats don't like high temperature food (they say), so a person who doesn't fancy molten bits in their mouth is called nekojita.

For me it means waiting a little while for my soup or tea to cool down before I can enjoy it. Since I was in the "temperature-sensing-projects mood", I decided to make yet another one, this time without any conductive thread.

With a learning sensor, I built a smart coaster that can monitor the temperature of my drink: green means it's just right, red means it's way too hot, and white means I'll need to heat it up again.

The wonderful thing about the learning sensor is that I don't have to hook it up to a computer to reprogram it. If I wanted to monitor (say) a cold drink, I can recalibrate it by tilting the coaster, which activates the tilt switch.

The coaster goes to sleep after 30 minutes, and wakes up when picked up. (next: a version powered by heat from the drink)

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Ingredients:

temperature learning sensor
red surface mount LED
green surface mount LED
white surface mount LED
tilt switch
plastic battery holder
large battery (CR2032)

You'll also need

some acrylic (or something similar)
a soldering iron
wire-wrap wire
a laser cutter (lucky us :-)

Design & Preparation

First, figure out where you want the lights, and your desired disc diameter. I used my favourite cup as a guide.

To embed the LEDs into the underside of the acrylic, measure and etch three rectangular bevels. There are three more on the other side in case the future me wants indicators there too. Drill holes for the learning sensor too.

Next, cut out three doughnuts that will be stacked up...

to clear the battery holder.

Some LEDs don't have indicators to tell which end is which. Connect a 3V battery to each one, and mark the minus... it makes things much easier.

Prepare the wires by stripping and tinning them. The exposed wires are very short so they'll be flush when soldered into the learning sensor. "H", "M", and "L" go to the red, green, and white LEDs respectively. More details.

Solder on the wires: I used white for "+", black for "-", purple for "calibrate," and so on.

Pass the wires through the holes until the sensor is flush.

Since the solder connections are fragile, apply some epoxy and fill in the holes (you don't want to resolder through acrylic if any break). Glue down the LEDs as well.

This is also a good time to glue down the battery holder... use one of the doughnuts as a guide. Make sure you'll have clearance for replacing the battery.

For multi-drop connections like ground, I prefer to expose a long arc of wire and connect with smaller pieces as needed.

Hook up all the "-" of the LEDs to the negative terminal of the battery holder.

Here's a tip: when you want an exact length for wire, you might run into trouble with the regular wire stripper, as you may stretch or even cut the wire by accident.

In these cases, I like to burn off and trim the plastic with the soldering iron because there's no risk of breaking.

Finish wiring up the components.

After wiring the tilt switch, angle it so that it will be off when the coaster is flat on the table. 30 degrees should do it. Glue it in place.

Stack the disc and doughnuts, and glue them together. You can use more exotic acrylic glues, but since this isn't load bearing, a thick epoxy will do.

It works! Turn on and calibrate

When the smart coaster is turned on for the first time, any of the lights may be randomly lit, depending on the temperature.

Fill a cup with the "right-temperature" beverage of your choice and let it sit on the coaster for a minute. Remove the cup and tilt the smart coaster so all three lights start flashing - this recalibrates the sensor.

Enjoy!