This coaster uses TMP006 Infrared temperature sensor. This sensor measures indirectly the surface temperature by its infrared radiation. This is perfect IC for my idea, since there is no other way to measure temperature of a cup/plate of different sizes, shapes and materials.
By placing TMP006 under the cup, I can perfectly measure its temperature. Moreover the power consumption is very low :1µA in shut down mode and around 200µA in active mode.
EFM32 Zero Gecko fits here perfectly, since it also consumes very little power.
Here are µCoaster main features:
-Stay in Stand by mode consuming 10-20µA
-Each 10 second check the ON button.If touch is detected wake up.
-It is possible to set the desired temperature by using according buttons.
-Additional button resets any action and puts µCoaster back to Stand By
-Additional button allows you to read current temperature and set it as desired temp for the future (In case you dont know exact preferable temperature, you can just measure and save the one you enjoy)
-The temperature is shown by 8 red LED in binary, which are controlled by 8 bit shift register.
-There is green LED and a buzzer for status notifications
-3,3V boost converter ensures there is enough voltage for bright LED's and buzzer , even if battery is low.
Here is first prototype.
After the concept has been proved, I started to work on custom PCB:
While PCB were in production, I had some creative time working on case :D
Meanwhile the PCB arrived:
I made a few newbie design mistakes, but this is also my first big project...PCB looks cool, but the production quality is pretty poor(made by Ragworm, UK)
Anyway, time to assemble and test:
After many sleepless nights and few dead geckos, it finally works!! Time to calibrate the TMP006 using
Greisinger GMH2710 (Accuracy +-0.1 °C )
The sensor is calibrated for the clear glass material, but it actually works pretty good with many other surfaces like ceramics, cardboard,plastic etc, since these materials have pretty similar emission constant.
You must avoid materials with very low emissivity, like shiny, polished metals.You also have to consider the thickness of the material which effectively isolate the liquid. In case of glass bowl, the temperature of water is pretty much equal to the temperature of the surface outside.On the other hand, I use big white mug (see video), which has very thick wall. The temperature outside differs from the inside by about 6°C. In my code I compensate this temperature loss.With calibrated sensor, the sensing accuracy is +-2°C which is more then enough for this purpose.
The system is powered through single CR2032 battery. The power consumption in stand by is around 12µA. While performing temperature measurements the µC stay most of the time in sleep mode and consume 4µA.
The TMP006 Sensor consumes 1µA in stand by and around 240µA while performing measurement. the temperature is measured each 10 s, in between the sensor stay in stand by. Overall a single coin battery should be enough for more then six months of operation.
I made this project to enter Low Power Design Contest by Silicon Labs:
Here is my demonstration video:)