CoffeeSense: Proof of Concept

Fall 2015

When getting caught up in your work, it can be easy to forget about simple things; your coffee, for example. You make a cup of coffee, set it down to cool, get caught up in your work, and before you know it your coffee is at an awkwardly lukewarm temperature. Boom, time and coffee down the drain. My team tried to solve this problem by creating a coffee cup that lets you know when it feels neglected. Just a few minutes before the coffee gets too cool, our mug sounds a tone letting you know that it’s time to chug that caffeine before it’s too late.

This is the wiring involved in the project. The Proof of Concept is protoboarded together to fit inside the plastic disk. In the circuit, the input is the temperature of the coffee and the output is the alarm that lets you know to drink your still warm, caffeinated liquid.There were two fundamental challenges with this project:

(1) Converting the DC power into AC so that the speaker would work. I tackled this obstacle by hooking the DC current and speakers up to a circuit routed through a 555 timer integrated circuit. The way we wired the inputs and outputs of the chip allowed it to be used as an oscillator, which drove the speaker circuit. The values of the resistors and capacitors affected the frequency produced by the speaker.

(2)  Getting the alarm to sound when at a particular threshold with increasing resistance. I was able to make this happen by creating a voltage divider transistor circuit. I used an NPN MOSFET transistor to connect our speaker circuit to Vout and connect our thermistor from the gain to ground. Setting an appropriate 13 kiloOhm resistor as the Vcc to gain resistor, I effectively created a voltage divider that would only supply the 1.8 volts needed for the resistor to turn out with a Vcc of 9 volts. I set it up so higher resistances turned the circuit on and lower resistance turned the circuit off, seeing as we wanted the alarm to sound when the coffee was getting cold (and hence when the resistance of the thermistor was going back up past a certain threshold).

The Proof of Concept was succesful for a standard cylidrical coffe mug and produced a buzzing noise when the temperature of the coffee lowered. One of the challenges of putting this into production would be devising a way to have the thermistor be in contact with any sized cup, including cups with curved sizes and extruded bases.