Autonomous Popcorn Maker

Background

The desired outcome for this design is to minimize human interaction with a popcorn making process, whether that be over a hot plate or in a microwave. It was observed that when popcorn is popped on a stove, the popping starts slow, becomes more frequent, and then tapers down to slow pops again. Therefore, the code utilized counters in a real time operating system to match the slower and more frequent popping patterns.

In this case a hotplate is used as the heating element, the microphone and microprocessor account for the popcorn pops, and the buzzer sounds when the popcorn is done popping. Furthermore, the design can be transported to different pots or pans with different heating elements. Some possible problems with this design include safety, and durability issues. This project involves a relay that switches on and off AC wall power, and misuse can lead to death by electrocution.

Technical Description

The basics of the system include the pot (or pan) which rests on the hotplate, with a microphone installed on the lid. The microphone, which connects to the 3.3V pin in the microcontroller, listens for the pops. The popping noise corresponds to some output voltage, which correlates to an analog input value that the microcontroller reads in. Notice the setup in Fig. 1:

Figure 1: System setup.

The popcorn pops were measured with the same microphone using an oscilloscope to see the voltage peaks. Notice, in Fig. 2, the voltage peaked at approximately 2.7 volts. The threshold of the pops was set around 2.7 voltage (analog input value of approximately 800). Each pop lasted between 1.5­3 milliseconds, which allowed the RTOS timer to be used.

Figure 2: Popcorn voltage spike.

With the hotplate plugged in, power source on, and the microcontroller on, a switch on the microcontroller will turn on the hotplate and initialize the microphone counter. Once the popcorn starts popping, the microphone will listen for pops that have spaces less than 0.5 seconds in between each pop. Once this happens, a count is added to a counter, thus once this counter reaches 3, another counter is introduced. This new counter uses the microphone to listen for pops that have a space of about 3 seconds in between each pop, however, if there is for any reason a pop in between a space of 1 second occurs, this new counter will reset back to zero automatically. This is to ensure that the new counter does not reach its threshold of 5 counts that turns off the hotplate. The idea behind the two counters is to accurately control when the hotplate turns off exactly when the popcorn stops popping. It was observed that when popcorn is popped on a stove, the popping starts slow, becomes more frequent, and then tapers down to slow pops again. Therefore, the counters are put in place to match the slower and more frequent popping patterns

Prototype & Design

It is also worthy to note that a better system for switching on and off the hot plate is recommended due to the high amperage requirement when the hot plate is running at full power. A better solution would involve controlling the switch inside of the hot plate, rather than splicing into the hot plate power cord and adding a relay. Additionally, a vibration system or pot shaking mechanism must be implemented because the popcorn at the bottom of the pan burns if not shaken or stirred.

Figure 3: Nomenclature of design and parts used.

A 10 amp relay was used in conjunction with one optoisolator for this task. Figure 3 shows 12 V from the power supply, which is regulated to 5V and run through the optoisolator, which then sinked in the microcontroller. On the other side of the opto isolator 12V is ran through going to the relay. A buzzer is also on the opto isolator which goes off once the popcorn is done. On the other side of this circuit, a microphone listens to the pops in the pot and sends data as voltage spikes, which is converted to an analog input that the microcontroller reads.

Code

Levon Markossian - 2021