The big picture of processes organization inside a soldering station system is depicted in the Figure 1. It shows the process blocks and their interconnections: temperature data acquisition, key switch reading, display multiplexing, display switcher, temperature controller, and powerline PWM actuation. While each block represent a state machine, the rounded corner block represent the shared data. The interconnection from and to the data represent the reading and writing operation, and the interconnection from one machine to other machine represent the operation of calling the other machine’s public function. Here are the functional descriptions of each blocks:
- Temperature Data Acquisition. There are two temperature sensors, the first is a thermocouple for measuring the temperature difference between the hot soldering iron and the cold junction, and the second is an LM35 temperature sensor chip for measuring the cold junction (which is the ambient temperature). The temperature reading from the sensors produce a very noisy signals, a digital signal processing (DSP) algorithm for noise filtering will be applied instead of analog filtering in the hardware. This digital signal processing would be helpful in reducing hardware complexity.
- Key-Switch Reading. There are only two button in this system: the up and down button. These buttons should be enough for setting up the temperature reference (the expected temperature). A brief press of up button will increment the setting by 1 ºC, and long press (press and hold for a second before release) will increment the setting by 10 ºC. The down button would do the decrements in the similar way. Reading the switches seems very simple, but it need to handle the false signal known as “bouncing-effect” produced by the mechanical switch.
- Display Multiplexing. Three digits of 7-segment displays is used to display the temperature setting and the reading of the achieved actual temperature. Multiplexing is used to keep the number of the required digital output pin as low as possible while maintaining hardware simplicity.
- Display Switcher. Since the 7-segment display shows only one temperature value by all of its digits, a switching mechanism is provided by this task to manage which value has to be selected to be displayed.
- Temperature Controller. An automatic control algorithm computes the actuation signal based on the expected temperature value and the actual temperature reading. Here one of the classic automatic control algorithm will be used: the proportional-integral controller.
- PWM Powerline Actuation. Pulse-Width Modulation (PWM) is used to control the power of electricity supplied into the solder heating element. The low frequency requirement for accommodating the zero-crossing control of the TRIAC prevents the use of the Arduino’s built-in PWM controller.
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