Arduino Nano [reference 1] is chosen for the digital soldering station project since it has complete features to implements all functions. Other microcontroller can be used as long as it has the following features:
- Eleven digital outputs to drive three units of seven segment displays, 3 outputs for activating the common pins (the employed seven segment displays can be common cathodes or common anodes), 7 output to drive the segments, and 1 output to drive the opto-isolated powerline driver.
- The digital outputs are able to drive at least 20 mA current both in sink and source modes, since the 7 segment display is driven directly (through a resistor) by microcontroller’s port.
- Two digital inputs to read the up-down buttons/key-switches.
- Two analog inputs to read the thermocouple sensor and the ambient temperature compensation sensor with at least 10 bit resolution.
- Internal counter for timing, and this timer or counter should run independently without cpu control. An interrupt handler that process the timer/counter’s interrupt (for example to produce longer bit counting/timing) is acceptable.
- Internal EEPROM or non-volatile memory for storing temperature setting.
The circuit schematic diagram shows no power supply circuitry, which should consist of a small adapter from 220V AC to 5V DC required by the Arduino microcontroller. We can easily built this small power supply using 100-350 mA 220VAC to 9-15VAC transformer, bridge diodes, capacitor, and 7805 linear voltage regulator.
The 7-segment displays are actually 3 independent units, which are connected in parallel except the common pins. The dot pins are not used, so only the 7 segments pins are paralleled. The digital inputs for the up-down button (S1 and S2) are A0 and A1, which are analog inputs by default of Arduino initialization, but here must be set to digital inputs by custom initialization.
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