- Power supply: It could be used with many sources of power supply: 24VAC, 24VDC or below.
- Power control of the heater: using MOSFET to switch on and off supply current through the heater with PWM signal from Microcontroller to modulate the MOSFET.
- Display: Using a 4×7-segment LEDs. Actually, I will use the 5461BS since I already have many of them in stock. I chose red LED over LCD because with a soldering station, we need a clear bright display and the LCD characters maybe too small for my eyes.
- Temperature adjustment: through an analog variable resistor (pot). I just don’t see why they use button to adjust temperature. It’s probably faster and more convenient to adjust if we use a pot. Imaging you have to push the UP button 20 times to get to 200°C (assume 10°C/step). Pfff! Really, I will go with the pot a hundred of the time.
- Heat sensor amplifier: I don’t know any better choice than an op-amp. The op-amp has to be a single rail power supply one. After considering 2 cheap op-amp LM358 and LM324, I chose LM358 because I only need 1, maybe 2 op-amp unit, while LM324 has 4 units.
- Additional functions: ambient temperature sensor using TMP101 – an I2C temperature sensor chip from TI; 32768Hz crystal for real-time display while on stand-by mode; a brush-less fan used for sucking soldering smoke.
- Most important: the brain of the system. As I said on my previous post, I have many PICs in stock. So I will choose a PIC that has enough pins required, cheap if possible. I considered PIC16F88 (18 pins), PIC16F873 and PIC16F876A (both 28 pins). I finally chose PIC16F873. If the firmware need more space, we can always migrate to 876A with double programming flash.
That’s it. Next time I will go over all requirements, one by one.