Towards a reflow oven controller

So, I found a suitable enclosure 😛 and went through my box of bits. I found an ATmega328 board that I must have built a while ago, that will be the microcontroller for this project. I dug out an LCD display and an Adafruit I2C backpack. The thermocouple and AD595 amplifier were already kicking around. What else? Ah, LEDs, you can’t have enough LEDs 🙂

The "concept"

The “concept”

The above shows the basic idea (yes, “enclosure” = “cardboard box” 😳 )

The red LED will reflect the oven state, green will just show that the unit is powered on and yellow will do, er, dunno, something or other.

I’ll use the bottom line of the 16×2 LCD to show what the buttons do. That’s all to be determined but you can bet that the red button will be “STOP” whenever the controller is running. Top line will show current status (IDLE, PREHEAT, SOAK, REFLOW, COOL) and the oven temperature. There will also be a programming mode (I think) where you can enter the desired temperatures and durations and save them to the on-chip EEPROM, not sure about that yet as I don’t know what I’ll need.

Power and button de-bouncing board

Power and button de-bouncing board

The lid now has the LCD and backpack connected. The buttons are also in place, as are the LEDs. Inside the box I’ve placed a board with 2.1mm power input (7-15V approx), voltage regulation and a button de-bouncing circuit consisting of 10KR pull-ups and 10uF capacitors (via 1KR) connected to an inverting Schmitt trigger IC. Press a button and the line goes “HIGH”.

Work in-progress...

Work in-progress…

I’m using the Pololu crimp pins (and the excellent, now I know how to use it, crimping tool) to make most of the connections between boards – it’s easier than soldering even though it will probably look a bit of a mess. The advantage of cardboard is that it’s easy to make holes for connectors – see the power connector at the bottom of the box.

Programming connector and thermocouple cable

Programming connector and thermocouple cable

Here, I’ve used a 6-pin connector for programming – compatible with FTDI programming boards and cables. The thermocouple just comes straight out the side of the box.

Nearly done

Nearly done

The inevitable hot glue is used to stick everything in place. There’s not far to go in the above, nearly all done…

I've seen worse (well I probably made 'em!)

I’ve seen worse (well, I probably made ’em!)

I’ve tested everything and it’s all working. Programming the 328, the thermocouple, talking to the LCD and flashing the LEDs – even the buttons. Now I just need to fettle the oven, add the GND and the wire for controlling the solid-state relay, only just remembered those 😳 , and put together the firmware. There are lots of firmware examples out there so it looks easy enough to get something together. I’ll concentrate on the PID aspects since good control of oven temperature is really what I need. Then I’ll go to town on the “bling” 🙂

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