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For hobbyists and engineers involved in prototyping, a reflow oven can be a very useful tool. With the ability to properly control the reflow temperature profile, a high quality solder joint can be achieved for SMD components and BGAs. Unfortunately the price of a commercial reflow oven is prohibitive for most of us... 

There are many implementations of the toaster oven reflow concept, however they generally have the MAX6675 (or 31855) IC in common. This is a thermocouple-to-SPI converter. This part is expensive and SMD only, meaning it requires its own PCB. A more practical solution for many is to use that old RS232 enabled multimeter which has been lying around the house. It even came with a thermocouple, remember?

Many low-cost multimeters use the same protocol, check this page to see if yours is listed. It is very easy to tell with a serial terminal application if your DMM uses the same protocol. Just look for fourteen repeating bytes in this sequence: 

0x1X, 0x2X, 0x3X, 0x4X, 0x5X, 0x6X, 0x7X, 0x8X, 0x9X, 0xAX, 0xBX, 0xCX, 0xDX, 0xEX 

where the X represents a random digit. The baud rate used is 2400bps. Yes, 2400.

Here is how it works: The DMM's opto-isolated output is connected to the UART RX input on the HU320 PCB, with the +5V line powering the photodiode in the opto isolator. Normally this is powered from the RTS line in a serial port. As data is received by the HU320, it is buffered internally and shifted to the host when requested. These requests happen automatically, and the user simply reads the data from a byte buffer in .NET, or subscribes to a data received event. 

The oven is controlled by interrupting the mains power line with a mechanical relay. A triac or SSR would of course be a better option, but the relay is fine for this demonstration. A single GPIO pin is used to toggle the relay, via a transistor. The control algorithm is simply a hysteretic type, with the relay output switching when the measured temperature deviates by more than two degrees. Despite the simplicity of this system, the control is precise enough for our purposes.  

Here is the complete interfacing code required to drive the relay and receive data from the DMM

The control quality acheived from even this simple implementation is quite impressive! Check out the plot below. The reflow profile is not quite right, it should be a bit quicker, however the oven used in this exersize is only 700W. A more powerful oven should be able to handle a steeper curve.