There are several ways of doing this, but one very common way is using the Modbus protocol. The protocol doesn't specify the eletrical interface. I'm going to use RS485. For processing, I'm going to use an ATMEGA8, since it has hardware USART and I2C. The I2C I'm going to use to interface with the sensor, a LM75 temperature sensor. Other hardware bits are LM7805 linear regulator with a Vmax of 30V and a MAX485 for the serial interface. All components except the block connector are surface mount.
The board was laid out in EagleCAD, which was a first for me. I wanted to learn how to use EagleCAD, since so many other hobbyists do. The board came out nicely, I even added a little silk screen art to the bottom in form of a QR code.
The board outline is 5cmx3.5cm, and double sided. I opted for a 1.6mm PCB thickness for rigidity, and green PCB, white silk-screen and HASL for my wallet. I sent away the design to a PCB prototyping service.
A stack of PCBs arrived at the post office a couple of weeks later. By then I had sourced all the components needed to build the ten boards. Well, there was one component I couldn't source, and that was a the one-way SMT DIP switch. If I make another revision of the board, I will make that one through-hole.
I took me a while to hand solder, as I don't have the equipment to reflow solder. Perhaps it is time to invest in one. Anyway, I think the board turned out well.
As you can see, the 1-way DIP switch is unpopulated. If I make another board revision, that would be one of the things I would change. The QR-code works well thought!
Next up is the code...
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