19 Februari 2013

PICMAN

For a long time now I've been wanting to make my own microcontroller-based prototyping board. My original motivation was the unjustified high costs for even the simplest boards (a basic Arduino for $30 - why???) and the challenge of designing something that anyone can make at home within a few hours, with parts that can be cheaply obtained on eBay.

Eventually, I came up with the PICMAN. It is: Based on Microchip's PIC18LF4553 - a 12MIPS microcontroller with 12-bit A/D, plenty of I/O, built in USB transceiver and tons of other coolness.A single-layer PCB design - ideal for DIY toner-transfer etching fabrication.Small form-factor that nicely fits on a solderless breadboard.Can be powered by USB/external 5V/external 8V-35V using on-board 1.5A regulator.Has a reset button, a user button a power LED (blue) and 3 user LEDs (red, yellow, green).Needs zero external components to work.Programmed with a bootloader, making it possible to download a program via USB.Can implement any USB device using Microchip's USB stack.Less than $7 total with easily obtainable parts (not including shipping costs, which are usually low if not free, and assuming that some of the small parts are bought in quantities).I made two pieces so far, each took a couple of hours' work, requiring some SMD soldering experience. I did the initial programming (bootloader image) with a PICKit2 programmer, after having to struggle a little with the Microchip provided bootloader firmware code. It really works nicely - I like it much better than most Arduinos that cost 5+ times as much and it was really fun to build.
Some ideas that I used in the design:
Mount the PIC at the bottom of the board, between the header pins.Use the copper layer for text to make it easier to locate individual pins.Use a mini-B USB connector for smaller form factor (than B) and availability of cables.Some assorted design/fabrication tips:
Edit the final PostScript file generated by Eagle with a text editor, replacing the last number in every line ending with "h" with 2000. This effectively resizes all holes to 0.2mm making them perfect for centering the drill bit.Add a cool logo on the final PDF with Illustrator.Print in 1200DPI on a transparency with a laser printer.Cut board with a large paper Guillotine.Use lamination machine for toner transfer. Let fully cool in air before gently removing transparency. Verify before etching, or otherwise scrape off toner and retry.1 part HCl, 2 parts 3% H2O2 for etching.Final cutting of the board to shape with tin snips.0.6mm holes for PIC and ceramic capacitor, 1mm holes for L7805 and all mounting holes (switch and USB jack), 0.8mm holes for the rest.1k resistor for the green LED makes it just as bright as the other ones with 330 resistors.Solder USB jack first. Have a lot of patience ready.
Here is the schematic, the layout and the final artwork (mirrored). The Eagle files, firmware images and programming software can be downloaded from here. I'll happily share eBay links, where all these components can be bought cheaply, just ask if you can't find any of them.
If anyone has any constructive comments, or has built one and wants to share it, feel free to comment below.

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