Step 7: The Process: Rapid Prototyping

In reality, what I did was look through the Digikey catalog, where I only found the DN7418 inductor (the other one was somewhat hidden in the RF inductor section). And it showed up before the Mouser box, so I spent an hour or two making up a prototype.

The circuit itself is simple, I want one large electrolytic cap for low frequency smoothing on the battery, and an output cap pair (electrolytic and one ceramic cap for high freq. smoothing). I also need the chip, a reference voltage capacitor, the inductor and a schottky diode to finish off the boost regulator. I happen to have some 1N5818's, which are often used as schottky diodes in boost regulators. I also need a USB type A female jack, of course, and two holes to solder the battery pack into. You can compare the schematic to the topology diagram in step #3 keeping in mind that this chip has an internal transistor switch.

All these parts must fit into the space left over from the battery pack. I make EagleCAD library parts for the inductor and chip (the rest are already there) and lay out the board. I'm not going to detail making library parts in eagle or pcb layout, others have done so already. Use whichever software you want, I like Eagle because there's a free version available for download if you're just making small PCBs.

Since I am know this is just a prototype version, I make the PCB single sided -- for easy etching. I also make the traces really large. I print out a paper version of the PCB and punch the parts through to verify that they're the right shape/package.

I get my etching setup together, turn on the heater for the etching tank, and print out a bunch of tiled PCB layouts on toner transfer. I transfer the toner onto a single sided PCB and etch it in the tank

Then I clean off the toner transfer, drill the holes with a dremel drill-press with carbide drill bits, and cut out the shape.

Then I solder the parts in, and fit it into the case with the battery pack, using double-sided foam sticky tape to hold down both the battery holder and the PCB without shorting the PCB to the metal tin.

OK, done!

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