So you want to design and build your own accessory. lets base it off the atmega 328 with bootloader on it (installing a bootloader on a blank is possible but lets face it for your first one its always good to know it will work) this is a “dip” package which is bigger but means you can solder it straight on or easily add a socket to swap the chip.
Parts you need:
Designing your board:
The usb host takes (D10-13) 4 pins [the spi communication pins]
- 10 Digital IO pins 0-9 [usually used for out but can do digital on off]
- 6 Analog in 0-5
Materializing your design:
Theres two ways to end up with your circuit ready to use in your hands.
- comercial pcb manufacture
Comercial pcb’s are nice, they can make the board more precisely, do more than two layers. add a stencil layer. and coat it to prevent corrosion.
Homemade are a bit more limited. but like a finger painting you accept the imperfections since you made it by yourself ! also you can turn out a single run ready to solder board in a hour.
Comercial is recommended for products you want to sell or mass produce.
If wanting to make the board yourself the transfer method is recommended. (there is also a light transfer method)
Etching using the transfer method can be quite easy, the hard part is getting a nice transfer. make sure the copper surface is clean from finger prints, if not clean with acetone or rubbing alcohol. Its possible to do a double sided board at hone with a laser printer. printing out each sides layout lining up and then ironing paper toner to the correct copper side. the paper will become stuck to the copper board and its best to submerge the whole board in water then gently
rub away the paper hopefully leaving the toner stuck on the copper board.Check over the design for any imperfections and fix them with a etch resist pen. cut the board to size and its ready to be etched
- Ammonium Persulphate
- hot water.
- non metal stirrer
it’s best to pick a bowl the right size for the board your board so that the board will be able to be submerged easily. use the thermometer to wait for the temperture to be xx degrees then add the required amount of ammonium perisulfate and stir vigorously agitating, it is possible to etch at room temp it just takes more like an hour. amonium perisulfate is dangerous it will stain clothes so take care while handling and mixing.
after a few(or many) minutes you should start seeing the edges of the copper disappearing. and keep going until all the exposed copper is gone.
Now fish the board out and drop it in the water bath. to clean it and pull it out and dry it. acetone is excellecnt for rmoving toner form the board. now just drill your holes and solder up the board.
22g per 100ml
Design suggestions: for circuits that require 3.3v sensors and 5v sensors.
Make use of the solder jumpers “SOLDERJUMEPR_2WAY” to change between a 3.3v and 5v for sensors less chance of frying a sensor that demands 3.3v. a made up board will have no default. with you or the end user having to solder bridge the jumper makes you stop and think which to use. the cool benefit is if you want to change sensors at a later date you can just de-solder one jumper and bridge the other voltages gap.
Having the senosors as 3 pin jumper holes allows sensors to have long leads leaving applications of your product flexible. such as if you need a thermal sensor a distance away from the board.