Lack of BOMs on OSHW projects is frustruating....
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Re: Lack of BOMs on OSHW projects is frustruating....

by westfw on Mon May 07, 2012 6:50 pm

You could easily mention on your BOM that values between X and Y are acceptable for this part

I don't think you can, without confusing more people than you help. The possible values for the Arduino RESET pullup resistor range from infinity ("you don't need it at all") to quite low values (ye old 2.2k TTL standard?) People with some background can see that, people without the background would just be more confused.

labeling all parts in the schematic for power dissipation and voltage rating

So which of the resistors on an Arduino or Chumby Hacker Board need to have power dissipation or voltage ratings? Every TH Arduino I've ever seen has a PCB layout for 1/4W resistors (~2x7.5mm), but I don't think any of them NEED to be 1/4W (vs 1/8W. Supported by the SMT versions...) OTOH, "more than 1/8W" is supposedly possible to draw from an AVR pin...
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Re: Lack of BOMs on OSHW projects is frustruating....

by Technobly on Tue May 08, 2012 1:04 am

inventorjack wrote:
FightCube wrote:I just created a spreadsheet for my Arduino Pro Mini v12/v13 (5V) BOM. If you are interested check it out here:
https://docs.google.com/spreadsheet/ccc ... mE1YlN5MUE


I liked your idea enough to start building something similar for my own projects. If anyone finds it useful, feel free to use and adapt to your needs:
https://docs.google.com/spreadsheet/ccc ... jY0LW8xRWc


Thanks! I like your take on the BOM coloring... and the package size column is a good idea.


westfw wrote:
You could easily mention on your BOM that values between X and Y are acceptable for this part

I don't think you can, without confusing more people than you help. The possible values for the Arduino RESET pullup resistor range from infinity ("you don't need it at all") to quite low values (ye old 2.2k TTL standard?) People with some background can see that, people without the background would just be more confused.


You like to complain I see. Agree to disagree. Certainly if you posted a mini explanation next to each "range" of values on a schematic... people wouldn't be confused. However the schematic would be "busy". Even your "explanation-complaint" for the range of RESET pull-up values is not confusing per say. A simple design like an Arduino Pro Mini would be trivial to write up a design description for, that could explain everything without cluttering up the schematic. I'll race you to writing one... ready... go!

westfw wrote:
labeling all parts in the schematic for power dissipation and voltage rating

So which of the resistors on an Arduino or Chumby Hacker Board need to have power dissipation or voltage ratings? Every TH Arduino I've ever seen has a PCB layout for 1/4W resistors (~2x7.5mm), but I don't think any of them NEED to be 1/4W (vs 1/8W. Supported by the SMT versions...) OTOH, "more than 1/8W" is supposedly possible to draw from an AVR pin...


I would almost never place voltage ratings on resistors.. that's kind of silly actually. More like capacitors, zeners, diodes (more so in high voltage designs), mosfets, bjts, tvs... I know we are kind of talking a lot about Arduino's in this thread.. but my comments were more general in nature, as is this topic. It is typically a good idea to label the components I mentioned... especially if you don't list all of the MFG part numbers somewhere first.

You are missing the point on your own example... sure, the 1/4W TH resistors don't need to be 1/4W... but actually... that IS the footprint that is on the PCB. And if you are looking to buy those resistors, and didn't know what should go there... 1/4W labeling would help get you in the right direction. Sure you could also stuff a 1/2W in there without problems to the circuit (if you think like someone that knows electronics and not someone that knows ART or MUSIC), but it might have to be a little crooked or tipped up on end... or it might not fit in the holes at all.

The point is... "Lack of BOMs on OSHW projects is frustrating"!

Provide a BOM with as much information as you can bare to document... end of story.
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Re: Lack of BOMs on OSHW projects is frustruating....

by westfw on Tue May 08, 2012 2:04 am

So would it be better to supply part numbers that you THINK would be correct, or not, if you haven't actually tested them?
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Re: Lack of BOMs on OSHW projects is frustruating....

by adafruit_support_bill on Tue May 08, 2012 6:49 am

(if you think like someone that knows electronics and not someone that knows ART or MUSIC)

I wasn't aware there was a difference. :D
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Re: Lack of BOMs on OSHW projects is frustruating....

by Technobly on Tue May 08, 2012 7:33 am

adafruit_support wrote:
(if you think like someone that knows electronics and not someone that knows ART or MUSIC)

I wasn't aware there was a difference. :D


I.e., if you don't KNOW electronics, then yes there is a difference. Can you see an ART major recreating one of your OSHW designs, not having a BOM, and THINKING hey... "I can just substitute this 1/2W resistor for this 1/4W one without any problems" or "I can just substitute this 25V 10uF cap for this 16V 10uF cap, and I know not to put the 6.3V 10uF cap in it's place".

That said, ANYONE can learn electronics... if they have a desire to ;-) Learning how to substitute components properly is achievable by all, but still the point here is NOT without a BOM. Without a BOM I would say you need to have mad electronics skills to figure it out.
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Re: Lack of BOMs on OSHW projects is frustruating....

by westfw on Tue May 08, 2012 4:25 pm

Provide a BOM with as much information as you can bare to document.

I don't have any problem agreeing with that. I'm just pointing out that that may be harder than it sounds, and "as much as you can" might not be very much.
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Re: Lack of BOMs on OSHW projects is frustruating....

by Jack52 on Wed May 09, 2012 8:28 am

Some customers will be beginners and it is important provide them with details. I just spent 2 days in email exchanges with a customer trying to convince him to just solder all of the ground wires to one point and finish the project.

I usually include a complete BOM with my projects, and even a text file that they can use with the Mouser Project system. Also, I will include a graphic that shows how to connect the off-board components, as I find this is where a lot of novice builders have a problem.

Email support is critical too. I've debugged many a project from looking at closeup photos of the finished pcb.

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Re: Lack of BOMs on OSHW projects is frustruating....

by mctaylor on Thu May 10, 2012 5:27 pm

My thoughts are that BOM have several uses themselves, the first being they are the long-life project document, and can be thought of as being associated with a particular schematic revision, and that the BOM's first goal is that it should form the bases of what your purchasing department (if you had one) needs to know in order to place an order for the correct parts.

The second (internal) goal is document enough details that alternative yet as far as your product's use of the actual physical parts can be selected if necessary due to availability, or obsolescence. A MMBT3904 SMT transistor is electrically equivalent to a 2N3904 plastic through hole part, and would past schematic or spice design rules check, but it's a real pain to rework the part into even a single one unit of your product.

The parts list from a schematic drawing or PCB layout drawings may often not include all of these, typically banal, details necessary to keep your business going. Leaving you scrambling to remember correctly select an alternative part before the shipping deadline in twenty minutes before a long weekend begins...

I suspect the biggest lack of BOM is either existing kits not being designed by experienced professional engineers with actual product life-cycle (concept to being shipped), not just design, experience. I know that a number of kit makers are software or IT professionals, and as one of those keyboard cowboys, I can assure you I had to rediscover a fair bit of engineering best practices that would taken as being so self-evident it wouldn't need to be specified as a best practice.

Another possible reason I can think of would be that perhaps kit makers don't have it as a discrete document, but either combine it with, or split its would-be contents across, PCB documentation and purchasing & inventory records.

I take it most medium to large companies use component databases to manage a lot of these details for their products, but a possible "solution" would be to specify both the component's basic or root name (enough to get the correct datasheet, not a marketing "series" name) - which may sourced from multiple vendors, and the specific manufacturer's full part time, as would be used to ensure the correct part was ordered (e.g. PIC16F84A-04/P which includes the part revision, clock speed, and package type). And if you specify an URL for the datasheet, then extend that idea to include a link to octopart.com or adapt and extend AdaFruit's own PartFinder databases which gives key parameters for a given part, such that an alternative could be used.

Example to try to clarify the terminology I used: Replacing a National LM340T-12 with a TI UA7812, which are considered pin-compatible as far as I know, and I believe basically identical as far as technical specifications and assurances, or using Linear Technology's L7912 to replace your the LM340T-12, would be just about always a drop-in replacement, but it isn't considered an equivalent part, but a (pin-compatible) replacement.
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