Dec 14, 2011

Halloween Build Part 5: Bill of Materials

In this final installment of my build log for my Halloween LED Flasher project, we will look at the BOM tables for this build. When I first started out in electronics, I often used BOMs from others online to get a sense of how the circuit worked as well as a feel for what are considered “jellybean” parts. If you don’t know that means, it basically refers to parts that have become the defacto standard when you are trying to create a particular piece of your circuit that is well researched and has been perfected over the years. For instance, switch-mode power supplies often use the TL431 programmable shunt regulator as a replacement for an op-amp in their feedback paths. The part gives you all the functionality of an op-amp with lower power consumption and, in most cases, faster transient response. It is widely recognized among power electronics engineers as a necessity in voltage-mode feedback power supplies.

My hope is that with this last piece I will have given enough information for someone to easily reconstruct this project to meet their own needs. I have tried to give out every piece of information I could think of related to my build, but leave comments if you are looking for more (type of solder??). Now…the final tally…

Bill of Materials
Large Craft Pumpkins $4.00 4 $16.00 Oriental Trading
Plastic Art Containers (LED Diffusers) $1.00 4 $4.00
Artifical Pumpkin Carving Kit $4.00 1 $4.00
0.33uF Electrolytic Capacitors $0.26 4 $1.04 Allied Electronics
LT1121CZ-5 Voltage Regulator  $2.60 4 $10.40 Digi-Key
JST Connectors $0.75 4 $3.00 Adafruit
PCBs $2.19 2 $4.38 Radio Shack
SPST Switches $3.19 2 $6.38 Radio Shack
Papers Bowls $1.98 1 $1.98 Walmart
Matte Black Paint $0.97 1 $0.97 Home Depot
9V battery holders $1.19 2 $2.38 Radio Shack
Machine Screws (#6-32) $0.47 2 $0.94 Home Depot
Nuts for #6 machine screw $0.48 1 $0.48 Home Depot
Enercell 9V battery $11.00 1 $11.00 Radio Shack
1/4W Resistors $0.64 2 $1.28 Parts Express
Various Colored LEDs $0.89 12 $10.68 Evil Mad Scientist Labs
20-pin IC socket $0.59 4 $2.36 Radio Shack
Microchip PIC16F690 $1.19 4 $4.76 Microchip
Heavy Duty 9V Snap Connectors $3.00 1 $3.00 Radio Shack


A few notes on this BOM before we wrap this project up completely. First and foremost, notice that I have listed two different prices for each of the elements in this design. The first is the cost of the package to get the components regardless of the number I used. For instance, I put 1 for the ENERCELL batteries because I got four 9V batteries in 1 pack. Therefore the cost associated with that pack is for four batteries instead of one. Likewise, I listed the total price of two packs of ¼W resistors even though I only used one full pack and two from another pack. I was only able to buy the resistors in sets so the unused parts still contributed to my overall project cost. In the past I have ignored these kinds of costs and just used the “per component” cost in my BOMs. From now on I am going to list the total cost to procure each item regardless of the quantity they are sold in.

Secondly, you can save a fair bit of money by switching to the MCP1702 voltage regulator from Microchip instead of the one I used here. With that regulator the total project cost drops to $80.55, just under 10% lower than the current BOM estimates.

Be prepared to spend some money on shipping if you decide to tackle a similar project. I did not include shipping costs or tax in my BOM because a lot of it varies based on location and method. Plus, I am way too lazy to look back and figure out how much I paid for shipping on each item. My personal feeling is just that it doesn’t make sense to include shipping costs in these sorts of things.

That’s all folks. I have taken this project from an idea to a tangible piece that I can improve on and build from in the future. I think we can finally put Halloween behind us at To The Rails and move on. Cheers to the next step…


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