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
ITEM BASE PRICE QTY TOTAL PRICE LOCATION      
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









TOTAL PROJECT COST:      $89.03


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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