Tuesday 17 May 2011

Component Selection - The rest

Bloody Blogger! I just spent a few hours summarising how I spent a few weeks selecting components for the prototype BlueFlyVario, describing in detail why I choose what I did, etc. The ultra nasty autosave thing in Blogger saved over everything when I accidentally selected all of the text and deleted it. I really dislike trying to replicate my thoughts so this post is now going to be only a quick summary.

Microcontroller – I chose the PIC24F16KA101. It does everything I need (UART, I2C, Digital IO, Analogue input, etc), comes in DIP and SSOP packages and is programmable in C with free compilers. I got this (less than $3) along with a PICKIT2 programmer from MicrochipDirect.  Awesome speedy shipping.

BluetoothModule – I choose the Roving Networks RN42. This is going to be the most expensive component ($20) but provides a bunch of features that I like (adjustable baud rate, Bluetooth name etc, LED status output). It is well documented. If I choose to expand to making these varios for iStuff then there is an apple friendly version.

Voltage Regulator – Not sorted yet. I just used an old component I had lying around for the prototype. I will choose a SMD component in the next few weeks, probably a Microchip one (like the MCP 1702 – less than $1).

LiPo Battery – Also not sorted yet. I will get a 3.7V LiPo or LiIon battery, probably one I can get in quantity from DealExtreme (about $5 or so). About 1000 mAh should give a life of over 20 hours (the Bluetooth Module is the most power hungry component, about 40mA while transmitting). For the prototype I just used an old smart phone LiIon battery I had lying around.

Recharge Circuitry – I used the breakout from Sparkfun for the MCP73831T for the prototype. I can get this IC from MicrochipDirect for less than $1 for production prototypes.

SMD Resistors and Capacitors – I got one of those packets of 5000 resistors and 2000 capacitors in a wide range of values from ebay for less than $20. For the prototypes in the breadboard I used old components I had lying around. For low rate production prototypes the cost of the SMD components is negligible.

Circuit Board – I have not done anything on this yet. BatchPCB seems to be the standard for these kinds of projects. In volume production a double sided board with vias, etc. is likely to cost less than $1, but in the prototypes I expect it to be about $5 or so.

Case – Not chosen yet, but I know I need to prior to designing the circuit board. I reckon about $4 is about right.

LEDs , switch, usb connector, etc – I have just used old components to date. These should all be less than $3 in the production prototype.

In general I have chosen everything to be 3.3V compatible (same as the pressure sensor, meaning less power switching about.

Adding all of the above production prototype costs up (including the pressure sensor on the previous page - $15 for the MS5611) gives a total cost estimate of $55 for the production prototypes. 10% cost growth already!

Some of the components

Shipping Detritus