May 18th 2007


Use 3 push buttons made from QTC pill as inputs for emoticons.
Use 4 low power LED (those can run at only 2mA) for force level display.
Supply PIC with 3.3 V instead of 5V.
Since we need to migrate everything to surface mount, I found a Bluetooth module which seem appropriate which is BR-C40 ( )

I’m still thinking about which battery to use.

Quick measurement of the current drawn from the input circuit we have now:

set up Current drain measured Current drain expected
1 sensor at max drain 0.52mA 0.4mA
PIC circuit w/o bluetooth 10.58mA 10mA
circuit w bluetooth idle 18~22mA 27mA
circuit at max 1 sensor 142~145mA

Note: when we change the LED type the total current drawn would decrease much more.

Battery type Capacity Voltage design solution Pros/Cons
N Alkaline 2000mAh 1.5 at least 3 cells in series Small cell size but need 3 cells. Capacity and voltage varies with load thus making it unstable
AAA NiCad/NiMH 1000mAh 1.2 at least 3 cells in series Rechargable. Voltage decrease gradually. 3 cells will increase the size factor.
AAA LiIon 300mAh 3.6 1 cells Rechargable but quite dangerous if mishandled. Small size factor. Stable performance

So now if we use normal alkaline battery we can supply the current circuit up to 50 hours at rest and 13hrs if sensors are pressed constantly. However normal battery will have their voltage drops after time. Besides 3 cells design will make our device bigger.

Using rechargeable battery, especially using LiIon battery will give us 3.6 V with just 1 cell. However the capacity of the AAA LiIon is not high. We can consider using prismatic LiIon cell like those in mobile phone which give us same voltage but around 900mAh. Also we can consider use the AA LiIon which give 2400mAh and 3.7V output, but its weight is around 47g which is much heavier than the AAA one.

It seems that LiIon battery is the best choice since with just 1 cell it can deliver 3.7V for our circuit.
I found very nice reference design that use polymer LiIon cell. That is the iPod shuffle, the smallest iPod.

and here is how the polymer LiIon prismatic cell is placed.


However there is one issue with the LiIon battery, it may explode if left over charged or short circuit occurs. Hence if we are adopting this type of battery, we will need to add in the protection circuit.

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