June 24th 2007

Current measurement for the second input circuit prototype:

Component Current drawn
Bluetooth 18mA during scan mode; 28mA during data transferring at 9600bps
PIC 3.25mA
LED 1.35mA/ 1LED
Sensor 0.28mA/sensor at max switch current
Total 31.25mA when sensors are not activated; Ranging from 32mA to 52mA when all 12 sensors are activated and all 8 LEDs light up.

So the expected load will be around 50mA. With this result, I had considered several type of voltage regulator and battery solution. Finally, I decided to go for 1 cell of 1.5V battery and for that the voltage regulator I used is MAX1676 from MAXIM.
MAX1676 is a step up DC-DC converter with guaranteed start-up at 1.1V input and 500mA current limit. And with 1.2V input its efficiency is max when load current ranging from 10mA up to 100mA, which is exactly our input circuit current:
MAX1676 has an internal synchronous rectifier which means it does not require an external diode. Although this is a really nice feature which helps us to reduce the circuit size, I still use an external diode. This is because with a external diode, it is guaranteed that the start up voltage will be within the range from 0.9V to 1.2V when load current ranges from 10mA to 100mA. Hence, we can be more flexible with battery options.
So the supply circuit for input part would look like this:

I hope the new components will be delivered by this coming week so that I can test them out be for designing the final head PCB. In the mean time I need to work on the sensor design more carefully and design the final version for the main PCBs. I am targeting end of this coming week to finish the final PCB design.

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