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Add sleep mode

A project log for TheSixthSense

Develop an extra sense for a better orientation.

Sebastian FoersterSebastian Foerster 09/06/2015 at 21:130 Comments

woohhhooo... My project made it into the Top 100 chosen semifinalists. How great is that?

A big thanks to all supporters and especially a big big thanks to Reiner for his video support (coming soon) and Reinhard for his English lessons and video support :)

I have recently added a new software feature: a sleep mode when the battery gets low. This is important to improve the overall battery lifetime. Of course, a good battery like the one I use has a deep discharge protection. Unfortunately the protection trigger is a bit late ( ~ 3 V). It's always a good idea not to discharge a Lithium-Ion battery below 3,4 V.

For TheSixthSense PCB it's of major importance to use a LDO with a very low Input Quiescent Current. The Microchip MCP1700 is a good and cheap solution. The 2,5 V version has a typical Iq of 1.6 µA and at higher temperature only 4 µA @ Iout = 0 A.

The next crucial part is a good schematic design. You should design a voltage divider to measure the battery voltage with a "disconnect function". A P-MOS at the positive voltage rail of the resistor divider is used here.

The main advantage of parts that have good sleep modes is that they don't need to be disconnected from the voltage supply. Like the LSM303D and the ATXMega32E5:

LSM303D: Current consumption in power-down IddSL = 1 µA typ.

Atmel ATXMega32E5: from Datasheet Figure 37-17. Power-down Mode Supply Current vs. Temperature -> @ 25 °C, ~2,5 V => 0.15 µA (no Watchdog, no POR, no RTC)

Okay, there are 0.15 µA + 1.6 µA + 1 µA = 2.75 µA in theory.

How is it in practice?

It’s not so easy to measure those low values, but I measured it with my cheap and out of range multimeter (2000 µA range). The result is a toggling value between 2 and 3 µA ! :)

You can easily verify this reading with 1 Meg Ohm +- 10 % resistor as additional parallel load.

Toggling 2 to 3 µA reading:

verify the multimeter reading with an additional 1 Meg Ohm load (2,5 V / 1 Meg Ohm = 2,5 µA additional):

~ 2.5 µA current consumtion is almost low enough because a LiPo has a standard self discharge current in this region and higher.

Check out my new git commit for the software update….

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