Power Analysis

A project log for Star Trek Communicator Badge

In the true spirit of Star Trek, this communicator badge is completely autonomous, while fitting in the form factor of an original badge

JoeJoe 07/02/2017 at 21:150 Comments

I am looking at bringing the system power down to extend battery life. The power breakdown in idle mode is as follows:

ComponentActive RX PowerSleep PowerAvg. PowerNotes
RFM69HCW17mA0.7uA9mA50% sleep duty cycle
Teensy LC5mA150uA2.5mA50% duty, w/ bootloader
Microphone155uA155uA155uAAlways on
LIS3DH11uA11uA11uAAlways on
Totals22.16mA / 73.1mW317uA / 1mW11.66mA / 38.5mW

The battery I have chosen has a capacity of 150mAh @ 3.7V. The system runs at 3.3V so energy rating of the batter at 3.3V is 150*(3.7/3.3) = 168mAh @ 3.3V. This means the standby time is 168/11.66 = 14.4 hours. That's a full day of standby time!!!

I think we can push the sleep duty cycle longer than 50% to save even more power. Probably by as much as 80% duty.

Now to measure the full system during streaming mode:

Here, I put the whole system into stream mode and measure the supply current (measured on the battery voltage rail).

ModePowerMax Duration (based on 150mAh)
STREAM_RX37mA @ 3.7V4 hours of talk time
STREAM_TX (31 power setting)39.6mA @ 3.7V3.8 hours of talk time
STREAM_TX (15 power setting)38.9mA @ 3.7V3.85 hours of talk time
STREAM_TX (0 power setting)38mA @ 3.7V3.9 hours of talk time

Wow, 4 hours of talk time! There must be something missing... there is, the speaker. The speaker driver can source as much as 120mA when driving a lout signal. Therefore The talk time is limited to more like 1 hour of receiving.


12+ hours of standby

1+ hour of talk time

Actually, not too far off from an old Nokia bar phone...