senseRev4_Power_Calculations

Introduction

This page will give information about average current consumption of SensePi Rev4 in different operating states. This data is used to calculate the battery life of SensePi Rev4 for 2 AA Ni-MH Batteries. We cannot directly measure average current with Multi-meter over a longer period of time. So for that we use uCurrent. uCurrent is a device which gives voltage proportional to input current without inducing high burden voltage. To take measurements in the 10s of mA and uA a modification has been made in the uCurrent.

Block diagram

As shown in block diagram, uCurrent is connected in series between battery and SensePi Rev4 board. At output side we will get voltage proportional to input current. This proportionality constant can be changed according to requirements. There are 3 different voltage current ratios available for this modded uCurrent are :

1. 1mv/nA
2. 1mV/uA
3. 100mV/mA

We have used 2nd and 3rd ratios for different calculations.

The average value for current consumption is calculated over 5 sec for different states of SensePi Rev4.

Snap shots of tests conducted:

These are the snapshots of voltage waveform at output port of uCurrent in different states. For this 'Analog Discovery 2' is used as an oscilloscope. Here we can observe voltages proportional to current consumed by SensePi Rev4.

Formula to convert voltage reading in current :

I = (conversion factor) * voltage (unit)

1. Advertisement:

This is the first state related to Bluetooth communication. Here green and red leds are working along side with BLE(Bluetooth Low Energy) stack. In this state SensePi Rev4 is ready for connection.

For this uCurrent was set in a mode where it was giving 100mV for every 1mA. So conversion factor will be '0.01' and unit will be 'mA'.

Here that triangular shape that we observe is due to PWM. As we change duty cycle of digital signal, it also changes the analog amplitude of signal. So here Duty Cycle is first increased linearly and once it reaches 100% then it's decreased linearly to 0%. And this cycle is repeated over and over again.

PWM signal is given to both Red and Green LED. By combining both color we get Orange color.

2. Connected:

This is the second state related to Bluetooth communication. Here only green LED is working with BLE stack. In this state SensePi Rev4 is connected and communicating with mobile device.

For this uCurrent was set in mode where it was giving 100mV for every 1mA. So conversion factor will be '0.1' and unit will be 'mA'

Here the triangle you observe is also due to PWM only.

PWM signal is given to Green LED only. Therefore lower current is consumed compared to both red and green LED combined.

3. Sensing only:

This is the mode where SensePi Rev4 will be working most of it's time. Here it's in sensing mode but there is no trigger has occurred. This mode is supposed to consume minimum and almost constant current. So for that we took 2 readings to see the difference. First reading is taken over the time span of 100ms and second is taken over the time span of 1s.

For this uCurrent was set in mode where it was giving 1mV for every 1uA. So conversion factor will be '1' and unit will be 'uA'

Sensing for 100ms:

Sensing for 1s:

As this can be observed that average current consumption in both the cases is ~18uA. And this was verified using Multi-meter.

4. Sense and Trigger with LED feedback:

This is the same state as above. But here average is taken over 5 sec. In this window of 5 sec, device is triggered by motion sensor once. On every trigger processor wakes up to serve that trigger. And in first 10 mins if it detects any motion it'll blink the red LED as feedback.

For this uCurrent was set in mode where it was giving 100mV for every 1mA. So conversion factor will be '10' and unit will be 'uA'

5. Sense and Trigger without LED feedback:

This is the same state as before but after first 10 mins SensePi Rev4 will not give any feedback and hence it'll not consume as much current as before.

For this uCurrent was set in mode where it was giving 1mV for every 1uA. So conversion factor will be '1' and unit will be 'uA'

Average current consumption in different states:

All the snaps above gives values in mV only. Those values are converted by using proper conversion factor and units and represented in tabular format.

State	Average Current Consumption	Unit
Advertisement	7.5449	mA
Connected	2.1738	mA
Sensing Only	18.2	uA
Sense and Trigger with LED feedback	365.9	uA
Sense and Trigger without LED feedback	20.4	uA