After building the sensor node, it's time to check the power consumption and improve it !

Reminder

The first circuit is composed by:

• An Arduino Pro mini 3.3V @ 8Mhz (external oscillator)
• Ultrasound distance sensor HC-SR04
• NPN2222 + resistor
• Level shifter (3.3V / 5V)
• nRF24L01+ radio
• DC-DC Step up MT3806, adjusted to 5V output
• The circuit is powered by 3 AAA battery cells

First circuit enhanced:

• NPN for switching on / off the step-up converter, powering the HC-SR04 sensor;
• Power LED of the Arduino pro mini removed;

So, the source code and the hardware are a little low consumption oriented.
Let's see how we can go further !

First results

With the first circuit, I got those results (with full batteries).

Max consomption

• Arduino in normal mode
• nRF24 in normal mode (radio.powerUp())
• SR04 sensor and level shifter power up

Max power consumption: 11mA

Min consumption

• Arduino in sleep mode (with LowPower.powerDown(SLEEP_STEP_DURATION, ADC_OFF, BOD_OFF);)
• nRF24 in sleep mode (with radio.powerDown())
• SR04 sensor and level shifter power down

Min power consumption: 40µA

Duration

As an AAA batteries is around 860 – 1,200 mAh, the device could run in sleep mode more than 2 years if we assume that the battery will be entirely used.

Limitations

This first circuit uses 3 AAA batteries. As I already said before, when a AAA battery is full, the voltage is about 1.6V. The battery can be considered empty when the voltage goes down to 0.8V.

With this first implementation, V_CC is between 4.8V and 2.4V; I will not be able to use the batteries entirely, as my Arduino pro mini use a step-down regulator (to 3.3V), and I cannot predict the behavior if the current goes below 3.3V (even a little above, as the regulator drops the voltage down to regulate). Moreover, I'm not sure that the Arduino pro mini can run at 8Mhz with "only" 2.4V.

That's why I published some blog posts about the Arduino Pro Mini running at 1Mhz / 1.8V.

Next step

The next step will consists in:

• Use only two AA batteries to power the device (more compact, more power)
• Use an Arduino Pro Mini @ 1MHz (internal oscillator) to use the batteries as much as possible
• Find an other ultrasound sensor @ 3.3V to avoid the level shifter and the level converter