We can optimize power consumption based on the use case at hand. Even for the discussed solutions, alternate hardware can provide a more optimized implementation.

Let's discuss the power consumption of the above solutions under various settings.

a) Power Specification of Components

First, lets look at the power specification of each component used. Based on the measurements by RaspiTV and Pidramble [15] [16], here are some estimates:

• Raspberry Pi Zero (Idle) = 80mA * 5V = 0.4 W[Watts = Voltage x Current]
• Raspberry Pi 2B (Idle) = 220mA * 5V = 1.1 W
• Raspberry Pi 3B (Idle) = 260 mA * 5V = 1.4 W
• Raspberry Pi 4B (Idle) = 540 mA * 5V = 2.7 W
• RPi Zero 1.3 & Pi cam = 180 - 230 mA = 0.9 - 1.1 W (For 720p - 1080p)
• Raspberry Pi 3B and Pi cam = 460 mA * 5 = 2.3 W (To shoot 1080p video)
• Raspberry Pi 4B and Pi cam = 640 mA * 5 = 3.2 W (To shoot 1080p video)

You can drop the frame resolution of Pi Cam to save some power. RPi Cam draws 260 mA to shoot 1080p while only 180 mA for 720p. Raspberry Pi Zero 1.3 with Pi Cam draws only 0.9 W power while 720p video is being shot.

Now consider Movidius NCS and LIDAR,

• Movidius NCS 2 = 180 mA On 5V USB = 0.9 W

Based on RPLIDAR A1 Power Supply Specification [17],

• RPLIDAR A1 M8 = Scanner system + Motor system = 300 + 100 (Work Mode) * Voltage = 400 mA * 5V = 2W

For Door Access Control and Indoor Navigational Assistance use cases, you can usea Proximity Sensor instead of LIDAR.

• HC-SR04 Ultrasonic sensor = 15 mA * 5 = 75mW
• Grove Ultrasonic sensor = 8mA * 5 = 40 mW

For Luxonis OAK-D DepthAI Hardware, the total power consumption usually stays around 800-900ma, with cameras and the DepthAI SoM. The power expense of OpenMV Cam H7 is less than 150 ma andPico4ML is 40 ma in idle mode and 60 ma while running ML models.

b) Power Requirement of Solutions

The solutions with mathematical hacks are feasible to execute on Raspberry Pi Zero 1.3 at least power cost. It is trivial to port the above solutions across various models of Pi or OAK-D, as it supports OpenVINO and MYRIAD. Even to Depth AI hardware like OpenMV Cam H7 or Pico4ML, it is easy to port some use-cases with serious power gains.

The power requirement of ported, OAK-D, OpenMV Cam & Pico4ML are given under "Ported", "OAK-D", "Open MV", and "Pico4ML" heads respectively.

1) ADAS - Collision Avoidance System on Indian Cars

Better run this on a recent RPi model, as response time is critical here.

• Current: RPi 4B + Cam + Movidius + LIDAR = 3.2 + 0.9 + 2 = 6.1W
• Ported: RPi 3B + Cam + Movidius + LIDAR = 2.3 + 0.9 + 2 = 5.2 W
• OAK-D: RPi 3B + OAK-D = (260 + 800) * 5 = 5.3 W

2) Indoor Robot Localization with SLAM

If no point cloud visualization, it is enough to use Raspberry Pi 2B or 3B.

• Current: RPi 4B + LIDAR = 2.7 + 2 = 4.7W
• Ported: RPi 2B + LIDAR = 1.1 + 2 = 3.2W

3)Touch-less Attendance & Door Access Control

This solution can run on Raspberry Pi Zero 1.3with Movidius, though the frame rate can fall from 12-20 to 4-8 FPS which is decent enough. LIDAR can be replaced with an ultrasonic sensor for depth perception.

OpenMV Cam H7 has Haar Cascade Face Detection, but better to use DL for Face Recognition in a security use case. However, OpenMV Cam is a good alternative as eye-tracking & optical flow is handled in the hardware.

• Current: RPi 3B + Cam + NCS + LIDAR = 2.3 + 0.9 + 2 = 5.2W
• Ported: RPi 0 + Cam + NCS + U-sonic sensor = 80 + 180 + 180 + 8 =2.24 W
• OAK-D: RPi 3B + OAK-D = (260 + 800) * 5 = 5.3 W

4) Indoor Navigational Assistance for Blind & Elderly

This solution can easily be ported to Raspberry Pi 2B but it's ideal to use OAK-D here as it can do depth sensing, object detection, and tracking as well.

• Current: RPi 3B + Cam + Movidius = 2.3 + 0.9 = 3.2W
• OAK-D: RPi 2B + OAK-D = (220 + 800) * 5 = 5 W

5) Smart Cam with Gesture Alarm for Women Security

Our efficient solution based on linear algebra can be executed on Raspberry Pi Zero. However, OpenMV Cam is ideal for this use case, as it can do circle detection, and blob centroid detection based on color, in the hardware. However, you can fetch a huge power gain, if you use Pico4ML itself as the gesture object, as it has an IMU onboard.

• Current: RPi 3B + Cam = 460 mA * 5V = 2.3 W
• Ported: Raspberry Pi Zero 1.3 + Cam = 0.9W
• OpenMV: Less than 150 mA * 5V = 0.75 W
• Pico4ML: 50 mA * 5V = 0.25 W

6) Security Barrier Cam using Shape Context

The mathematical hack based on Shape Context can be executed on Raspberry Pi Zero, by replacing Pearson's chi-squared test with cosine distance to compare log-bin histograms.

• Current: RPi 3B + Cam = 460 mA * 5V = 2.3 W
• Ported: Raspberry Pi Zero 1.3 + Cam = 0.9W
• OAK-D: RPi 2B + OAK-D = (220 + 800) * 5 = 5 W

7) Monocular Social Distance Tracker

The depth inference and distance compute runs on another remote machine.

• Current: RPi 3B + Cam = 460 mA * 5V = 2.3 W
• Ported: Raspberry Pi Zero 1.3 + Cam = 0.9W

8) Worksite Helmet Monitoring

More industrial situations can be handled by adding Depth AI to this solution.

• Current: RPi 3B + Cam + Movidius = 2.3 + 0.9 = 3.2W
• OAK-D: RPi 2B + OAK-D = (220 + 800) * 5 = 5 W

WiFi may be required for some of the solutions above. Note that the Pi further consumes 170 mA when the wifi is turned on. To compensate, you can drop the frame resolution from 1080p to 720p to save nearly 100 mA. So the power expense remains more or less the same.

The speaker is an optional component in most of the use cases, even otherwise you can use a low-power USB speaker or audio HAT like this for sound. The Blinkt and LED SHIM are also optional components. Hence, they are not accounted for in the power expense computations.

c) Uptime Estimation on Battery

Let's try to estimate the uptime of the above implementations on battery, based on the power estimate under "Current" and "Ported" (considered same, as we can use the same code in another Pi model).

Battery uptime depends on the current draw. For 1000 mA current draw, a 10 Ah battery can last 10 hours.

Uptime in Hours = Battery Capacity (mAh) ÷ Current (mA)

Thus, on a 10 Ah battery, the above implementations,

• ADAS - Collision Avoidance System: can run 10 hours
• Indoor Robot Localization with SLAM: can run 15 hours
• Touch-less Attendance & Door Access Control: can run 22 hours
• Indoor Navigational Assistance for Blind & Elderly: can run 15 hours
• Smart Cam with Gesture Alarm for Women Security: can run 55 hours
• Security Barrier Cam using Shape Context: can run 55 hours
• MonocularSocial Distance Tracker: can run 15 hours

For some use cases, the actual up-time can be slightly lesser, as RPi draws more power under work mode. The OAK-D solution variant can last longer in real, as it draws only around 700-800 ma in work mode and much lesser in idle mode. Moreover, neural inference runs on OAK-D, freeing the host device resources.

However, note that both Gesture Cam and Security Barrier solutions qualify for always-on use cases, as they draw the least power, despite doing on-device sensor data analytics. Moreover, Gesture Cam can run 66 hours on OpenMV Cam H7, and around 200 hours on Pico4ML.

However, Pico4ML requires itself to be used to signal gestures, which may not be practical. Hence, OpenMV is the most ideal choice for Gesture Cam because it can do circle detection, and color mask on the hardware itself.