Technical Specifications

How a Sentinel adaptor functions and interfaces with a trail camera

The Sentinel was designed as an easy-to-use module that connects to commercial trail cameras to upgrade them with data processing and transmission capabilities, without the need to modify the trail camera. Sentinel devices contain a custom printed circuit board (PCB) including a low-power system operated by a microchip SAM D21E microcontroller (MCU) (Microchip Technology, Chandler, AZ, USA) and a high speed Google Coral System-on-Module (Coral SoM) with integrated Tensor Processing Unit (TPU) (Figure 1.1a), enclosed in a protective weather-proof plastic housing (Figure 1.1b). The PCB can optionally include several communication modules including cellular (Blues Wireless module; Blues Inc., Boston, USA), LoRa (Notecard for LoRa; Blues Inc., Boston, USA), and satellite (Swarm module; Hawthorne - California, USA) in addition to WiFi. In the performance results presented herein, we focus on a hardware version that only includes the cellular module connecting. Similar to a Nvidia Jetson Nano, the Coral SoM is a single board computer with a specialized neural network chip (in this case a tensor processing unit - TPU) capable of processing trail camera images with neural network models. The Coral SoM runs a linux operating system (Mendel, a Debian derivative) that supports the TensorFlow lite framework. Furthermore, the PCB contains custom power management components, a trail camera trigger detection circuit, connections between communication and data processing modules, and a micro-SD slot to store captured images. The Sentinel is powered by 2+5 rechargeable 18650 lithium ion batteries and offers the possibility to be powered from an external solar panel. A series of LED lights allows users to track and verify correct functioning of the device in the field.

The Sentinel directly connects to the SD card slot of the trail camera via a USB-to-SD cable (Spypoint, Victoriaville, Canada; Figure 1.1b). The cable connects via the Mini-USB connector to the Sentinel PCB and replaces the normal SD card in the trail camera. This architecture allows for basic camera functions such as trigger speed, image quality, and battery life to be unhindered, and eliminates the need to manipulate the trail camera. Most major trail camera brands have been tested for compatibility, including different models of Browning, Bushnell and Reconyx. Connection problems are mostly related to correct positioning of the SD card in the slot and the cable pulling the card out while closing the trail camera door.

Figure 1.1: Sentinel hardware components and assembly. (a) Front and back of the custom PCB with all major components numbered. (b) Assembled Sentinel with trail camera. Sentinel and trail camera are deployed in close proximity due to the Spypoint cable connecting them. (c) Deployed Sentinel and trail camera.

Basic Function

The Sentinel registers the camera trap triggering. If the camera trap tigger count surasses a certain threshold or if a set time thrieshold passes, the Sentinel 'wakes up': In the case of cellular communication, which we focus on for this short description, the Sentinel microcontroller (MCU) first activates the cellular module, which attempts to connect to a 4G LTE tower. If no 4G signal is found the cellular module will attempt to fall back to 2G (GSM). This process consumes ~72mA and typically lasts 60s although longer durations can be set depending on the environment e.g. dense rainforest. The MCU then turns on the Coral SoM, which mounts the cabled SD card (in the trail camera), transfers all images from the SD to the Sentinel microSD before clearing and unmounting the SD (~860mA). The Coral SoM then runs inference on all transferred images (~965mA) before compressing classification results together with additional information into insights. An insight typically contains the species label, image crop, classification confidence, and a timestamp. Compressed insights are then queued for cellular transmission along with device data such as number of images processed, battery voltage, and space remaining on the microSD (~860mA). After cellular transmission completes, the Coral SoM and all communication modules are shut down and the system returns to deep sleep (~1mA).

Once cellular data is received, insights are decompressed and stored in a cloud SQL database from which they are served to different endpoints such as the custom user dashboard (https://mysentinel.info/) or the EarthRanger platform (www.earthranger.com).

Electronics

• Google Coral SoM (System-on-Module)

  • NXP i.MX 8M SoC (quad Cortex-A53, Cortex-M4F)

  • Google Edge TPU coprocessor (4 TOPS (int8); 2 TOPS per watt)

  • 8GB eMMC flash storage

• Microchip SAM D21E Microcontroller

• Spypoint Cell-Link Cable

  • 512MB SD to USB interface

• Communication Options

  • Wi-Fi (802.11b/g/n/ac 2.4/5GHz)

  • Satellite (optional, SpaceX Swarm M138)

  • Cellular (optional, Blues Wireless Cellular Notecard with external SIM slot)

  • LoRaWAN (optional, Blues Wireless LoRa Notecard)

• Power Options

  • (preferred) 2x 18650 rechargeable lithium-ion batteries

  • (optional) 5x 18650 rechargeable lithium-ion batteries

  • (optional) External solar-powered battery pack (note this replaces all 18650 batteries)

  • JST connector

  • Micro-USB

Other

Performance

• User-defined Photo Processing Periods

  • Photo count reached (every 10 photos, 30 photos, etc)

  • Set time period (every 6 hours, 24 hours, 3 days etc)

• Battery Life

  • Sentinel can handle ~800 cycles per charge with 7 batteries (max capacity)

• Model Support

  • YOLOv5 (including MegaDetector v5)

  • EfficientDet

  • MobileNet

  • ResNet

  • FOMO (Edge Impulse)

• Camera Support

  • Most commercial camera traps with a standard SD card slot

• External Memory Support

  • MicroSD (up to 1 TB)

  • USB (up to 1 TB)

• Power Consumption

  • Sleep: <1mA

  • Image Processing: 600mA-1A

  • Peak (for microseconds): 3A

Power consumption during activity states