Ignition Interlock Camera
To design and build a camera add-on to the existing in-car breath analyzer and ignition interlock system (used to prevent intoxicated drivers from starting their cars). The camera needed to take a picture of the person blowing into the analyzer and store the image for future retrieval by a trained technician for use as court evidence. The camera sub-system required storage of at least 1,000 pictures and the retrieval of pictures over USB to a PC. Lockout of unauthorized users of the USB interface and an interface for commands from and statuses to the car’s breath analyzer system were also necessary. The per unit target cost was a substantial challenge.
A Multi-point Control Unit (MCU) was selected to control the camera sub-system. This solution provided the most peripheral interfaces at the least cost and space. A Freescale ARM MCU was chosen for its inclusion of a camera interface. An SRAM and flash chip were chosen next to provide temporary program data space and permanent code space. To store the large amount of picture data a 32MB NAND flash was chosen. This amount of space and the data format allowed for the storage of up to 1,800 pictures. To save space, a picture size of ¼ VGA was chosen as the size format with JPEG as the compression format for black and white color. This approach provides adequate quality with a small file size.
The camera is tethered away from the processing board, which is located under the dash. A small power board and the camera are housed inside a small plastic box attached to the end of a 50-inch cable. This makes it easy to mount the system in all types of vehicles. A USB interface was added to the sub-system for connection to a laptop PC. The ignition interlock system commands the camera sub-system to allow the USB interface to function. This keeps unauthorized users from accessing the interface. The PC will see the camera sub-system as a mass storage device. This approach was selected because it allowed the use of a PC with no additional software. Using this approach, the PC can copy pictures off of the camera sub-system, but it cannot erase them. Only a command from the ignition interlock system can remove the photos. The images are stored in the NAND flash with a flash file system as this helped protect the file system from sudden power low. The file system is translated into a FAT file system interface so the PC can retrieve the images.