A Few Car-Counter Pictures

The sign on Olmsted road, establishing the maximum officially sanctioned vehicular velocity.

The housing for the IR detector, made of sections of PVC pipe. The larger diameter part cut at an angle is a sunshade. Not obvious in the picture is the elevation adjustment screw; azimuth was adjusted by simply twisting the entire assembly on the supporting vertical pipe. It was all painted green to blend in with the surrounding ivy. The receiver used a lens 4 cm in diameter, giving high optical gain but requiring careful aiming.

The data logger was a Blue Earth microcomputer which uses a 12 MHz Intel 83C51FB, with the optional 128 kB SRAM for data logging. The cpu, memory and A/D are all low-powered, but the external memory address decoder burns 20 mA, more than everything else combined when in 'sleep' mode. It was either redesign the whole system to power-cycle, or use a brute-force solution... hence the large batteries, two 6V 7Ah gel-cells in parallel. It was all housed in a surplus waterproof box (olive drab) and hidden under a tree by the road.

Action photo: unsuspecting cars go by. Their speed is recorded by the system. The receiver is invisible in the ivy, next to the lamp post on the far side of the road. Actually that's my roommate there on the left looking for it. The IR transmitter is in the short vertical rod to the left of the taller post in the foreground. The transmitter used an IR LED driven with a short duty cycle in front of a 2 cm lens. This fit inside a section of PVC pipe with the lens looking out the side of the pipe near the top. This was in turn mounted on another section of pipe, which supported the transmitter and contained three D cells for power (good for 3 weeks continuous operation).

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