This was designed for a radio controlled car track using cars of 1/10 scale. It works by measuring the time it takes for an object to pass 2 sensors placed exactly 200mm apart. In this way the speed can be calculated in "MPH" and ms-1.
In fact 2 versions were made with the second designed exactly the same but the case was designed for indoor use.
Designed and built by Phil Townshend 2008
The principle is simple. Take the time it takes to travel 200mm, multiply this by 5 to get the time for 1 metre then take the reciprocal.
eg: Time taken to pass 200mm = 0.1s. Then 1 metre takes 5 x 0.1 = 0.5s,
so speed = 1 / 0.5 = 2 m/s.
The speed is displayed in metres/second by default, but pressing SW2 will display the speed in MPH but at a scale speed so actually it will read 10x as much.
The Math routines I wrote I am very pleased with. They can multiply 16bit by 8 bit numbers and are based on long division and multiplication. I wrote them myself - honest!!
The sensors used were opto-transistors and the light sources were some 1mW keyring laser pointers (I found a deal on 30 for £10 from Japan - free shipping - bargain!) The lasers and sensors were positioned such that a beams were directed across the track exactly 200mm apart. The cars would break the beams as they drove past.
Boxes were made up from 6mm MDF to protect the sensors and Laser modules and also to ensure accurate positioning.
The Lasers were mounted in wooden blocks that swivelled and could be adjusted by bolts from the sides.
Speed Trap block diagram
Thanks to the PIC, most of the circuitry is interface control, such as buffers and drivers. The last 8 display digits are multiplexed, where only one digit is on at any one time, but all very quickly. To maintain brightness the LEDs are driven much harder than usual so protection was put in to detect if the clock ever stopped because of the PIC locking up for some random reason. In this way as soon as the display is frozen, the outputs from the shift registers is switched off. and thus in turn the display.