Speed sensor - any comments?

[Humbleworker]

I'm going to construct a speed sensor and this is my idea. I'm going to use two lasers. The distance(d) between the two lasers will be a constant. I'm going to extract the time(t) it took for the object to go from laser A to laser B. Then I'll put them in the formula: Speed = d/t. The answer to the equation will then be displayed on an LCD or seven segment display. Does this sound viable? Any tips? What do you think?

[BobK]

Hi Humble,

Why don't you just try and see if it will work?

When I get and idea for something I might look at some of the forums that are out there and look for something similar. But I will always give it a try before seeking help. To me that's the fun part of my hobby, learning to do something on my own!


BobK

[Pic_User]

Hi Humbleworker,

That sounds like it would work.

What type object? Size? Speed? Distance?

Emitters:
Do you need lasers or would simple infrared (IR) emitters (LEDs) and detectors be okay?

Photo detectors:
IR phototransistors are more sensitive, but photodiodes are much faster reactors.

Do you have bench testing equipment available? A “counter-timer” would allow you to test the idea. Then you could replace it with a PIC that would do the timing and calculate for you.

Have fun.
-Adam-

[Humbleworker]

FYI:
The objects whose speed are to be measured should be any opaque object(small or big) that has no gap in between (a gap where light could pass through).

It should be able to display the speed of a really slow thing such as a person walking to a really fast thing such as a car zooming past.

The distance between the two lasers will be really small(like half a centimeter) so that it can be easy to move around.

The laser I'm planning to use is a conventional laser pointer that's wired to a regulated power supply so that it would automatically stay on as soon as the device is turned on.

I'm still deciding whether to use a photodiode or transistor.

P.S.
Nice point BobK - I'll ask for help after trying to solve any problems I encounter myself. But I'm open to suggestion.

[Humbleworker]

Let's say I want to measure the speed of a car that passes by, I would have to attach the phototransistors on the other side of the road to the microcontroller on the other end without putting anything across in the car's path. I wonder if it can be done using IR or another laser or using a radio frequency. How do I setup these methods of transmitting data? I really am a noob and have no friend who knows this stuff.

[dragons_fire]

i would pulse the lasers on one side with a 555 or a micro, then recieve that pulse on the otehr side with a second micro, that way you wont have any wired going across the road, and oterh light sources wont interfere with the results...

[skimask]

i would pulse the lasers on one side with a 555 or a micro, then recieve that pulse on the otehr side with a second micro, that way you wont have any wired going across the road, and oterh light sources wont interfere with the results...

You wouldn't need the laser to be on the other side of the road, just put the emitter and the detector on one side, seperated by a few inches or whatever. A car goes by, you'll get a reflection. You may need a bit of a wide angle detector for this though. And make sure that there isn't a mirror or something like that across the road...

[dragons_fire]

with lasers, i dont think he would get much of a reflection off of the tires (if the sensor is that low) but it would probably work off of the body... he could put a mirror on the other side, and then look for the missing signal...