Hi

I have been playing around with one of my other hobbies lately RC cars.

I am designing speed sensor as accurate as possible.

Basically my design is going to be 2 light sensors with a exact distance between them probably 1meter. As the rc car passes point 1 a timer starts and counts how long till point 2 is reached.

I am thinking laser pointers pointed at phototransistor which is connected to a pnp mosfet. When the light is blocked by the car breaking the laser the pnp turns on which will be hooked upto a interupt enabled pin on the MCU. Or the transistor hooked directy to the MCU i will need to do some testing to see what works best.

The speed will be displayed on a big 7seg style home made display until the next car passes through the speed detector.

This is just for bashing not for use in a race track where multiple cars may be passing through at the same time or continuously.

What I am seeking advise on is I need this to activate as quick as possible. I am not sure how quick a phototransistor is. Before I purchase any parts does anyone have any suggestions for this?

If this makes so sense I will draw up a basic diagram and post it.

Ill post pics and complete code etc when done.

Pavement or dirt? Dirt tracks could pose a problem with dust. I don't think the reaction time of the phototransistor would be that much of an issue and as long as you can get the alignment correct and can prevent ambient light from effecting it, you should be able to make that work.

Both mostly dirt but there wont be much dust around till an rc car comes flying through flicking it everywhere. I have some ideas for allowing adjustments to the laser and the diode will be inside some sort of tubing to block out ambient light.

I discovered my camera has high speed 220fps I can do some testing on how quick the photo transistor reacts. ie. making the the MCU light an led as soon as it detects an interrupt request.

Now i just need some laser lights, they are really hard to get into australia. I ordered some keyring ones a few months ago, customs sent me a letter saying they were being disposed of as they are not allowed to be imported. These were $1 delivered cheapies off ebay lol.

I think i have some photo transistors somewhere I will have a play and post some results.

I have that laser/photo transistor setup on my scoreboard as an input to change the scores. I have a red laser pointed at a photo transistor, which is connected to a transistor. When the beam is broken, the transistor turns on. I am sending the signal through a debounce chip then to the atmega128. Works great. If you want, or get stuck, I can share the layout. May give you a starting point if you need it.