Method to track shadow from light over a 5 cm sensor strip

[burt46]

Hi,

This is my first post as i'm hoping someone could help with my specific sensor problem.

I would like to measure the speed of a shadow as it progressively covers a sensor area. The problem is that i need at least 5 cm of distance which a photoresistor doesnt give me. I have searched the internet and not found any sort of photoresistor or photodiode with a length of 5 cm. They are all round and up to around 1 cm.

Does anyone have an idea if there is a sensor (ideally analog) that is in a continuous strip format to allow me to measure the shadow position accurately? Open to any ideas. A simple sketch is attached of the idea.

 

[(*steve*)]

Why not have two sensors about 5cm apart and measure the time between one sensor falling into shadow and then the next?

 

[burt46]

Thanks Steve, but I cant do that as the speed varies over the 5 cm distance (the shadow changes as the light angle changes over the 5 cm distance). With two sensor is just get the start and finish times which i can use.

 

[(*steve*)]

Then use as many sensors as you need to track a number of points. I guess you could also use a linear ccd or even a ruler and a camera.

 

[Colin Mitchell]

The ones on ebay give 1mm They cost $1.00 for 20. I hope this is not too much for you.

 

[burt46]

Colin, good idea, but logging 50 sensors would be a pain in the backside if i can do it with one single data value.

I may have the answer, which i am currently testing. House a single LDR in a tube with a slit down one side that is exposed to the light/shadow. The light entering the tube through the slit will change the photo resistor at one end. Bit cumbersome, but it may just work.

 

[Colin Mitchell]

The problem you've got is the fact that a bright day at the beginning is the same as a dull day over half the sensor. That's why you can't use a single sensor.
You need a "reference."