Detecting a seed dropping through a plastic tube

I'd like to implement a seed monitoring system for my 6 row corn planter. The seeds are blown through a plastic tube with about 1/2" ID. I can handle writing the code and designing the system but I'm unsure how to detect the seed. Optically would be difficult IMO because of the large variance of seed size to tube size. Anyone have any ideas?

Optics would be the way to do this.
I would think that even a small seed would change the light intensity some what.
Feed this intensity change to a comparator to get 0V to 5V logic levels.

Optical is the way to go IMO. You don't need the seed to block the beam, just to reduce its intensity enough to be detected. You could use an RC network to average the phototransistor output, and a comparator (e.g. the analog comparator in the Arduino) to detect significant deviations from the average.

What type of sensor would work for a 1/2" tube? It almost seems that with a tube that large the seed could pass the sensor and never block any light.

collimate the beam, through a slit in tube, focus on photo transistor,

Alternatively, put a single IR phototransistor with a beam half-angle of between 40 and 60 degrees a short distance from the wall of the tube (which I am assuming is glass), and put several IR photodiodes around the opposite side of the tube. No extra optics required, but you will need to surround the whole thing with opaque material to block out sunlight.

How do the seeds get into this tube ?

The ones I have seen have a small wheel device feeding them in.

I'd be inclined to abandon the idea of trying to see them in the tube, and instead consider counting the turns of the small wheel.

michinyon: I'd be inclined to abandon the idea of trying to see them in the tube, and instead consider counting the turns of the small wheel.

But that only tells you the rate at which you hope the seeds are being delivered, not the actual rate of delivery (or whether they are being delivered at all). It's better to measure outputs rather than inputs when you can. I think the optical approach should be entirely workable.

or this -

There might be something here that could work or be adapted to work with your project.

If I were you, I would have tried this one with 5 mW laser & shielded LDR.

suntop: If I were you, I would have tried this one with 5 mW laser & shielded LDR.

I respecfully disagree. A laser provides a narrow beam of light, but what is needed here is a wide beam to cover the entire width of the tube (which is why I suggested several LEDs). To use a laser, you would need to bounce the beam back and forth between two or more mirrors. And an LDR might be to slow to react to the speed of the blip in strength of the light when the seed passes.

Commercial units use a phototransistor or photodiode and IR LED. Larry's suggestion of using a comparator would be the way to go; then you'd just need to twiddle a potentiometer to get it tuned right. It's well under a dollar in parts.

http://www.buildcircuit.com/experiment-with-lm358/

Really depends on what kind of a planter you have as to what would be the best way to arrange the LED and phototransistor but it appears that commercial units typically have the emitter on the opposite site of the tube facing the receiver. You're going to need to experiment and see what works best.

what Chagrin says is ok, some years ago I had the same problem, with affort you can do it