Hello, I came here looking for help. You might know about this project: "swimming microbe silhuettes projected onto a wall" using a green laser pointer and a siringe full of pond water. I want to make a circuit that can automatically produce a new drop when the one hanging from the needle end falls. I figured it can be done using a simple photoresistor and a DC motor with gears used in robotic projects. The motor will turn a screw pressing the siringe piston whenever a missing drop (that looks like a light bulb when paser beam passes through it) triggers resistance change in photo resistor placed close to the needle tip. Sorry for my bad english
Example:
http://www.gizmag.com/diy-laser-pointer-microscope-reveals-secret-life-of-water/16146/

 

Very cool! I can't wait to try this myself.

The electronics part of what you want to do is simple. The hard part in my opinion is getting a reliable mechanism to produce a drop. If you can solve that part, turning it on and off will be easy. If you are detecting off-axis, 90 degrees to the beam, there is a bright spot when the drop is present but nearly dark when it isn't?

I think using a gravity feed and a valve may be simpler than controlling a pump. Or maybe a linear actuator pushing on a syringe. Lots of options!

 

Here is a senior version of a syringe driver which delivers 60 ml over 5 minutes. Maybe a simpler version using 3 ml syringe advancing about .06 ml / drop. With needle removed, forms nice drops. A 1 mm pitch lead screw, high ratio gear head motor , 555 IC timer & misc. hardware should work. Detect drop with IR LED & IR photo transistor.