Low flow drip irrigation drip sensors
Drop can be big or small or minute. You need to quantify a "drop", which is easier said than done. It is better to measure the amount of water you need to use for irrigation. With that in mind, letting it out as few drops over a time period could be managed.

Raspberry Pi's timer could be used. 

Another easy idea is to borrow the used IV drip (comes with Glucose / saline bottles) from healthcare industry, ask some friends who work in hospital. 

Integrate Raspberry Pi with the dripping mechanism. 

Use timer with the IV drip mechanism (now modified for water) and a mechanical stopping. This will be cheap and easy to use in kitchen garden. Hope this helped.