STC 2P 025, 2 mm orifice
STC 2V 035, 1 mm orifice
The stainless steel piston has the orifice seal on the bottom - the black circle is like neoprene (I have no clue what). You can see the orifice that is molded into the body, and the piston sits on that. The brass tube is non-magnetic, and the O-ring seals it, and the plate holds it in. The magnetic coil force opens the plunger, and the spring forces it closed again. The valve simply opens, and the water runs through it, determined by orifice size, head pressure, and pulse duration. 24V may make it faster opening, but it wont change the flow. STC adds a LED in the top housing, visible in the 2V picture. These STC valves are only $19 ( sourced here ). My opinion is that the 2 mm orifice is too large for water drops (requiring the Mariotte bottle to be mounted low to reduce pressure (a little lower than the valve), with only about 4 inches of overall head. The Shako PU220AR valve has a 1.5mm orifice, which seems good to me.
With a 3/32 or 1/8 inch ID nozzle, then:
STC 2P - 2 mm orifice - works best at about 4 inches of head (bottom of mariotte top tube to solenoid plunger and orifice)
Shako PU220AR - 1.5 mm orifice - works well at about 12 inches of head (via about 5 inches of tubing).
STC 2V - 1 mm orifice - I have not been able to find a level good for water drops, but about 6 to 12 inches of head seems the best try. Drops may not even flow at only 2 inches of head on the 2V valve.
Too much water is no good for drops. OTOH, too little pressure loses control.