# Modulating a (diaphragm/direct lift) solenoid valve for proportional output

I am attempting to build a water control system which allows proportional control of the water output flow.

I had initially thought of using servo-controlled ball valves, but it was suggested that solenoid valves can be modulated using PWM or similar input and thus end up with a somewhat variable flow rate.

However, I just tested a diaphragm/direct lift solenoid valve (like this one from ebay) using different input frequencies and duty cycles, and was unable to achieve anything other than a "full on" and "full off" condition.

Must I use a "direct acting" solenoid instead, in which the solenoid plunger directly controls the flow?

or does someone know if it's possible to modulate the diaphragm-type valves? If so, what frequencies and duty cycles would be most effective?

Valve requirements

• Standard (US) municipal water source (50-100psi)
• 3/8" to 1/2" water supply line
• 12VDC (or less) input voltage, with minimal current draw (1A or less)
• Normally closed
• Low heat output - can stay open for extended periods (hours)
• very inexpensive - less than $10 each (preferably less than$5/ea)

Thanks for any insight! (Would this be a better question for the Engineering section?)