This (the black thing on the right with the wires coming out) is the solenoid valve that turns the CO2 on and off.
It is controlled by the pH meter/controller. On the left you can see the metering valve ("needle valve") that is used to fine-tune the bubble count.
Manufacturer: Snap-Tite/Allied, Valve Division, Erie, PA 16506
Part number: V273X-9
Orifice: 0.250 inch
Voltage: 12 volts DC
Note that this valve is rated for only 5 pounds per square inch. My understanding is that most regulators work best at 15 PSI or more. This valve occasionally sticks at that pressure. (That's 3 times its rated working pressue, so it's not surprising.) If I had it to do over, I would have gotten a different valve with a higher pressure rating. This one has worked fine for years at about 12 PSI, however. I got it at a boat supplier and was told that people use them to turn the propane supply on and off.
The valve consumes 11 watts when it's turned on. That's quite a bit, and it gets too hot to touch at the base. It is drilled and tapped at its base, and I'm sure that if it was mounted on a metal base which acted as a heat sink, it would stay cooler.
11 watts at 12 volts requires almost one amp of current (11 watts / 12 volts = 0.92 amps). I run it off an old "wall wart" type power supply, but it's a big one that's rated for 1.5 amps. You can't run a valve like this off a tiny little power supply.
I can't find this valve listed anywhere anymore, so I assume that it's no longer being manufactured. (I bought it in 1999.)