Sometimes the best solution for driving solenoid valves it to use AC valves and then control them with TRIACs. You can significantly reduce noise in a system by turning the TRIACS on and off at the zero crossings of the AC signals. The zero crossing switch controls can be implemented in 3 ways...

- Use a ready made solid state relay that has built in zero crossing switching. (Most cost and easiest).

- Use a triac trigger IC between the 8051 and the triac gate that has zero crossing detection built in. (Mid cost and a little more work in the PC board design).

- Build a direct gating control for the triacs, preferrably via optocouplers, and then have the 8051 provide the zero crossing timed drive signals. The AC can be coupled to like the INT0 pin through an optocoupler to permit the 8051 software to know when the AC is at the zero level and initiate changes in the on/off levels of the triac control signals. (Lowest cost but has additional software work).

Michael Karas