It occurs to me that if we had a spring loaded valve (a cone shape with a soft iron pole piece and behind it a single coil, so that the coil pulls the soft iron against the spring and the cone away from the seal) so that 0 current is zero flow, we could have a variable flow rate in addition to ON/OFF. The first picaxe circuit I posted could accomplish variable flow using the PWM output to drive the coil. A small voltage just cracks the valve open, and full voltage opens it fully. The flow rate could be set by the laser pulsing on and off with say 0.5 second pulses. The picaxe would count the pulses so that 1 pulse just cracks the valve and 10 pulses opens it fully. Hold the laser on for greater than 1 second would close the valve and reset the valve count. In this way, we could demand ON and OFF and the flow rate. This could all be accomplished with the first circuit I posted with a software change and a couple of tweaks.