Hi Ante, Many thanks for your help. What I want the circuit to do is upon, say, 14psi being applied to the pressure transducer (46.7mV by my calculation) I want to adjust a potentiometer to vary when the transistor allows 12v to flow to the solenoid and hold it open (it is normally closed). This would be the primary circuit. A secondary circuit would also be there, same as the first, but also with some sort of astable 555 circuit that would be activated (much in the same way as the first, I thought, with a pot/transistor) and then it would be possible to vary the Hz between say 1 and 50Hz which would then cause the solenoid to beopening and closing upto 50 times a second. I have read elsewhere that it may not actually fluctuate at high Hz but just partially open but this is fine aswell. I have also read that it may be necessary to put a relay in line to protect the transistor. Hopefully this makes a bit more sense of what I am doing and I do apologise for my ignorance but I am only just learning about electronics. Thanks John