24v Solenoid question - Help please?

Hello.

I am working on a project using 6, 12 and 24v solenoids which are being switched on and off via transistors. The wiring and components are the same as in this tutorial: http://luckylarry.co.uk/2009/07/arduino-control-a-dc-motor-with-potentiometer-and-multiple-power-supplies/

My problem: The 6 and 12v solenoids are working fine - controlled from max/msp via simplesystemmessage - beautiful. However the 24v circuit doesn't work - the solenoid is just constantly on.

Do I need to use a different rating of resistor or transistor when dealing with larger voltages?

Any help would be appreciated... I'm very new to all this!

Thanks

Any chance you could post a photo of your project. Have you double checked the diode is the right way round?