Hi Grumpy-Mike
Would using a transistor require an external supply as well as the 5v from the Arduino I saw a previous diagram on here using a BC183 to get a 10V output which was smoothed with a RC.
This circuit had the Arduino PWM port connected to the Base of the BC183 with a 4k7 resistor in in series. The emitter was connected to Gnd and the Collector connected to a 12v+ supply with a 1k Resister in series. A 680 resistor and 1 uf capacitor in series was used to smooth the output from the collector.
I was wondering if this is the circuit you were getting at in your last post and if so i cannot get the BC183 but there must be a replacement. something like the 2N2222A or a BC547C possibly ?