Thanks very much Ken. I will experiment a bit.
I'm surprised to hear that the driver circuit won't need adjustment. To take the example of a 10k resistor on the LED, with a 12V supply this will saturate at about 1mA (10V drop across the resistor). Assuming hFE of about 100, that means a driving current of 0.01mA to fully switch the LED on, which means a .5V drop across the base resistor, so the LED fully lights when the output hits just over 1V.
I am new at this so my calculations could be completely wrong! I will fiddle about and see what I come up with.
Is the idea to have the LED pulse along with the DC output, so that with a slow cycle you would see the light go from off to soft to bright and back? And, for experimental purposes, is the output supposed to be 0 - 5V?
Thanks again for any help you can offer.