Making a forward only speed control reverse

[sanddrag]

Hey all you bright CDers.

I've got a challenge for you (actually it's more of a problem for myself). I need your help. I need to find out if there is any possible way to make a forward only speed control (where the power positive goes straight to the motor positive and the ground is what is being switched) into a forward and reverse speed control. I thought of putting a relay after it (before the motor) and using some fancy code. Would this work? Any other ideas?

Thanks