Microprocessor controlled variable voltage regulator

I want to be able to control the output voltage of the LM317 with an Atmel micro controller.

I just do not know what to look for. In my words I want a variable 5k resistor that can be controlled by a Microprocessor. Does such a thing exist, or is there another way.

I have tried switching an array of 8 x NPN transistor controlled by the Atmel, but get conflicting results (voltage output is not linear).

Ray.

It's called a digital potentiometer.

A grid of resistors can solve the problem.

I found some reading stuff for you here:
**broken link removed**
**broken link removed**

If you have PWM on your uC you can use that to drive an RC filter, buffer that with a simple opamp and feed the output into the ADJ leg of the lm317. You can get very good output voltage resolution (heaps of tiny steps) with this, and it requires minimal parts.