There are software controlled variable resistance ic's.
I know Maxim makes some Mixed-signal and digital signal processing ICs | Analog Devices
Or
If you really want to get into the electrical side, you could use an OP amp and PWM.
FYI
AVR is the term (brand) for Atmel's 8-bit Reduced Instruction Set Computer (RISC) chip micro.
I believe AVR stands for "Advanced Virtual Reduced Instruction Set Computer".
The AVR is one of the most powerful tools for it's price range.