you could use a multiplexer, preferably 4:16. and on each output pin of the available 16, you could connect a resistor that suits the speed you want to attain at different intervals. these outputs will be connected to your motor, the multiplexer will have to be bidirectional(both positive and negative half cycle) and will have to have a high current withstanding capacity. for this i suggest using power transistors at the output of your multiplexer.(the resistances previously connected will act as base resistance to limit the base current) , and therefore you will have a safe and precise control of your blower....