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I bet you could use an lm3914 to do this, it is designed to drive a led bargraph which represents the input voltage, so all you'd need is to hook up a transistor to the outputs, then hook that up to a relay, putting a protective diode across the relay of course. You would put a resistor of say 10k or so on a 5v line to the input voltage pin, then connect your unknown resistor from that pin to ground. This creates a voltage divider that will be analyzed by the lm3914 which controls the relays.
5v--/\/\/\--0 <--input pin of lm3914