electrical help please???

Hello list.
I am trying to come up with a device that I can feed 100-280 volts DC
into and then if the voltage is less then a preset voltage (around 120
DC) then it would inturn connect a separate DC power source in one
direction to a DC motor, If on the other hand the voltage was higher
then the preset 120 DC it would connect the same separate DC source to
the motor in the opposite direction.

This would inturn drive the dc motor clockwise if voltage is to high and
counterclockwise if the voltage was to low. If the voltage is the 120
volts +/- 3 volts nothing happens.

I tried this idea with relays but it didn't work out because the relay
would turn on at one voltage and turn off at a different voltage.

Does anyone here have a good way to make such a device?
Thanks
Stan