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"...the voltage of the input (at the base) must be higher than that of the high-current load placed across the emitter and collector."
I have been working this out, and I think I have just about got it. I am thinking of using a 4-10 line decoder from logic to select different resistors for R2.
Then I will put 2 transistors between the output from the 555 and the H-bridge. These will be selected by the logic, and will select the motor direction. when one of the transistors is on, the pulse signal will get to the H-bridge and will pulse on both sides of the motor (is this okay). i.e, it will pulse to both A + D or B + C (M is motor)
If I take 2 TTL lines to the line decoder I should be able to select 4 speeds an the direction at a cost of 4 control lines per motor.
If I draw a diagram would you guys mind taking a quick glance over it?