hi everyone
I spend all the last week searching on the web for how to drive a bipolar 6 wires stepper motor with uln 2803a without finding a result 
can anyone know how to do that 
shematic and arduino sketch, thanks in advance
The usual thing with a 6-wire stepper is to ignore the connections to the centres of the coils and treat it as a bipolar 4-wire stepper.
Post a link to the datasheet for your motor.
Why not use a specialized stepper driver such as a Pololu A4988. See stepper motor basics.
...R
An ULN2804 can only driver unipolar, so you would connect the centre-taps to the positive
supply and each of the other wires to an ULN2803 output. You can pair up channels for
double the current too.
BUT, what is your motor's ratings? If its a low impedance(*) stepper you cannot use it
this way as it needs current drive, not voltage drive.
(*) an ohm or two, as opposed to around 30 to 50 for a high impedance motor
Hi,
Do you have a part number or data for your stepper motor?
Tom.... 
i use a stepper motor from the apple image writer
lamineoriska:
i use a stepper motor from the apple image writer
Hi.
This part number point to a : unipolar with two coils, each with a center-tap.
Just identify common wire for each coils. But UlN2003-2803 ??? 500 mAmp. max.
Your stepper ask for 900 mAmp.
Use a multimeter to check each resistance, common wire should be half of resistance.
As I said you can double up the channels on an ULN2803 since it has 8 channels, giving
1.0A per winding. Check the overall current/power limitations in the datasheet to make sure
though.
MarkT:
As I said you can double up the channels on an ULN2803 since it has 8 channels, giving
1.0A per winding. Check the overall current/power limitations in the datasheet to make sure
though.
thank you i did what you say and i doubled up 4 channels to gat 1A to drive the stepper motor
and thnaks for all of you for helping