This circuit is suitable for driving uni-polar stepper motors at up to 1.5A per winding at a voltage up to 50v.
The main driving stage is provided by IC1 - an ULN2075BN. This, in essence, changes the TTL levels at it inputs (pins 3,6,11,14) into high powered outputs (on pins 1,8,9,16) and provides the same 'sense' values as the L298 did for the bipolar driver. Since this chip has no concept of 'inhibit' then we have also added IC3 a Quad AND gate which uses the inhibit outputs from the L297 to disable the output from the L297 from the inputs to IC1.
Diodes D1 to D4 are fast Schotkky diodes. The exact part number will depend on the voltage you are supplying to the motor and the current passing through the coils of your motor. But you will notice that this requires half the number of diodes compared to a bipolar motor driver.
Capacitors C2 and C3 that were used on the bipolar circuit are effectively no longer needed - since the motor supply voltage is only applied to the center taps and has no other interaction with the circuitry.
I have also included a jumper to allow the selection of various options. Of course you can dispose with the jumpers and just hardwire them. If you are breadboarding the circuit first then you can find the best jumper settings for your motors/situation and hardwire them accordingly. Otherwise use a 3 pin header and a jumper to make the selection at runtime.
J1 is used to set pin 19 of the L297 high or low and controls whether the chip emits a half-step or full-step drive.
The bipolar motor driver had a second jumper on pin 11 but this is not required for the unipolar circuit and pin 11 is just tied low.
Approximate price to drive one motor:
1 x L297 @ $9.90
1 x ULN2075BN @ $2.85
1 x 4081N @ $0.45
4 * diodes at say 50 cents each $2.00
C1,R3,R4,R5,R6 you may have in your spares bin but lets say $1.00
Trimmer R7 say $0.30
R1, R2 - the main thing to note is the current going through your coils will go through these resistors. Remember Watts = Amps * Volts and Volts = Amps * Resistance. So if your motor requires 1A per coil. Remember the reference voltage on pin 15 can be a max of 2.5V so lets assume we go with 2v. Then the resistor value should be:-
Volts = Amps * Resistance
So Resistance = Volts / Amps = 2 / 1 = 2 Ohms
The resistor power value would be: Watts = Amps * Volts = 1 * 2 = 2 Watts
On this basis then 2 x 2 Ohm 2 Watt resistors would be about 30 cents each = $0.60
So a total of around $16.50 per motor.