I use the driver L293B for motor control. Faced with the problem of insufficient voltage to the motor. Power battery 5V until the engine reaches 0.5-2.5V
code example
digitalWrite(9, HIGH);
digitalWrite(8, LOW);
analogWrite(11, 254);
On 9 and 8 pin Arduino connect 2 and 7 feet driver. On 16 and 8 driver at 5V. 11 Arduino signal PWR on 1 leg driver and 6 pin on the motor. 4 and 5 on the ground.
I need to use transistors or to solve a problem with the driver L293B?
The scheme connect without diodes. Perhaps because of this reverse current lower the voltage at the motor?
Diodes are absolutely required with the L293B. See the manufacturer's data sheet for suggested wiring diagrams. http://www.st.com/web/en/catalog/sense_power/FM142/CL851/SC1790/SS1555/PF63140
The L293B can handle at most 1 ampere per channel and since it is old technology, has a high internal voltage drop of up to 3 volts. If your motors require more than that you will need a different motor driver. Pololu has a nice selection of more modern motor drivers Pololu - Brushed DC Motor Drivers
And as you can see approach if the driver l293b to my engine. Earlier, the scheme was only transistors and is also powered from 5V. Everything worked