I am making an autonomous car from an old RC car. For the steering I made my own H-bridge, consisting of 2 PNP and 2 NPN transistors. When I connect everything, the motor works fine. But it hasn't got enough power to turn the steering wheel (the steering motion works with gears). When I connect the motor directly to the 5V and GND of the arduino, the motor has enough power to turn the steering wheel. Does anyone know how to get more power out of my H-bridge? I tried decreasing the resistances on the bases of the transistors and I also tried using a 9V battery, but both didn't work out.
The 9 V battery is likely to be the problem, since it will not supply much power. Try lithium batteries, e.g. two Li-ion or Li-polymer cells in series, to make ~8.4 V.
Is there really a need to make your own H bridge? There are plenty of very cheap and compact modules available. The one I use for a lot of projects is TB6612FNG (15 V, 2 A maximum, 2 channels).