I have to agree with RuggedCircuits about the PNP transistors.
As for this statement…
I still don’t know how to read the datasheets of the transistors to calculate saturation voltage.
What you are looking for is a a value typically called “HFE”. It describes the DC CURRENT-gain of the transistor. So your statement about saturation “voltage” is fundamentally flawed. This is because–with bipolar junction transistors–saturation is a result of the current through the base and not voltage. Now that being said, you should know that there are typically several values for HFE(all unit-less) found in data sheets. The one that is important for calculating saturation current is the minimum HFE. You want to saturate the transistor with the lowest possible current to save energy (or whatever), but you also want to ensure that it completely saturates. If it doesn’t, the transistor may get very hot when passing large amounts of current. Anyway…
Bear in mind this symbol is for NPN TRANSISTORS ONLY while your bridge also contains PNP’s too
HFE= ICE / IBE This basically means that the current through the transistor(ICE) is equal to the base current(IBE) times HFE. From experience, I can say that a lot of transistors will have HFE’s ranging from 100 to 300. Before calculating, you should understand that there is an intrinsic diode between the base and emitter which does require a voltage drop of roughly 0.7V.
From here, you need to know the following:
Maximum collector current
With these known values you can calculate the value of the series resistor to the base.
In your case voltage is 3.3V and maximum collector current is 0.5A(according to bc327-40 data sheet). As a safety factor we will supply five times as much current to the base as needed to ensure full saturation of the transistor. The voltage affiliated with the base current will be 3.3 - 0.7 = 2.6V.
IBE = (5 * ICE) / HFEMin
or IBE = (5 * 0.5A) / 100 = 0.0025A or 2.5mA
Remembering V = I * R
R = 2.6 / 0.0025 = 1040 ohms (so a 1K should work)
I wouldn’t be terribly surprised if I screwed something up in my calculations so I welcome any criticism.