I'm trying to learn about transistors and H bridges towards a final goal of controlling a bipolar stepper motor with Arduino.
I've done the following half H-bridge in the process of learning:
My first question: Why can't I use a single resistor to connect the base of both transistors to ground?
Thank you for your feedback.
I'm not sure that this schematic works... the led isn't reverse polarized?
It works.
I've read about how H-bridges work. I understand the general concept. What I'm having issues understanding is how transistors work. Here I'm just using them like switches.
What I've shown previously was, as I've said, half an H-bridge.
Here's the full H-bridge:
I've tested this on my breadboard and it works ( I can make a video if necessary).
What I've noticed on the full H-Bridge is that I have 2,77 V on LED1 and 3,40 V on LED2. I don't understand why. SInce the two halves are basically mirrored, I expected them to be the same.
As others have said, the reason for different voltages is that different coloured LEDs drop a different voltage across them for the same current, LEDs are current devices. in your circuit as mcb1 has said, i'm surprised the LEDs have not burned out, either the batteries are limiting the current or the gain of the transistors is insufficient with 330R base resistor to allow the full current to flow, wither way the LEDs are being driven badly.
If you do not yet understand transistor then start with that, not an H Bridge. in all cases above you are using the transistor as a switch, and you need to know what that means first. Then build on that to a Half bridge, then a Full Bridge
I would also suggest looking up how LEDs work as well as without that knowledge you will be burning a few out in short order 
Making smoke is not a bad thing, we learn a lot from it (Making mistakes is more educational than everything working first time) but all of the devices in the schematic you have above have limitations and you need to understand them in order to work within them (Prevent the Blue smoke of learning )
Data sheet for the BC558:- https://www.fairchildsemi.com/datasheets/BC/BC560.pdf shows a max current of 100mA and a volt drop of between 200 to 600mV across the transistor and you have 2 transistors and an LED across the supply
As others have said, the reason for different voltages is that different coloured LEDs drop a different voltage across them for the same current, LEDs are current devices. in your circuit as mcb1 has said, i'm surprised the LEDs have not burned out, either the batteries are limiting the current or the gain of the transistors is insufficient with 330R base resistor to allow the full current to flow, wither way the LEDs are being driven badly.
If you do not yet understand transistor then start with that, not an H Bridge. in all cases above you are using the transistor as a switch, and you need to know what that means first. Then build on that to a Half bridge, then a Full Bridge
I would also suggest looking up how LEDs work as well as without that knowledge you will be burning a few out in short order
Making smoke is not a bad thing, we learn a lot from it (Making mistakes is more educational than everything working first time) but all of the devices in the schematic you have above have limitations and you need to understand them in order to work within them (Prevent the Blue smoke of learning )
Data sheet for the BC558:- https://www.fairchildsemi.com/datasheets/BC/BC560.pdf shows a max current of 100mA and a volt drop of between 200 to 600mV across the transistor and you have 2 transistors and an LED across the supply
