Dear friends, in my Enterprise Model I have put in six LED-groups to simulate the working of the spaceship. For every function a wire exits the model.
For powering every group I wanted to use NPN-transistors from which the 'base' is controlled by a Arduino Uno/Atmega328P. However, as a technical newbee, in the model I have bundled ALL the grounds so only one black wire exits the model. I know I cannot use a 2n3904 NPN-transistor unless I put the LOAD (LED+resistor) on the 'emitter'-side. Is that a serious option or a 'not so smart' option? Do I have other options, like maybe a PNP-transistor?
There is also an attachment with a picture of my problem.
Since you bundled and tied all of the low sides low side switching is about out of the picture. Low side switching is most commonly done using an NPN. Next design you will plan better. Additionally your mention of a 2N3904 which is fine for use as a low side switching transistor, just make sure in any case, high or low side switching your transistor is suitable. The 2N3904 has a max current and you need to work within that range and leave some overhead.
Thanks Ron_Blain and runaway_pancake for your help. Learned about emitter-follower and low and high side transistor switches.
I seem to have two options: as 'runaway_pancake' states, by using the emitter-follower principle I can move the load to the 'emitter' side. The other option/suggestion from Ron_Blain is a high side transistor switch with a PNP-transistor.
As I don't have PNP-transistors I will use the first option (emitter-follower principle).
Several good answers here already. I just want to add that I always prefer to use MOSFETs for switching. Yes, they are bit more expensive but they are lower loss and to me that is often key. My go-to N-channel MOSFET is the IRF520 and my go-to P-channel MOSFET is the IRF9530.
runaway_pancake:
Good gosh.
Those aren't "logic level" MOSFETS.
Look for an IRL520 at least.
The IRF520 turns on at around 3-4V and the on-resistance on my IRF520's at 5V is extremely low (~100mOhm from my measurements). I only work with 5V logic and for me they work fine all the time. Sure, logic level would work better. In my case, I never switch high currents (>100mA) so in those cases, a logic level is probably better to ensure it is fully saturated to reduce power loss. Good point! But for lower currents they will work excellent!
amaruk:
But for lower currents they will work excellent!
Which is why for such low currents the eBay modules using these devices are indeed usable, - but there are much better modules available for much the same price if you look.
