Help with Transistor, Motor Pad Wire

@Bobz the actual smd transistor I will be using is MMBT100

I did notice that the package for the transistor had the pins reversed for emitter and base, so I fixed those and wired it correctly…This is a bit brute force at the moment on the pcb design…

thanks again!

Did you notice, @John_Patemans circuit emmiter and collector for the transistor are swapped. I dont know where your base terminal is connected to but there is a reason why he swapped this terminals what you should understand for your design.

I referenced the data sheet for the actual transmitter I’m using and wired it based on that. So I swapped the pads/nets and ensured the connections are going to the correct place.

This is what I meant in one of my other responses about brute force. I have a working circuit on a breadboard and while I’m not great at drawing the schematics yet, I wired the components the way they are on the breadboard…so hopefully it will work!!

Thanks for the tips @John_Pateman! I’ll redraw with the supply on top and ground on bottom.

I’m building an Adafruit Feather Featherwing (Arduino add-on board), so my current incarnation of the schematic has the header pins in the middle of the page and I’ve run lines from its ground and power. Everything should attach to the pins from that layout. I’m not sure putting all inputs on the left will make the most sense visually, but if it’s the correct thing to do I’ll do it. The only outputs are a motor and a led. The inputs are a button and. Power switch.

I wasn’t sure if it was appropriate to upload the actual schema file to the forum, which was why I shared a small screenshot of a portion of it. Additionally, this kept things focused to the problem at hand. I suppose it would be better to share the overall schematic for feedback on how to doing things a bit more correctly.

Thanks again!

Zooming in helps, as the details give useful hints.

As mentioned above, the open square green boxes tag a not-terminated schematic wire.
Notice you have solid green circles on a ‘good’ connection.
ie try to eliminate all open green boxes, by redrawing.

On the PCB, when zoomed, notice the netnames are NOT exactly the same (M1 vs D2), so they will not connect. This problem stems from incorrect SCH terminate.

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The thing about a schematic is that it is an abstraction of the actual design - it really doesn’t need to look like the actual component, it simply has to facilitate a logical view of the circuit. So, for example, here is an Amega 328P-AU from the KiCad library. The symbol looks like this:

but the actual chip has a completely different physical layout. It doesn’t matter and, in practice, you often edit your symbols to fit your schematic layout more neatly - for instance you might swap the PORT C and PORT D blocks around, split them up or invert the order so that it fits your design layout better - it will still connect just fine as the pin numbers define the physical connections.

Screen Shot 2020-02-16 at 18.31.43

You can stick your connector symbol anywhere convenient and just connect to the pins you want to use. You don’t even need to run a wire between the the pins you wish to connect, you can simply attach a label on both of the connections you wish to make which declutters the layout further.

Screen Shot 2020-02-16 at 18.44.37

You may benefit from using a template for this board - a quick search turned up this which includes a link to, but like all things, you probably need to verify that it meets your needs.

and the repo


Thanks @John_Pateman I actually started using labels but when I was working through ERC errors I thought it might be best to connect directly.

I’ll checkout the template you found (thanks for that!). I actually took Adafruit’s schema and pcb for one of their Feathers and deleted everything I didn’t need. This was especially useful in the pcb design to ensure the edge header holes and the board edge cut would match their board…so I’m not starting from scratch…

I’ll try redrawing (it will be good practice) and attempt to do as you suggested with the labels, which would even alleviate the need for a power bus and ground on top and bottom.

Thanks again!

I redid the schema and have attached an image of it.
I am seeing 3 warnings and am not sure how to resolve them. Since they are warnings are they truly a problem?

Does this layout look better? Any other suggestions?

Thanks again!!

That motor circuit doesn’t look right. If I flip it vertically so I don’t get distracted by the GND being on top, you look like you’re implementing an emitter follower. But for that to be true, it would have to be a PNP transistor and I assume you know it’s a NPN. Therefore I suspect your emitter and collector are swapped in the schematic, and it’s just in common emitter configuration.

@kenyapcomau thank you for that catch…

I just fixed it and moved the GND to the bottom for readability…

I think I should be using a different symbol for the momentary button. When wiring a button to the Arduino you use 3 of the 4 pins. I think I should be using a 4 pin symbol so that I can wire the pins the way they need to be wired. The way it currently is I can only depict part of how the button is wired.

Is this typical? Am I misunderstanding that symbol?

Finally…I cannot guarantee this will work, but I do not see anything definitely wrong with this small portion of the schematic.

I do not know what sort of signal is coming into R3 (or what is driving it) but you might want to connect another resistor from base to emitter of Q1.

If the MCU output is push pull as they often can be, then this won’t been needed.

Your symbol of q1 has at least some of its pins set to electrical type “unspecified” which always results in an ERC error. See Electrical type of schematic symbol pins (KiCad 4 and KiCad 5)

@Rene_Poschl I read your article on the pin types but I was still unsure with how to label the Transistor pins when used as a switch.

I finally was able to get the ERC issues to resolve by setting the Transistor pins as follow (I’m not sure if this was correct however!):

Pin 1 - Output
Pin 2 - Power Input
Pin 3 Open Collector

Does this seem correct?

Thanks again!

I looked at some transistors in the library and generally B is input, and E and C passive. Remember the classification has to take into account the configurations the device can be used in, i.e. common (emitter, base, collector).

Thank you @kenyapcomau I made those changes and there are no ERC issues.

Yes that is why I was not too forceful in my wording. It is almost free to include the pads for the added base-emitter resistor on the pcb (possible exception of a little bit of space) and you can always leave the component location unpopulated if desired.

This has become a long conversation. The pcb view shown earlier by scampa123 shows what look like some unnecessarily thin traces. I usually like to use traces as wide as can easily fit. Wider traces reduce stray inductance, are less susceptible to accidental damage, and of course are suited to higher current. There are exceptions for impedance control (I have not done that) or rarely where stray capacitance is an issue. But I doubt that either of those situations apply in this case.

Thank you to everyone for their input and feedback. I’ve re-written both of my boards schemas and designed two different boards which have been sent to OSHPark.

I’m anxiously awaiting their arrival to see if everything works!

Thank you again!!

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