Help with Transistor, Motor Pad Wire

I’m trying to wire my pcb and have pads for wires for a small motor to be soldered.

I’m having an issue with Pad 2 on the motor pads in that I need to connect a wire from that pad to the Collector pin on my Transistor. The wire keeps pushing around the wire connecting to the Collect instead of joining it.

I’m assuming this means there is something with my schema so I’ve included a photo of both the schema and the pcb…

I’m a newbie, and am still learning here, so any info would be appreciated.

Thank you!!

Screen Shot 2020-02-15 at 12.06.29 PM

What symbol are you using? It looks strange or maybe you are overlapping things? You seem to be trying to connect to the wrong part of the symbol. Note the bottom extension on this one. The pads are the circles.

This is the one I found in the official libraries.
image

Also, did you change the grid settings from the default 50? Things seem to be a bit of a mess.

Small green rectangles identifies the points where wires are ended without connection.
In schematic use grid=50. Change it to lower only for text positioning.

I would start by redrawing this part of your schematic - I would suggest you start a new project and try out some of these suggestions and only then come back to your project. I think this would help you significantly.

There are a number of conventions that have evolved over many years about how to draw schematics. Most are not set in stone but when a schematic doesn’t follow normal convention, it causes a lot of head scratching.

Generally, the supply rail is drawn at the top of the drawing and the ground at the bottom. Inputs are on the left, outputs on the right. You connect to a component by the small square, usually using a wire - not simply by running two components together. So, where you see the small squares (GND, emitter of your transistor and cathode of your diode), none of these point are actually connected. You can rotate ((R key) or mirror components (using the X and Y keys). The power flags should go on where the power actually comes from (not the motor hopefully). Give yourself more space!

Screen Shot 2020-02-15 at 19.21.42

I suggest running ERC. It looks to as if there are a few wires that are not properly connected (They still have their small square at the end that indicates an open end)

I have written a beginners tutorial that could be of help: Tutorial: Introduction to PCB design with KiCad version 5.1 (Getting Started)

1 Like

Aside from repeating the point about keeping the 50 mil grid:

Your schematic does not make sense from a circuit design point of view. The emitter of a bipolar transistor should point in the direction of positive current flow, from + to -. I suspect that D1 is intended to be anti-parallel to the motor so as to clamp back EMF from the motor. But your polarities are screwed up.

Yes the convention of locating positive rails on top of the schematic drawing make it easier to read. Maybe that got you confused but it is not the worst aspect of what you are showing us.

ADDING to the above: Perhaps you just grabbed the first convenient bipolar transistor symbol, but a BF199 is a 25V 50 mA rated RF transistor. I cannot quite guarantee that it will not work, but it is a bad choice. If you need a TO-92 packaged NPN transistor to drive a tiny motor up to about 30V and 300mA, try a 2N4401 or MPS2222.

3 Likes

Hi @hermit,

I’m using Transistor-SMD:SOT96P237X111-3N as the symbol. That might have been from the Sparkfun library I installed, but I’m not 100% sure.

I figured out the issue in the PCB view. I had to update the pad for the motor for it to allow me to wire to it.

I’m working from my breadboard design, and I’m still working through properly drawing schemas and designing the pcb.

I did change the grid as well…

@Rene_Poschl I did run ERC and everything is clean at this point. I worked through the issue I was having by changing the pad’s net value.

I’m mentally struggling with wiring the components as they are on my breadboard vs how I’ve designed the schema.

I’ll check out the tutorial too!

@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.

1 Like

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

2 Likes

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?