Solution
This issue was caused by the choice of SOT-23 symbol when creating my PCB in KiCAD.
As you can see in the picture below, there are a number of symbols associated with transistors in the KiCAD default library:
When mocking up my circuit I chose the first option 'Q_NPN_BCE'. The key elements here are the last three letters which define the pinout in KiCAD. What I didn't know at the time, but know now, is most common SMD transistors have a 'BEC' pinout rather than 'BCE' (or 'CBE', 'CEB, etc...).
If you are running into a similar issue, you can verify which pinout your transistor is by referring to the datasheet.
The pinout of a SOT-23 package is counted from the first of the two pins going in a clockwise motion:
To determine which symbol you should pick in KiCAD, just read out the pins in the same order.
In my case it's Base, Emitter, & Collector, or 'BEC'. So when choosing the symbol from the KiCAD library, I should choose the 'Q_NPN_BEC' option.
Hopefully this can help someone having a similar issue in their circuit design!
Original Post
I guess I should preface this with the fact that I'm definitely a beginner when it comes to electronics and circuit design/debugging. Thanks for the help!
I recently developed a circuit board for use with my Raspberry Pi 3 A+. The purpose of the board is to accept various inputs, but also toggle some relays (two solid state, and two regular). My current issue is that I can't seem to turn relays C and D on. I'd hoping someone could help me find out where I went wrong.
Here's a snippet of my schematic for reference:
This is what I know so far:
- All the outputs on the pi are working properly (I can see ~3.3v at R2 - R5 when I toggle the respective pins on).
- All channels of the optocoupler are working properly (I can see ~5v at R6 - R9 when I toggle the respective pins on).
- I'm unsure of exactly how to tell if a transistor is doing it's job with a multimeter, but if I plug a resistor and led into J8 or J9 I can verify those channels are working as the led lights up when I toggle the respective pin on.
- I can say it's not the protection diode installed wrong, as removing it had no effect.
- If I run a wire from GND to pin 2 of either of the relays, they click.
My best guess right now is that I chose the values for R6 - R9 incorrectly for the application.
Datasheets:
- https://www.vishay.com/docs/83512/tcmt1600.pdf
- https://www.diodes.com/assets/Datasheets/ds30036.pdf
- http://www.smc-diodes.com/propdf/1N4148WS%20N0572%20REV.B.pdf
- https://datasheetspdf.com/datasheet/SRD-05VDC-SL-C.html
Update
It was brought to my attention that the collector and emitter of my transistor may be flip-flopped. After reviewing my design and the datasheet I'm inclined to agree. Here's a picture for reference: