Hi Everyone,
I could really use some help here. I’ve designed a project and it’s just not working according to plan. A little background: I’m a biologist and never understood or liked MOSFETS. I’ve simply never gotten them to work as expected. I’ve had great luck with all sorts of relays, but never mosfets. I’m trying to figure out if the issue is with my board and wiring or my understanding of how the mosfet should have been wired.
The issue I’m having is that when I try to drive the mosfet the led (or laser) dims, but does not shut off completely. So I think there is something wrong. There are 3 boards and 4 mosfets per board. All mosfets behave the exact same way, so it seems like an issue with my design. These are N type MOSFETS COM-10213
I’ve used Sparkfun’s Eagle tutorials to design the board (fantastic tutorials!).
All components are from Sparkfun.
The board is from OshaPark.
This is an image of a Sparkfun Tutorial that I used to design the mosfet circuit.
From here I interpreted this as power going to the device unimpeded, through the device, and shorted to ground at the mosfet (meaning the mosfet must be activated to complete the circuit to ground). I added the mosfets in Eagle and used these schematics to guess what are the drain, source, and gate. Gate was intended to go to the Arduino pin. Drain is supposed to connect to ground on the device and source goes to the actual ground to complete the circuit. I used these schematics to design the board.
Now this is the PCB board design in eagle. Does this look correct or did I botch something here?
And here is a zoomed in look at just one of the mosfets.
Here is an image of the actual PCB. My interpretation of the MOSFETS is that they go Gate, Drain, Source from left to right.
When I test my connections with the multimeter, some things make sense to me, and some unfortunately don’t. The left pin of the mosfet, Gate, does indeed always connect to an Arduino pin. So I think Gate is good.
Drain should go unimpeded to the devices ground, in my case the left ground pin of the jst connector. But I do not get a connection. I am not sure why. According to the eagle schematic, I thought I should. Drain instead appears to go to the ground of the power supply and not the jst connector. I am not sure why.
Source seems to go to the ground of the device. This now appears to me as if the pins on the mosfets are just not accurate? Any help is greatly appreciate. I fear I may need to print all new boards and get new components. 
Sincerely,
FB
