Question about a PJ-102A Power Jack Connector


I’m sure that I’m missing something terribly obvious here, but I’m hoping folks here can help a noob out. I am working on a design that will be using a 9V battery connected to a board via a PJ-201A Barrel connector. There wasn’t a footprint for this in KiCad, so I found one that fits, but I think I may have a problem (footprint image below for reference).

According to the schematic and some other comments I found online, pin 2 (side) is a shunt and not connected to pin 3 (center/inside) when the plug is inserted. Does this mean that 2 and 3 should not both be marked as ground, as in the footprint, or is it irrelevant as long as I create a net between them on the PCB? I’ve not worked with this type of connector before and I’m a bit confused because I’m not getting a consistent voltage reading from my DMM when this is connected to a battery, and nothing is happening on the board.

I’m willing to admit that my problem lies elsewhere on the board, but I figured I’d start at the source and go from there. If this is working, as designed, I’ll keep debugging…




Hey Brandon, I’ve used a similar barrel jack for some of my projects: this one. It looks like it has the same schematic as yours from the datasheet. When I’ve wired it up in the past I’ve always just left pin 3 disconnected. I think pin 3 can be used for signaling if the jack is plugged in or not, but I’ve never used that feature. Here’s an example of what I’ve had success with in my own projects.

Schematic view:

Footprint view:

Hope that helps.


Sorry, I just realized my above solution doesn’t do you any good if the board is already made.

I’ve had that issue before where I plug in a board and there was a short from vcc to ground somewhere on the board which made the voltage drop because the power supply couldn’t keep up with the current demands. If you have access to a current limiting power supply, that might be the safest bet for powering it up to see how much current is being drawn through the circuit. Is the voltage inconsistent if you measure it right at the barrel jack terminals?


Thanks @jonthomasson both of these replies are helpful! I think you’re right on about a short. I’m getting a consistent reading at the jack itself now, but I suspect a drop elsewhere. Onto more debugging! Thanks again!


Hey @jonthomasson, I’ve attached the schematic for the circuit in question. I’ve got it working on a breadboard, but I’m sure I did something wrong when mapping that design to this schematic. Does anything in the below image stand out to you as incorrect?


That looks good to me! Is the battery connected into the barrel jack? The only thing I would suggest if that’s the case would be to swap out the battery symbol in the schematic and replace it with a barrel jack. That way it matches more closely with the footprint that’s on the board.


Thanks @jonthomasson, and yes it is. Good idea, I’ll adjust accordingly!


Looks much cleaner, thanks! I also noticed that I had my R1/R2 and R3/R4 resistor values swapped from the breadboard implementation to the schematic, so we’ll see how it goes now!


If you are still having problems try to be a bit more specific about the what the circuit does or doesn’t do, including scope shots (assuming you have a scope) of the output of U1 as well as the dis/charge cycle of C1. And the exact part number of the 555 you are using.

As for the barrel jack, pins 2 &3 for a switch that is usually used to disconnect an internal battery when plugging in a DC adapter.


Thanks for the feedback, Rob, and point taken. I’m new to scope-land, but this is a good exercise. I’ll capture what the breadboard circuit is doing via the scope and then compare that to the PCB-version. Will share more here if I am still stuck or have questions.




ok, so I captured a few readings from the scope, both with the Breadboard circuit as a reference, and the PCB version. Here’s the breadboard version:

And on the PCB

So the Cap is charging, but not discharging. Could that be due to how I have the Discharge pin on the NE555P timer (TI 12AT68M) routed? I think I see a difference from how I have this laid out on the breadboard that might solve my issue, but if anyone sees anything that jumps out, feel free to let me know! :smiley:



That is bizarre. That 9v line is just as straight as an arrow! Was that scope measurement taken from C1? Just to help visualize things, here’s the same circuit setup in the falstad simulator. It may be helpful just to storyboard different scenarios in the simulator to see if we can duplicate what’s happening on the PCB. Similar to what they did in Apollo 13 :). On your PCB, do any of the LEDs illuminate? For the discharge pin, how do you have it routed on the PCB?


So… I learned long ago and early on in my career as a software engineer that, regardless of experience, I would always retain the ability to make bone-headed mistakes from time to time…

This trend has continued in my electronics journey, with today serving as another example. I was re-routing the PCB today and I noticed something on the LED footprint.


The square pad for the LED is negative, and the circular is VDD. Nothing out of the ordinary there, except it struck me that every time I soldered this board to test it, I was putting the LED anode in the square hole, and the cathode in the round.


The lesson for me today? Always keep the schematic and layout nearby when soldering.

Thanks for taking the time to help me troubleshoot this, @jonthomasson, I do appreciate it. If nothing else, I learned quite a bit more about how to use the scope features on the Analog Discovery, and that will come in quite handy in the future!




I’ve done that same thing before myself :slight_smile: . I’m just glad you were able to find out what was causing the problem.


Kudos! Troubleshooting is a bit like running a marathon. By the end you’re exhausted and don’t feel like there’s any energy left, but hopefully you’re left with a bit of a runner’s high :smiley:


That doesn’t explain why there is no oscillation at the capacitor. I suspect your troubleshooting is not over yet,


Thanks Chris, that’s a good, and accurate way to put it! :smiley: