PCA9685: Servo Motors not working with External Power

@markpritchett

Make sure that you have a common ground between the external power and the Arduino.

We'll probably need a wiring diagram.

@will thank you for your suggestion. I can't try just now, sorry. The wiring in the video and the online article do not have a common ground between the external power supply and the Arduino, so that may well be the issue. 

Here's Bill's diagram

Source: https://dronebotworkshop.com/servo-motors-with-arduino/

@markpritchett

Bills wiring diagram is correct.  Normally one would power the servo motors via the +- terminal block as shown.  The additional V+ pin on the side should be connected to the + on the terminal block and I think its normally used when chaining more that one PCA9685 together.   

Some boards come as just boards to which you solder the pins and terminal block.  First thing I would check (whether soldered by you or not) is the solder connection of the terminal block.

And in saying that I remembered I had a PCP9685 in my parts bin, and bu**er me, the GRN pin on the terminal block is connected to the v+ pin on the side connector !!  

 😎 Thanks for raising this problem, I think you may have save me from a future fry up.

Woops, cancel the fry up my probe on the the V+ also touched the vcc pin at the same time.  But I think a careful check with a multimeter on the bench (and not hand held in hamfisted hands) should show that the terminal pins are connected to the v+ pin and ground pin.  

@markpritchett

Whilst a check of the terminal soldered joints is probably a good idea,  the rest of my last post proves to me how little I know of the electronic side of things.  

I fished out a better multimeter as the one I was probing with (in continuity mode) was not issuing a nice beeb, more a strangulated warble.   However my other multimeter issues a nice load beep when I put one probe on the terminal ground and the other on any other pin including the v+  Maybe I have had a fry up after all.  Maybe its something to do with the reverse polarity ability that the terminal block has (as per @dronebot-workshop video).  

So a multimeter may not be much use in this circumstance, and maybe its actually detrimental.  I will find out when I get to use my PCA9685 😥 

Posted by: @will

I find it interesting that it shows the connections for SCL and SDA as A5 and A4 but neither connection is made as shown ...

I think you will find the A5 and A4 pins are repeated on the other side of the board where the connections are showing as being made in the diagram.   Now sing along - The A4 pins's connected to the A4 pins, The A5 pins's connected to the A5 pins,  Dem pins, dem pins, dem A pins ....  Oh Lordy.

@byron 

Dem pins, dem pins, dem dry pins ...

Yes, I haven't used a UNO for a very long time and afterwards I double checked the pinouts and deleted my message. Apparently not before you had a chance to respond o it 🙂

Posted by: @will

deleted my message. Apparently not before you had a chance to respond o it 🙂

Oh well I hope you had a nice sing song. 😀 

@byron 

I made a few notes of it 🙂

Posted by: @will

I made a few notes of it 🙂

I can imaging a sonorous baritone of a caveman gnawing at dem bones 

@byron 

Not to mention bangin' on the bongoes like a chimpanzee !

@byron, @will - What a hoot!

Posted by: @markpritchett

thank you for your suggestion. I can't try just now, sorry. The wiring in the video and the online article do not have a common ground between the external power supply and the Arduino, so that may well be the issue. 

I should be the last one to offer hardware advice.  I don't really have a clue about why its not working as expected, but I did want to throw in something I'm learning about 3D Printer Drivers.  I know that I need to keep a common ground and I noted that the A4988, 8825, TMC2209, etc drivers don't need explicit connection between the logic level ground and the motor level ground.  I just assumed that those two connections were tied together either inside the chip itself or on the little PCB.

VBR,

Inq

@will @byron in the video Bill says that the boards he is using have dedicated SCL and SDA pins. With other boards we should use A4 and A5

@byron Brilliant 😂

It's working 😀 

Common ground seems to have fixed the issue.

A big thank you to all the support, and humour, you've given in resolving this issue.

Much relieved

Mark