raspberryfriky
Posts: 3
Joined: Sun Sep 15, 2019 7:50 am

Can I use a PCA9685 like this?

Sun Sep 15, 2019 11:57 am

Hi there,
I have 14 servos (some of them work at 5v, others at 7v and others at 24v) and I'd like to know if I can use the PCA9685 (PWM Extension) like this:
pca9685_hypotesis.PNG
pca9685_hypotesis.PNG (230.57 KiB) Viewed 1268 times
So that the idea is to feed each servo externally and individually. Will it work? if not, do you have any suggestions?

Thanks in advance!

RattusRattus
Posts: 70
Joined: Sat Sep 29, 2018 11:27 am

Re: Can I use a PCA9685 like this?

Sun Sep 15, 2019 5:37 pm

Almost certainly your servos wion't like that...

You will need to level shift the PWM signal to match that of the input required for each servo. However you are correct in commonning the 0V lines together.

This could be off the shelf level shifters, or as simple as inverting your PWM output and driving a transistor with a pull up resistor between the output (collector) and your servo power rails.

I suspect, however, that the servos are also quite powerful. You may also want to opto-issolate the servos from your I2C PWM driver:

Code: Select all


  5V       3V3                           Servo Vcc
  |         |                 _____________|
  |         |                |             |
  |         |___________     R (10k)       |
  |         |           |    |__________ Servo
RPi --> PCA9685_________D    K             |              Where D  K is opto transistor Issolator
  |_________|                |____________ |
  |                                        |
  0V                                     Servo GND 

(Post edited to layout diagram)

/Andy

raspberryfriky
Posts: 3
Joined: Sun Sep 15, 2019 7:50 am

Re: Can I use a PCA9685 like this?

Tue Oct 15, 2019 8:00 pm

I tried it and succeded! My hypothesis works!
Thanks anyway.

blimpyway
Posts: 210
Joined: Mon Mar 19, 2018 1:18 pm

Re: Can I use a PCA9685 like this?

Fri Oct 18, 2019 5:32 pm

In general it is better to search and read documentation for each specific model (of servos in this case), There should be written what it accepts and what not.

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