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signal ground between two isolated voltage circuits

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rstofer:
In the Arduino IDE installation, there is an example called Knob.  This reads a potentiometer, does some scaling and produces an RC servo pulse.  It would be quite easy to load this into an Arduino Uno and generate 5V pulses.  What we don't know is whether there is an internal resistor on the LED portion of the opto.

I suppose you could put a 220 Ohm resistor between the BEC +5V and the white wire while measuring the current with a DMM in series.  You can then do a little arithmetic to figure out whether there is an internal resistor.  It would be useful to also measure the voltage drop across the 220 Ohm resistor.

Resistor voltage = BEC voltage - VF of the opto  This is 5.0 - 1.5 for a 4N35 opto coupler.  So, we have around 3.5V dropped across the resistors.  The maximum allowable current for the 4N35 is 50 mA but, in use, it usually runs around 10 mA.  So, Total R = 3.5 / 0.01 or 350 Ohms.  We can start with a 220 Ohm resistor and see how much current flows.  From that, we can work out the internal resistance.  If it seems like this internal resistor will limit the current to a couple of mA then we don't need an external resistor.

What I'm trying to avoid is just twisting the red and white wires together and dumping multiple amps into the opto LED.  It's best to test at a low current and try to rationalize whether there is an internal resistor.  An 'oopsie' will destroy the opto.

Look at "Forward Voltage" spec on page 154  = VF = 1.3..1.5V at IF = 10 mA

We almost certainly don't want the motor connected during these tests.

You'll know right away because if you use a 220 Ohm resistor, about 16 mA will flow if there is NO internal resistor and something much less if there is.  It's a quick test!

Take some measurements and post back.

grizewald:
The vast majority of Pixhawk based craft run a common ground. Years ago, Engine Speed Controllers could be bought with an opto-isolator built in, but these days, when an ESC is described as an "opto" ESC, it normally means that it does not incorporate a Battery Eliminator Circuit and not that it actually uses an opto-isolated PWM input.

Is there some special reason why you must have the motor power completely isolated from the Pixhawk's power?

You don't tell us exactly what parts you are using here, but I'll put money on the black, red and white wires from your ESC being:

Red: 5.5V
Black: ground
White: signal

There is nothing in your craft which uses -5.5V.

The reason why the BEC outputs +5.5V is to account for the protection diode on the Pixhawk's 5V power input. The diode will drop around 0.5V and therefore the output from the BEC is a little higher than 5V so that the Pixhawk actually get 5V power and not 4.5V

Unless there's a special reason for isolating the two supplies; then don't!

jberookc@gmail.com:
Thanks again:

I can't work on the project until Monday.  It is in the lab at my tech school.  I am a 71 year old student.  Now that you know a little about me, I hope you won't find that too amusing.  Yes I went back to school after retirement.  It's the best thing for me now.  It keeps me sharp as if that is still possible.  I'm not a complete novice at electronics, however I may as well be with the advent of miniaturization.

It is good to be around younger people and be able to share some life's work experiences.  I singed up for Cad Classes about five years ago.  Now I help out the instructors and do substituting for some of them as needed.  And.. I get to play with all the "toys"...3-D Printers and such.  As a plus, My instructor has let me build this project in our lab.  He wants today's millennials to get more interested in  hands on stuff' as he puts it.

So, thanks again for your help.  I have not been able to save the wiring schematic as a PDF as of this note, however I do have some snipits of my years long solid works rendition of the project sans wires.  I will post it just so it may be of some help and interest.

Hopefully, I will have the schematics converted on Monday and some results to share.

jberookc@gmail.com:
Here are two examples of the project in Solid Works.

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