Protecting microcontroller from damage due to voltages on comms wires

Hello,

I wasnt gong to post this schem, but it looks so bad, that i may as well, as its not likely to be useful to competitors.....

A contract electronics company has designed for us a LED lamp which can be flashed in synchrony with other LED lamps of exactly the same type.

I think they have made mistakes……do you think so too?

Here is the schematic surrounding the microcontroller that flashes the LEDs on and off, and communicates with the other LED lamps to allow it to flash in synchrony with them.

https://i45.tinypic.com/2cd8wf7.jpg

These LED lamps, could , for example, be mounted on a big lorry, etc.

My problem is that GP5 can never “see” a high input because of the 5V6 zener (D2) in series with it.
GP5 is supposed to be checking that when a (logic high) SYNC pulse is sent (by turning on Q1), that the SYNC pulse really does get sent……i.e., that P3 goes high.
…the reason to check to see if turning Q1 on takes P3 high is to check if the sync line is shorted to ground……
…but as you can clearly see, GP5 will simply never see a high input due to the zener diode D2.

…..Why didn’t they just use GP2 to sense whether the SYNC pulse is actually sent or not.?

It appears to me that the designers were worried about overvoltages destroying the microcontroller. This is why , I think, they used the 5V6 zener, D2.

The problem with sync’ing these LED units is that each lamp has a local ground, and of course, this may be different (due to I*R drops) than the ground local to the other lamps.

With 200 lamps potentially mounted on a truck, and each lamp potentially drawing 0A75, then theres potential to have well over 20V difference between the various bits of ground at each lamp……..so why did the designers think that a mere 5V6 zener diode would be enough to protect the microcontroller pin?

…….and why have they only protected against excessively high input voltages……..excessively low input voltages on the sync wire are just as likely…..so why not back to back zeners?

Anyway, do you agree that the only way to correctly do this job is to use like a "4 to 20mA" signalling approach, and use optocouplers to receive/send the sync signal?

I agree that the 5.6V Z-diode circuit doesn't pass a high level in normal operation. But the purpose of GP5 isn't obvious at all.

For a fail-safe circuit, two points are missing in my view:
- short circuit protection for the driver
- reliable overvoltage protection

I don't see a purpose in guessing why the circuit was designed the way it is.

Before making an attempt to correct or redesign the circuit, I would ask for a specification besides the said fan-out of 200.

The circuit was designed by contractors who have long gone.

We do not even know what spec they worked to.

They were asked to simply make the lamps flash in sync, thats it....thats all the microcontroller is supposed to do.

Also, they were asked to ensure that the microcontroller would not be damaged by any transients etc coming in on the sync wire.

Thats it.


I think they have made a deliberately confusing , (but useless) network of components , in order to "impress" us into thinking that they've done something really clever....................in other words, we've been ripped off.......do you agree?

Maybe they've designed for more than one level (maybe for future use?).
The sync out/in pins are maybe some collision detection or something? i.e. pull the line high (5V), but also to check if the line is already
pulled high or not.
The GP5 looks like it would not detect this level, but it would detect a higher level, e.g. 10V, maybe from a different device or a switch,
on the same blue line. The 100nF cap may support the switch theory a little bit (primitive debounce).
But it's just speculation (and that's not a good thing to do) - hard to figure out without knowing the intentions.