Arduino uno +5V/GND pins question

Development Tools IDE 1.x
1

Hi folks, new here and new to arduino too, so I have a question.

I connect the arduino uno to a H-bridge, (simple sketch, 5sec fwd, 2sec pause 5sec rev)

bridge has 4 input pins: "dir" "PWM" ; +5V and GND

I use
pin 7 to PWM
Pin8 to DIR
+5V to +5V
Gnd to GND

When I use a cheap chinese clone Uno R3 this works as intended

Now the Problem/question:

When I use a Different uno (without ch340) or a sainsmart mega and connect the exact same way using same sketch this does NOT work. only the led on the arduino is flashing once a second.

Why?

When I connect an external PSU to supply +5V /GND to the bridge input pins and additionally connect only GND from arduino to GND of the bridge this works as should - same for the mega.

I was thinking the bridge needs too much current - but the additional external PSU on the input pins of the bridge draws only 18mA... so not much current is needed.

I also checked if the sketch is running ok by measuring the output of the pins 7, 8 against GND of arduino with an oszilloscope, (without connected to bridge. signal is fine pins supply the correct

But when I connect to the bridge using only the arduinos +5V/GND it doesnt work

But it works fine with the cheap UNO R3 clone....?

arduino-problem1.jpg
arduino-problem1.jpg735×768 93.2 KB

2

For motors and motor shields never use the power rail from the board.
But do always have a shared ground.

Different boards especially clones are often built using cheapest components and there may be variations in the design to keep the cost down. They dont always perform as a REAL one would.