Problems getting H-Bridge to work as expected with electromechanical display

Hi guys,

Electronics newbie here. I'm trying to get some electromechanical vane displays (bought second hand from Latvia off eBay) to be controlled by an Arduino. I feel so tantalisingly close, but am thwarted. I'd appreciate any help I can get.

So let me fill you in with the details. First the 7-segment electromechanical vane displays:

I've got limited data on these. Essentially each segment is controlled by one coil. Place a current impulse (>50ms) across the coil and it will generate an electromagnetic field and flip the segment out. Reversing the current across to coil generates a magnetic field with opposite polarity and the segment flips in.

From what I can scrounge up, the coil requires a between 20-30V to work. Using an adjustable power-pack, I have found that 24V works fine. (the power-pack is rated 1A at 24V; the resistance across each coil is about 50 Ohms).

To interface the Arduino with the coils, I chose to use SN754410 1A H-Bridge chips (data sheet here: Using it in a Full H-Bridge configuration, I should be able to drive two individual segments out and in by reversing the polarity.

I constructed a simple circuit to test an individual segment: (NOTE I missed putting a wire in the diagram between +V from external power source and pin 8 (it's there in real life).

And used the following code for the Arduino

int segmentOUT = 4;
int segmentIN = 5;

// the setup routine runs once when you press reset:
void setup() {                
  // initialize the digital pin as an output.
  pinMode(segmentOUT, OUTPUT);     
  pinMode(segmentIN, OUTPUT);  

// the loop routine runs over and over again forever:
void loop() {
  //Sent a 50ms pulse to put segment to OUT position
  digitalWrite(segmentOUT, HIGH);
  digitalWrite(segmentOUT, LOW);
  //Delay 2s

  //Sent a 200ms pulse to put segment to IN position
  digitalWrite(segmentIN, HIGH);
  digitalWrite(segmentIN, LOW);
  //Delay 2s

Now, if I replace the coil in my circuit diagram with my multimeter, I can see impulses of current going alternating between +24V and -24V as would be expected given the code.

Unfortunately, if the circuit is connected to the coil, there is no flipping in and out of the segment. I have tried playing around with how long the pin is HIGH, but no luck.

I would appreciate any input. Have I done something wrong with my wiring. Am I missing something completely??

Cheers guys!!

24V 50 Ohms - thats about 1/2 Amp. What is the actual voltage across the coil when H-Bridge is energized?

Interesting .. it's only showing just under +4V / -4V across the coil when the circuit is energized. What do you think is happening to the other 20V?? (don't laugh - I'm a newbie).

With some help, I managed to figure out the problem was because I didn't have the grounds connected between the Arduino and 24V power supply. Once I connected the two ground rails, the circuit started working.

I've updated my diagram to show (and also fixed up my drawing errors previously - like forgetting the lead to pin8).

Thanks all for your help and input. If you can see any further errors in my design, please let me know.