74hc595 + uln2803 + unipolar stepper issue [SOLVED]

Hi.

I’m working on a wireless antenna tracking system (UP-DOWN, CC-CCW ) with 2 unipolar motor,
mitsumi 5v-1amp and a 28bjy 5v.
I’m using a shift register 74hc595 with a uln2803 to drive the steppers.
The schematic that I use is this one:

Just added some caps, 100nF for each IC and a 1uF for latch pin on the 74hc.
My issues is: before adding these caps, my mitsumi stepper act like crazy,
starting making noise like uncontrolled steps every time I power on the circuit. 5v 3amps power supply.
Now with caps, it happens much less, but some times it come crazy. With the 28byj, never got that issue.
This issue is not related with my code. Even disconnecting my arduino from the circuit, this occurs.
My 1st test was with push buttons and only the uln2803, and every thing was running as expected.

Something wrong with this schematics, or I´m missing something ?

Any help and suggestion are welcome. Thanks

and a 1uF for lach pin on the 74hc.

You don't put a large capacitor on a logic pin, they go on the supply pin to ground
for decoupling.

You should really be running the motor from a separate power source.
And the motor supply will need more like 100uF or more decoupling, its much
higher current. Without lots of decoupling on the motor supply you run the risk
of the inductive spikes on the supply well above the nominal voltage.

Thnaks.

MarkT:
You should really be running the motor from a separate power source.

Is not powered by the arduino, I uploaded the wrong schematic ( I just corrected that on the post ), I powered the circuit with additional power supply.

MarkT:
And the motor supply will need more like 100uF or more decoupling, its much
higher current. Without lots of decoupling on the motor supply you run the risk
of the inductive spikes on the supply well above the nominal voltage.

So I need to remove the 1uF from latch pin.
And for the motor supply should I need decoupling all 5 wires ( unipolar) or only the COM (5V) wire ?

Thank for help.

The capacitor on the latch pin is severely overloading the Arduino pin, its a complete
howler that's still out there in someone's online description.

Decoupling is always across the supply, here from the COM pin on the ULN2803 to GND.

Decoupling a signal is called "low pass filtering", and is done with analog signals, not
logic signals.

MarkT:
Decoupling is always across the supply, here from the COM pin on the ULN2803 to GND.

Hi

Thanks for that.
All most solved the issue; with 100uF and 220uF got stable results.
Now, it just occurs when I touch the board, or move my finger very close the 74hc595. Any clue ?
I read some comments about this kind of problem and the suggestion was to put some 10k pull-ups resistor on data line and clock.

Those signals should be being driven push-pull so resistors not required.

It you have long leads then you need to pay attention to the kind of cable. Any problems
are likely to show up first with the clock signal as reflections and glitches on this signal
will cause double-clocking or missed clocks.

Hi

MarkT, thanks a lot for your time.

I made a shoot and added a pull-down resistor, a 10k SMD in RCLK pin 12 on the 75HC595 to GND, and no more "random" crazy stepper sound.

You really help me to understand all details about this schematics to take consideration.
Is working now, as I expected, slower to react when I press the push buttons as it was before add the 74hc, but for the project is working fine.