Relay causes errors on MAX7219 7 segment displays

Yes indeed! Been putting it off for too long but it's the time to make a detailed schematic.

I am now using MOSFET IRF520N for switching LEDs and my problems seems to went away, no more display errors.

Now, the only thing that i am not sure about are resistors on MOSFET, in some schematics i see a resistor (usually 1k) is connected between Arduino PIN and GATE pin on MOSFET and somewhere 10k resistor connected between Arduino PIN to GATE and GND... to prevent floating pin state because of high impendence, which as far as i know is a problem on INPUT pins and not on OUTPUTs as in this case...

Confused...

apart from that, my rig is 99% finished!

Many thanks,
Alek

DrDiettrich:
In case you control the power to the entire display including the MAX, then you should disconnect not only power but allso all signal lines. After turning the display on again the MAX has to be initialized again.

No, i am using "shutdown" function tu turn displays ON and OFF, i am not breaking the circuit by hardware.

elcrni:
Now, the only thing that i am not sure about are resistors on MOSFET, in some schematics i see a resistor (usually 1k) is connected between Arduino PIN and GATE pin on MOSFET and somewhere 10k resistor connected between Arduino PIN to GATE and GND... to prevent floating pin state because of high impedance, which as far as i know is a problem on INPUT pins and not on OUTPUTs as in this case...

Ah, but you see, when the Arduino is reset the pin is an INPUT until your code actually starts up and your code makes it an OUTPUT, so the resistor holds the FET off until that happens.

It all depends on whether having the FET randomly switch on or switch on only partially as the system starts up, will cause any trouble. :astonished:

Paul__B:
Ah, but you see, when the Arduino is reset the pin is an INPUT until your code actually starts up and your code makes it an OUTPUT, so the resistor holds the FET off until that happens.

It all depends on whether having the FET randomly switch on or switch on only partially as the system starts up, will cause any trouble. :astonished:

Hmm, BUT, as soon as the program starts, "setup loop" tells it the pin is OUTPUT, or is that not soon enough?
In any case, FET only breaks the circuit for cca 10 LEDs, so nothing bad would happen in either state of the FET at reset or startup... also, my rig is not going to power up or reset often at all.
Many thanks,
Alek

And that is the point - if you don't care, it doesn't matter, but sometimes you really do not want the thing you are controlling to flash on for a fraction of a second. :roll_eyes:

Paul__B:
And that is the point - if you don't care, it doesn't matter, but sometimes you really do not want the thing you are controlling to flash on for a fraction of a second. :roll_eyes:

Yes, it's all clear now! When its really important to keep the pin LOW until you explicitly tell it it's HIGH, we need a resistor. Learned a lot!
Many thanks,
Alek