Shiftregister goes Haywire with load on relay

Discussion in 'General Electronics Chat' started by Raphael_DK, Jul 14, 2013.

  1. Raphael_DK

    Thread Starter New Member

    Jul 14, 2013

    My first post here, so bear over with me.

    Now, my problem.

    I use a Arduino Uno with a 74HC595 Shift Register and a 5v relayboard with integrated optocouplers.

    The problem i face, is that as soon as there is load on the relay, the shift register goes haywire.

    I'm aware of that it's caused by noise, but i strugle to figure out how to wire the relay / shift register to remove that noise.

    I found following threads with solutions, but i find the schematics confusing:

    Right now the shift register is wired directly to the arduino and the relay. No rectifier, resistor or capacitor is mounted anywhere.

    My questions is:

    How should i wire it "properly" to remove all the noise / problems I'm facing.

    I know this maybe a "simple" problem to solve, but i just can't see the solution. Maybe I'm stupid? hehe..

    Thanks in advance :)
  2. MrChips


    Oct 2, 2009
    Well get a darn 0.1μF capacitor across Vcc and GND of the 74HC595 plus a 10μF cap across the power supply.

    Add a 1N4001 diode across the relay.
    ErnieM and Raphael_DK like this.
  3. Raphael_DK

    Thread Starter New Member

    Jul 14, 2013
    Thanks for the very fast answer !

    I've got both

    0.1uF 250V 104J CBB Metallized Film Capacitor


    1A 1000V Silicon Bridge/Rectifier Diode 1N4007

    But i do not have a 10µf cap, is there anything else i could use instead ?
  4. absf

    AAC Fanatic!

    Dec 29, 2010
    Is you load connected to AC mains or a higher DC voltage higher than 5V?

  5. Raphael_DK

    Thread Starter New Member

    Jul 14, 2013
    My relay is connected to the 230v AC mains.

    I still experience some problems with the solution above. Maybe because i don't have and use the 10μF cap across the power supply. ?
  6. savignyexpress

    New Member

    Jun 22, 2017
    Hello everybody,

    I've got similar problems with 74hc595 used in a model railroad application. The turnouts quite high current peeks were probably the cause of the perturbations.

    As the shift registers were in separate boards, I've used shielded cables between the boards to carry the data, serial clock and output clock between the boards. And obviously the shield is connected to boards grounds. As shielded wire, pieces of Ethernet cable is perfect as it has enough wires insided for such an application.

    This solution greatly helped to suppress almost all the perturbations.

    Best regards.