Replication Steps:

  1. 595N shift register powered by arduino
  2. Each output on register connected to 1 of 8 relays in relay board
  3. Arduino programmed to loop over each 8, triggering 1 relay at a time
  4. Dedicated 5v power line connected to relay board as so enter image description here
  5. Begin loop
    • Expected: LED's light and relays make contact iterating over each of 8
    • Actual: Occasionally relay/led fires, then immediately goes out/disconnects OR occasionally relay/led x should fire, but instead relay x+1 fires
  6. Disconnect separate DC supply from relay board
    • Expected: Relays will not fire/make contact, LED's will continue buggy pattern from step 5 actual.
    • Actual: Relays don't fire (expected), LED's light in correct order always, no bug

Friend who is more hardware savvy thinks it could be simple matter of Separate DC supply needed more voltage as my supply is providing enough amperage to fire all 8 contacts simultaneously. However, this board is rated for only 5V correct?

What gives?

  • what voltage is -VE ?
    – jsotola
    Mar 23 '18 at 2:04
  • Please show us your sketch.
    – VE7JRO
    Mar 23 '18 at 3:35

you need a ground connection between the arduino and the relay board

  • same result whether or not a ground connection between arduino and relay board is present
    – Doug Clark
    Mar 23 '18 at 2:17
  • @DougClark common ground?
    – user2497
    Mar 23 '18 at 13:45
  • @user2497 yes the ground from the relay board goes straight into the arduino, thus common ground correct?
    – Doug Clark
    Mar 25 '18 at 13:56
  • @DougClark yes, that’s system COMmon, or gnd. each channel pulls about 5-10mA to turn on the optocoupler, and each relay will pull ~50mA for solenoids - so expect 400mA, well below USB2 current limit. I am unsure if the collapsing EM field is an issue on GND-JD_VCC. Have you verified that your shift register is working? It seems like the only tricky part.
    – user2497
    Mar 25 '18 at 14:10
  • @user2497 Step 6 Actual results (led's light correctly) as well as using a voltmeter on each output leg of register verifies the shift register is acting according to code. I have no idea how the independent act of powering the solenoids from an isolated power supply causes everything to fall apart
    – Doug Clark
    Mar 26 '18 at 15:24

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