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I am new to this forum and came across it trying to find a solution for what I thought would be an easy problem to solve.

I work in custom AV installations and I was hoping to find an off-the-shelf solution but either it doesn't exist or I am simply using the wrong terminology whilst searching for a product.

We have to install a TV with a motorized bracket. I am trying to avoid the need for an additional remote control to operate the bracket so I am trying to automate the process so that the bracket, which has a contact closure input, is triggered when the TV is turned on. I spoke to the manufacturer of the TV and they have confirmed that the 5V 1A USB output on the TV turns on/off with the TV and does not remain on when the TV is in standby.

Being clueless I hastily went out and bought two different 5V relays, thinking either will do the trick. I started testing them and only then realized I will need a secondary low level trigger. https://www.switchelectronics.co.uk/products/5v-1-channel-relay-board-module and https://www.switchelectronics.co.uk/products/5v-1-channel-solid-state-relay-board

I was hoping that a relay exists which changes the output state when it is connected/disconnected from it's own power source.

If anyone knows of anything 'ready to buy' that will do the job, that would be greatly appreciated!

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  • \$\begingroup\$ What you bought are not relays, but relay modules. The distinction is important. What you want is just a regular relay. \$\endgroup\$
    – Hearth
    Commented Nov 21, 2023 at 14:13
  • \$\begingroup\$ If you use a bare relay as @Hearth suggested, take note of the flyback voltage spike on powerdown, which can damage the USB port. I discuss this in my answer below. \$\endgroup\$
    – TypeIA
    Commented Nov 21, 2023 at 14:20
  • \$\begingroup\$ This is a Q&A, not a forum. \$\endgroup\$
    – JYelton
    Commented Nov 21, 2023 at 17:09

1 Answer 1

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The first board at least (I didn't look at the second) is "active low," so if you want the relay to be actuated anytime it's powered, just tie the logic input to USB ground. (If you tie it to VBUS, it will always be unactuated, either because it's unpowered or because the active-low control input is high = off).

From the photo, the relay itself appears to be a SRD-05VDC-SL-C and there's a link to a proper datasheet. The power consumption is worth taking note of. It will pull about 90 mA (0.45 W) from the TV's USB port, which is well within the base USB specification of < 500 mA.

Another thing to watch out for is flyback voltage spike from the relay coil. I don't see proper schematics for the board, but the photo shows a diode, so hopefully that's a snubber. If not, you may need to add one externally, otherwise the voltage spike on powerdown can be high enough to damage the USB port. You can check this with an oscilloscope if you have one (most multimeters are not fast enough to catch it).

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  • \$\begingroup\$ Thank you very much for your information. It looks like we have found an alternative solution. As it turns out the motorized bracket also has a 12v trigger. It looks like we will simply be using a 5v to 12v converter cable and trigger the bracket that way. amazon.co.uk/… \$\endgroup\$
    – digitalav
    Commented Nov 22, 2023 at 13:06

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