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I have a display monitor that is connected to a Raspberry Pi, all running off a battery. Most of the time the monitor is not used so I'd like to turn it on only when required, but I also would like it to be automated, i.e. to have the Raspberry Pi do the "turning".

I was wondering, if I use an N-channel MOSFET like in the scheme below, would an issue rise, since the monitor's ground would be connected to the transistor's drain through the supply wires, but as the supply is shared and there is also a ground connection in the HDMI connector it will also get connected to supply ground as well, through the Raspberry Pi.

schematic

simulate this circuit – Schematic created using CircuitLab

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3 Answers 3

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I was wondering, if I use an N channel MOSFET like in the scheme below, would an issue rise

Anywhere between likely and certain you will get problems with unwanted current down the HDMI 0 volts when the monitor is supposedly disabled by the low-side MOSFET. The worst case scenario is that you damage the monitor's HDMI port and the RaPi's port too.

I highly recommend that you use a positive rail switching transistor.

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    \$\begingroup\$ The most probable outcome will be the monitor getting ground from its HDMI port and RPi and simply not switching off. \$\endgroup\$
    – fraxinus
    Feb 11, 2022 at 10:02
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You can use HDMI "hot plug detect (HPD)" to programmatically disconnect the monitor from the source. Disconnect the monitor side of the HPD and connect your own circuit to the source side HPD. Your monitor will also go to power save when no signal is detected and the source will think there is no signal.

Just disconnecting the gnd from monitor is not a good solution, even if it would work in a certain setup with some luck.

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Similar to Ralph's answer, but using stuff how it's actually meant to - HDMI has a feature called CEC (Consumer Electronics Control) which is intended to, among others, do exactly this - switch the display between standby and normal operation. This is how you can use your settop's remote to switch the TV on and off.

While I haven't used it myself, this seems dead simple, with no additional hardware needed (example tutorial).

There are two downsides to this:

  • the display must support it, and not all do
  • a display's standby is not exactly free - but considering the power draw of a backlight it might as well be, if properly implemented
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  • \$\begingroup\$ I'm using a $10 LCD display driver module off of ebay, I'm highly sceptical it supports any hdmi feautres... And as I'm barely aware of how hdmi works I prefer to simply shut the power off and be certain that it is off :D \$\endgroup\$
    – php_nub_qq
    Apr 26, 2022 at 11:58
  • \$\begingroup\$ Nevertheless, thanks for the tutorial link, I'll certainly give it a try as it looks really simple. Will report back findings. \$\endgroup\$
    – php_nub_qq
    Apr 26, 2022 at 11:59
  • \$\begingroup\$ @php_nub_qq yeah, that might not work then. You can think of CEC as something akin to I2C or 1-wire running on a separate wire inside the HDMI cable. \$\endgroup\$
    – jaskij
    Apr 26, 2022 at 14:29
  • \$\begingroup\$ Well that's what I immediately thought too, but on second thought, they probably used some IC in that board that communicates with the devices which probably will support CEC. According to wikipedia CEC was implemented in HDMI 1.0 and my board claims HDMI 1.2 so in theory it should work. Unfortunately it's gonna be a good few months if not year until I can get a hold of it and try it. Still I'll report what happens for what it's worth. \$\endgroup\$
    – php_nub_qq
    Apr 27, 2022 at 18:01

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