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I have a HBaseT Matrix (EL-4KPM-V88-A2416) that has 3.5mm mono (signal+ground) plugs that is used to send IR codes to HDMI devices (it would usually be connected to an IR emitting diode placed in front of the HDMI devices) the signal is documented as 5v.

I'd like to connect this to a Raspberry Pi GPIO and use LIRC to translate the commands and then perform my own action when I detect certain remote control buttons have been pressed.

I know I can't connect the 5v signal directly to the Pi so I was going to add in a level shifter however I don't have access on this plug to 5v. From my understanding I need to power the 5v side of a level shifter from the device generating the signal and I can't power it from the Pi.

My question is that correct? Can I use the Raspberry Pi 5v into a level shifter? If not is there another way to connect this to my Pi if I don't have 5v available?

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You can level shift using a logic chip that can safely accept 5V on input while being powered from 3.3V, or completely unpowered.

But please note that if the outpuy is used to drive an IR LED, the output must be modulated with the IR carrier frequency. It means you might want to filter out the carrier as that is how most IR receiver modules work, they give out a demodulated baseband signal.

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By far the cleanest approach is to use an optocoupler (such as an H11L1). The upstream circuit drives an IR LED, the phototransitor/logic gate connects to the RPi power supply, and mirrors the upstream state (give or take an inversion). This gives you galvanic isolation, but signal passthrough.

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