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I thought it best be start with some context. We're trying to connect two "Tally" systems together - system that turns on a red or green light when a camera in a studio is live or on standby.

The main system we have has a GPIO output (a 2.5mm connector) that seems to be two open collector outputs, one for each LED colour, green and red. The manufacturer provided us with this diagram:

https://i.sstatic.net/bFGzN.png

I can get this to work no problem - BUT the LEDs we need to switch are switched positively. All have a common ground, and need the positive supply switched to turn the green and red LEDs on and off.

I've given this a real good go by myself first and doing a lot of research, but the closest I've got is having it work the opposite way. i.e. green light on when its meant to be off, green light off when its meant to be on. I did this by wiring up a 5V source to the open collector (top pin) with a pull up resistor, then wiring that to a MOSFET, but it did the opposite as I mentioned.

This is most of the circuit that we're building, but missing the part that connects the open collector outputs to the Positive supply that drives the LEDs...bit messy, I'm only DIY apologies.

https://i.sstatic.net/r5vht.png

What would be the best way to drive the LEDs? I want to avoid relays and the clicking so if have to go relays, solid state, but would prefer a solution like MOSFETs or similar.

I really appreciate your help and patience.

Proposed circuit:

https://i.sstatic.net/r5vht.png

Supplied by manufacturer:

https://i.sstatic.net/bFGzN.png

Products:

Main system: FlexTally - https://flextally.cerevo.com/en/other-functions/lamp-gpio-output/

LED we're trying to drive: DataVideo TD-3: https://resource.datavideo.com/Instruction_Manuals/TD-3_G082060738B1_A6.pdf

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  • \$\begingroup\$ Your tally light needs a 12 V power supply. Do you have one? \$\endgroup\$
    – CL.
    Commented Nov 16, 2019 at 11:44
  • \$\begingroup\$ We plan to use the 5V USB power source already in the circuit. We've confirmed with the manufacturer the tally light is meant to run on 3V-5V anyway. The 12V supply in the manual is for testing and the resistor drops it down to around 3 from memory \$\endgroup\$ Commented Nov 16, 2019 at 11:56

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For high-side switching, you need to add a PNP or P-channel transistor:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Note - the above won't work until you add a resistor between Gate & Source. I would also add a resistor in series with the gate. \$\endgroup\$ Commented Nov 16, 2019 at 17:27

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