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Goal: Use transistor as a switch to control an intercepted signal

I have a system for which the schematics/information are not available. I refer to it as a Blackbox (also labeled as such on my diagram). I've cut the wire between the "Blackbox controlling all peripherals" and "Blackbox controlling solenoid". I would like to join them together but with a transistor in the middle so I can use my RPi to either allow communication or block communication on this wire. For example enabling HIGH on RPi CTRL pin on diagram.

The dotted lines on diagram represent where I would make wired connections however I do not know what type of transistor or what a correct schematic for this purpose would look like?

I would like help mocking up a schematic that would accomplish my goal.

desired_setup_raspberry_pi

ar94
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    There is only one wire between these two boxes? – HandyHowie Feb 02 '22 at 13:59
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    And no ground connection on the two black box circuit? – Andy aka Feb 02 '22 at 14:10
  • @HandyHowie There are three wires between the blackboxes (red, blue and black), I only drew the one I cut which was the blue one. I connected this cut wire to a mechanical switch and it worked (circuit operated as normal when I pressed the button and nothing when I left button open) which indicates this is the wire I need to take control. – ar94 Feb 02 '22 at 14:56
  • @Andyaka There is a ground, likely the black wire I mentioned in my comment above. I suppose that should be my ground on the transistor as opposed to the ground from the RPi? – ar94 Feb 02 '22 at 14:57
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    Do you know the voltage of the signal between the black boxes? – HandyHowie Feb 02 '22 at 17:33
  • @HandyHowie yes, I measured 3.3V – ar94 Feb 02 '22 at 18:26

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