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I am trying to design a circuit which will switch between two RGB outputs into a CRT TV's neck board. Each of the two outputs has three pins. I have tried using the 2N3906 PNP transistor, and it works but I have hit a wall with trying to get it to switch to a second output without interfering with the prior output. My design doesn't also seem correct as per where I have placed the resistors across the transistors according to examples online that I have found, but it seems to be working.

Notes:

  • The output voltages are ~1.2V at 40uA.
  • An ATMEGA328 is being used to control the logic.
  • Pin 3 of the ATMEGA328 is what currently controls the original AV RGB signal.
  • Pin 2 of the ATMEGA328 is what will be used to control the additional output.
  • The "IN CRT" labelled R1, G1 and B1 are the INPUT to the CRT neck board.
  • The "OUT IC" labelled R2, G2 and B2 is the AV RGB OUTPUT.

So, to avoid additional confusion I simply need to duplicate the transistor array so I can control a second output.

Circuit with a single output (it needs dual-outputs)

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  • \$\begingroup\$ It’s working because you’ve directly connected all those Pins at CRT port to respective pins of OUT port. You bypassed the PNP transistors.Also this seems like this application requires ANALOG SWITCH IC. \$\endgroup\$ Commented May 5, 2020 at 3:24
  • \$\begingroup\$ Ah, right. So when the transistor is active, all it's doing it shunting the current though that 10K resistor thus seeming to me like it is working? \$\endgroup\$ Commented May 5, 2020 at 7:46
  • \$\begingroup\$ If the transistor is on and biased correctly then yes. \$\endgroup\$ Commented May 5, 2020 at 18:05

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You’re circuit is working but not the way you planned on it to work. It’s working because you’ve directly connected all those Pins at CRT port to respective pins of OUT port. You essentially bypassed the PNP transistors. Also this PNP switch won’t operate properly like this and it’s not best suited for this application. I recommend using an analog switch IC as this is what it can be used for. In your case you want to switch pins from one input port to either PORT A or to PORT B for example. To accomplish this task, you can use a Quad SPDT analog switch such as the ADG333A from Analog Devices. Here you can wire an input port to pins D1 through D4, output port A to pins S1A through S4A, and output port B to pins S1B to S4B. IN1 through IN4 are control pins.

I should mention that if you use this IC, it requires power sequencing if use dual supply(ie, power to positive power first then power to negative supply.) But you can also use different but similar functioning IC.

Shown below is an image of its internal function block:

enter image description here

Source: https://www.analog.com/media/en/technical-documentation/data-sheets/ADG333A.pdf

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  • \$\begingroup\$ Thanks for the informative advice and information! Funny, I was thinking that a Quad Switch might do the trick and be easier to use. Do you think I can use a 4066 Quad Bilateral Switch instead? I have one of those on hand already. Also, what type of resistance do you think I would encounter using a Quad Switch. ~5 Ohms? I was actually contemplating to use relays but they can have an internal resistance of up to 4K. \$\endgroup\$ Commented May 5, 2020 at 7:44
  • \$\begingroup\$ If you use the 4066, then you’d need two ICs. Good thing about Quad SPDT analog switch ic is you need one but it’s up to you. The 4066 can have high On state resistance of 400 ohm. I’d recommend a newer version which might have better specs. \$\endgroup\$ Commented May 5, 2020 at 18:12
  • \$\begingroup\$ Yep, you are correct. After checking the datasheets, the 4066 has a much higher resistance and I would indeed require two of them. As a test however, it's a good place to start. I will order some ADG333A's in the meantime. Many thanks Leoman12! Really appreciate your help :D \$\endgroup\$ Commented May 5, 2020 at 23:01
  • \$\begingroup\$ You’re welcome. Make sure to read the Datasheet for that part since It provides some good info. Good luck. \$\endgroup\$ Commented May 6, 2020 at 1:04

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