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Author Topic: RCA video signal and transistors?  (Read 1073 times)
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Warning, I'm a newbie.

I want to develop a project where a microcontroller switches the video input of a TV between two different video RCA connections. So my thinking was that I could use two transistors, each connected to its own RCA video signal on the collector pin. Each transistor base would be wired to a digital output pin on the Arduino. Only one transistor would receive a base signal at a time, which would then output the RCA video signal.

So would a transistor work with an RCA video signal? Or should I do something else entirely? And if so, what kind of transistor would work for this?

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I've made a video switcher using NPN transistor and 5k resistor on the transistor base. The requirement is that the arduino ground would need to be connected to the video ground on the tv and not the emitter of the transistor.
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No, you can't use BJT transistor as a switch,  it has saturation drop voltage (~0.3V). Distortion to the level of video signal (~ 1 V) way to high. JFET/MOSFET will works, but you have to design bias voltage circuitry so it stay reliable open/close - which is not easy task with arduino +5V.
Look for chip, some multiplexer IC should be o'k, only requirements it has to pass 5 MHz.
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Maxim makes a series of video multiplexers if your transistor attempt doesn't pan out.

MAX4158/MAX4159/MAX4258/MAX4259

MAX4310–MAX4315

-j
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or use the microcontroller to bias the bjt.

the bjt toggles a relay. common to tv, NO to one source, NC to the other source.

unless you intend to switch the signal a LOT...... a mechanical component should work just fine.
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No, you can't use BJT transistor as a switch,

I've done it with the simple Radio Shack NPN transistors, so I think you are floating some BS.  smiley-wink

You can test by putting an NPN transistor in the video signal line from the video source with the signal inputting the collector and exiting the emitter to the input of the video device (tv, capture card, etc). Video ground wire connected as usual between the video source and video use device. Connect a 10k resistor to the transistor base, then connect a 9v battery negative to the transistor emitter and the battery positive to the resistor supplying the base. Connect, video passes, disconnect and video stops.  
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Thanks for the guidance, zoomkat. I'll rig up a test on my breadboard like you suggested.

Just one quick question. I'm curious why the resistor is needed on the base.
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You can test by putting an NPN transistor in the video signal line from the video source with the signal inputting the collector and exiting the emitter to the input of the video device (tv, capture card, etc).
I didn't say it wouldn't switch video signal on. What I say, there would be significant distortion in the video.
 Have you tested signal after it pass throw BJT ? What level it has, how many gray/color level it has ? Probably, such switch be o'k for peephole B/W  video.
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I didn't say it wouldn't switch video signal on. What I say, there would be significant distortion in the video.

You from another plannet? You said "No, you can't use BJT transistor as a switch,".   smiley-roll
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Just one quick question. I'm curious why the resistor is needed on the base.

I used it to limit the current thru the transistor base and the video card video input. The actual power source to the transistor base was from a 5v parallel port pin. I used a 5k resistor as provided a decent picture from the web cam and limited the current to ~1ma. This currnt actually flowed thru the video input pin on the capture card which has a 75 ohm resistance. I looked at the video multiplex chips at the time, but the requirement for both + and - voltages to operate made them more complex than I was interested in trying.
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Just to say that this sort of arrangement is called a common base amplifier if you want to look it up and I have used it as the input for several video digitisers I designed in the 80s.
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