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I want to switch between 2 video sources (cameras) to a common video receiver. Is general purpose optocoupler suitable for that?

Here is the schematics: enter image description here

In more details, the video switch is controlled by a 3.3v MCU. It will select input video source before turning on the video receiver, so there will be no question regarding video synchronization. The cameras are of standard composite NTSC video type, have 12V power supply and 2 video wires (plus and minus, where minus is actually the common ground).

So, the question is if optocouplers work well passing analog signal.

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    \$\begingroup\$ Use a relay. It's way better than the opto-couplers. \$\endgroup\$
    – Krauss
    Dec 1, 2023 at 16:11

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No. An optocoupler is not a broadband signal switch. That will not work.

It's not clear what you'd need optical isolation for in such low-voltage signals, anyways.

Either way, you will have to use a broadband analog switch. There's dedicated video switch ICs (e.g., the TS3V330), but any analog switch IC with sufficiently high bandwidth (anything above ca. 6 MHz should do, but doesn't really hurt to go to say 20 or 50 MHz, really) and low insertion loss should do.

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  • \$\begingroup\$ Thank you. Since I'm completely out of this area of analog switches, could you please recommend the most suitable option of these: LMH6570, OPA875, LT1203, MM1111? They all are 2-to-1 video switches. In the datasheets there are such parameters as video signal level, 75Ω resistors, gain amplification, which I can't asses how they fit into my case. \$\endgroup\$
    – brigadir
    Dec 1, 2023 at 5:32
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    \$\begingroup\$ I literally gave you a recommendation in the answer. Anyways, if the recommended use case says "composite video", you're probably fine. The datasheets of any components you might want to use will come with an example schematic that illustrates how to use them. You'll basically be able to copy that. \$\endgroup\$ Dec 1, 2023 at 9:05
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Long live Never Twice Same Color - !

It might work, but if it does it will damage the video quality. A typical opto isolator is not designed to pass megahertz signals. Plus, if the video signals are back-porch clamped to GND, the sync tips will be a negative voltage wrt GND. The opto output transistors might have a problem with that reverse current flow.

There are dozens (hundreds - ?) of analog mux chips designed specifically for video. Maxim was huge in this field in the 80's and 90's. Linear Tech, Analog Devices, and TI all have legacy parts for this.

If GND or power isolation between the uC circuits and the video is an issue, power the analog mux's from the camera +12 V source, and use the opto's to control their address inputs.

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Optocouplers have significant voltage drop over the output transistor and limited current capability. The composite video signal uses low voltages, which would get heavily distorted by the voltage drop.

The ILD206T in the diagram has a saturation voltage of 0.4 V, while composite video signal has a maximum amplitude of 0.7 V.

I agree with Marcus Müller that if you do not need isolation, a video switch IC or an analog switch IC is more suitable.

If you do need isolation, consider photo MOSFETs instead. They do not suffer from a saturation voltage like BJT-based optocouplers do, though they do have limitations in on-state resistance and off-state capacitance.

Toshiba application note Points for photorelays in high frequency circuit applications has a comparison table of their own photo MOSFET products, of which for example TLP3431 would be suitable for composite video. Similar parts are available from other manufacturers also.

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