I want to install front and rear cameras on my car. However, the head unit only allows one composite in.

The circuit I am thinking of is below. I think the power side of the circuit is OK, but to keep things simple, I thought of just joining the composite signals with a simple rca splitter.

I only need to show one camera output at a time and am not too worried if the changeover is a bit rough.

My main concerns are… Will it work with just a splitter? Will I damage the head unit?

Any suggestions on a simple alternative would be welcome.enter image description here

EDIT. After the feedback (Thanks). I decided to go with a DPST relay on the composite signal wire with common earths. This seems to be the same as manual changeover switches. As this is in a car, I will use standard automotive relays for this Any comments?

  • \$\begingroup\$ A splitter is used to take one input and split into two ~identical outputs. You have the opposite need here: you need to accommodate two inputs. I would not recommend hooking up the two as you propose. You need something more like a selector that lets you choose which input (camera feed) shows up on the output of the selector (and therefore makes it to your head unit). \$\endgroup\$
    – gcr
    May 24 at 23:42
  • \$\begingroup\$ Why not use a relay with 2 changeover contacts? \$\endgroup\$ May 24 at 23:48
  • \$\begingroup\$ Thanks for the fast response. So I understand fully (I'm very new to this) GCR why would you not recommend using the splitter - considering that only one camera is powered at a time. Bruce - Do you mean, put a relay in the composite circuit? Can you recommend a relay - I looked around but couldnt find one. ... \$\endgroup\$
    – 1981TR8
    May 25 at 0:57
  • \$\begingroup\$ Use [at] to tag a person. E.g. @BruceAbbott. Tagging a person send him/her a notification so that he/she sees your comment/question. You can tag only one person in a comment. \$\endgroup\$ May 30 at 3:55

My main concerns are… Will it work with just a splitter? Will I damage the head unit?

It will probably 'work', and won't hurt the display unit. However you are feeding the composite video signal back into the output of the unpowered camera through the 'splitter' (which probably just has both output sockets shorted together) which may degrade the signal.

A better solution might be to power both cameras at the same time and use the relay to switch the video signals between them (signal wires only, not the shields - which should all be connected together). If you want to power only one camera at a time for some reason other than controlling their video outputs (eg. to reduce accessory power drain) then use a relay with two changeover contacts, or wire two relays in parallel.

Using separate relays may reduce the possibility of an accidental short between the the power and video signal wires (which could damage the display unit), and you could use a 'signal' relay for the video. Signal relays have contacts that are designed to work at low current and voltage, whereas power relays need to switch a relatively large current and voltage to 'burn off' surface oxidation and maintain low contact resistance.


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