Switching audio speakers when trunk opens

[jondavidf]

Hey all!

Ok, so I've been out of the car audio and electronics game for just a bit.... I used to be an installer at Circuit City if that tells you how long it's been, but I do remember some fundamentals.

So here's my story:  

I am building a small system for my 16 y/o son's '98 Altima.  I plan to put a single 12" sub in a rear facing box with rear panel that will snug up against the back seat.  I ended up with a couple extra pairs of speakers (don't ask...) and am thinking of mounting them on that back panel.  The issue is I really only want them to be on when the trunk lid opens.  I figure I can use an amp (audio source) and relays to reroute the audio from the internal speakers to the ones mounted in the trunk. 

This is my idea:

Use a normal 5-pin relay to switch audio from an internal speaker (Speaker #1) to an external speaker (Speaker #2).  The power source for the relay will be tied to a hood pin switch mounted in the trunk.  Then I'd wash, rinse, and repeat for each speaker I want to switch.  See attached image for a visual of the wiring setup.

My questions are:

1 - Do you think the above would work?

2 - If so, would I get noise through the audio by running the sound voltage through a relay?

3 - Is there anything like this already available on the market?

4 - Would there be any issues if I just switched (through the relay) the positive for each speaker while running the negative straight to the amp?

Thanks for the help!

Jon

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[NSTALN 10+ year member]

My first job as an installer was at the Circuit City in Oklahoma back in the 90's. I learned a lot and really enjoyed it.

Your plan should work fine but I would add a quenching (or flyback) diode across 85 and 86 to avoid voltage spikes. My co-workers/buddies from OKC CC used to compete and they would have different amp/sub configs for the SPL and SQ portions of the competitions, we would use relays to switch configs.

https://en.wikipedia.org/wiki/Flyback_diode

 

[keep_hope_alive Premium Member]

Your relay plan is fine assuming you are using the N.C. contact for interior speakers and the N.O. contact for external/trunk speakers.

Note that many hood/trunk pins switch to ground (the way they are mounted to metal) so it would be on the ground pin for the relay, tying the relay coil to ground.  if you put the hood/trunk pin on the 12V lead in, it'd just short out the source/fuse.  you'd need a pin with two internal, insulated contacts to wire them the way you show, but i'd still just switch the relay coil ground and not the hot (typical industry practice).  you could also have an override switch so he can defeat this feature (if desired) with a toggle switch in-line with the relay coil ground.

 

[jondavidf]

Thanks for the reply and info!  After ordering the pin switches I realized they had to go to ground so I understand your explanation perfectly now!  It turns out that the trunk is already wired for a trunk light so I just plan to use that 12V source to switch the relay(s).  Good call on the override switch, too!

Thanks for the tip on which speakers go NC and which ones go NO.  I thought I may have to trial-and-error that part.  Do you think it’s okay just to run the speaker positive wires through the relay(s) but run the negatives straight to the amp?

What do you think about needing a flyback or quenching diode across 85 and 86?  I understand the logic but don’t think I risk any harm to the audio system by not having them.  If you do think I should get them do you have a recommendation on which ones specifically to get?

Thanks again for all the help!

Jon

 

[keep_hope_alive Premium Member]

You'll need to switch both positive and negative since they are routing to different speakers.  You can get two-pole relays or four-pole relays to simplify wiring but they are more expensive. They can include relay bases so wiring is cleaner. 

The flyback diode is mostly when you're powering the relay coil from a circuit (op-amp, transistor, timer, etc.).  When I control relays from custom circuits, I use the diode.   When powering from a car's 12VDC, the quenching diode isn't necessary (but it can't hurt).