Automatic switching of speaker signals

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Cadence10m
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Automatic switching of speaker signals

Post by Cadence10m » Tue Oct 04, 2005 10:32 am

I have a sailboat that has a VHF radio and a CD player. The CD player has stereo speakers in the cockpit so I can hear the music while sailing. The VHF radio does not, and I can't hear it when the motor is on (and I can't even hear it well enough in the cockpit to understand the broadcasts when the motor is off). The radio has an RCA jack on the back for an external speaker so that I could hook one up in the cockpit and hear much better. But I don't want to cut another hole in my boat, nor have a 3rd speaker that really only gets occasional use. So my idea was send the VHF external speaker signal to the CD player's left speaker (it's only about 2" away) by somehow appropriately mixing or switching the signals.

The radio manual says "Use a speaker with an impedance of 4 or 8 ohm, with an RCA phono plug." Audio output is 4 Watts. I don't know the exact model of CD player but it's just a typical consumer car audio CD player. There's no additional amplifier attached to it and I don't listen to it very loudly.

My guess is the best solution would just be a two position switch that was triggered by the presence of a voltage on the radio's external speaker jack. Since it would just be a switch it wouldn't introduce any (or much?) distortion to the sound as well.

One problem is that the circuit (or switch at least, if I use a relay) needs to be able to handle the amplified signal sent to the speaker.<p>I'd like to build a circuit myself, but I'm not sure how to do that. Any ideas, or circuit diagrams out there that would work?<p>Any help is very much appreciated.

Craig

Dean Huster
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Re: Automatic switching of speaker signals

Post by Dean Huster » Tue Oct 04, 2005 12:57 pm

When you really get right down to it, about the only time you ever realize a stereo effect is when you're wearing headphones or sitting in the sweet-spot of a home stereo installation. Why not just mix the left and right channels of the CD for one speaker and run the VHF to the other?<p>It's odd that you seem to be placing more importance on maintaining the existing entertainment system installation than to the VHF radio while bobbing around out there on the high seas or lake or river, or am I reading you wrongly here?<p>Dean
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Robert Reed
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Re: Automatic switching of speaker signals

Post by Robert Reed » Tue Oct 04, 2005 2:46 pm

Caden--
Deans suggestion makes a lot of sense (especially since I am a boater myself). But if you really want to go ahead with this project, you could use a VOX (voice operated relay )on this channel, When audio is present on Maritime radio, It takes command by pulling in the relay from CD to VHF. These are available at many kit manufaturers sites (or Radio Shack). Try googling up said device and I am sure you will get a lot of sites to check out. These devices are simple to install and probably have thorough directions. One thing that would be a definate assett to these devices is a time delay on dropout ( 10 seconds ?) so as as not to be bouncing around a lot for transmission pauses-- inquire manufacturer. These are simple circuits to design even with delay if you are knowledgable in electronics.

Cadence10m
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Re: Automatic switching of speaker signals

Post by Cadence10m » Tue Oct 04, 2005 3:09 pm

Thanks for the advice guys. The VOX sounds like just the ticket.<p>To be clear, I'm not putting the emphasis on the music. On the boat I'm very much a "safety first" guy. In fact, right now I simply don't listen to music while underway on the boat, so that I can hear any VHF broadcasts. I also have a handheld VHF that I bought so I could hear broadcasts more easily in the cockpit (and as a backup in case the main radio dies). But the handheld isn't a great solution. This thread is just to find a solution where I CAN start to listen to music while sailing.

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Externet
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Re: Automatic switching of speaker signals

Post by Externet » Tue Oct 04, 2005 3:18 pm

Hi Cadence10m.<p>Open your VHF radio and hook a wire to the squelch node. It may be easier if there is a squelch led.
Bring that wire out of the chassis to a tiny relay to control which audio will go to the speaker of choice, or to both, or only one and muting the other to suspend music at all when the VHF receives.<p>This way you maintain your existing entertainment and give priority to the important stuff. And no extra holes... except one to pass the wire trough the VHF back panel, which you will seal with silicone.<p>Miguel ;)
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Edd
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Re: Automatic switching of speaker signals

Post by Edd » Tue Oct 04, 2005 4:19 pm

Fortunately there are some kit manufacturers that are generous with their schematics. This site happens to give you enough data for the build up of a VOX circuit from their info. Two options on the input circuitry to the 1st op amp: 1...either use it is, with a use of a small electret mike near the receiver for minimal extraneous back ground noise pick up.
2... If the receiver is left at an adequate volume setting, it might be possible to just inject its audio into the left side of C3 audio coupling capacitor and not building/or/ incorporating any of the circuitry of the first op amp feeding into that cap, since its gain and freq selective response is negated by inputting a squelch tripped audio signal into the circuit. Simplified, the parts count is then cut down to 16 parts. The final audio switching is accomplished via the SPDT relay.
Did you get the word ?...the word is :<p>http://www.ozitronics.com/docs/k126.pdf <p>BTW ...Miguel's is the simplest hardware solution, but typically would require a tad of current gain interfacing into that relay coming up from LED current activation level's..IF there even is a LED of that nature incorporated into your unit, otherwise you would have to be circuit savvy to even locate the squelch circuitry within the transceiver.<p>73's de Edd
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Re: Automatic switching of speaker signals

Post by Cadence10m » Tue Oct 04, 2005 4:21 pm

Externet,<p>Beautiful solution. Simple, and lets me use the existing squelch dial to control nuisance noise rather than real radio traffic! Thanks.

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Re: Automatic switching of speaker signals

Post by Cadence10m » Tue Oct 04, 2005 4:35 pm

Edd, <p>I bought that kit 30 minutes ago! If I'm feeling brave I'll pull the cover off my radio and see where the squelch circuits are, if not I'll use the kit.<p>I like the idea of not using the mic since I don't want conversations in the cabin to be inadvertantly blasted over the cockpit speaker. It just seems like a good idea to eliminate extra parts too. If the audio signal coming from the VHF turned out to be too much for the cicuit to handle at the C3 capacitor, I suppose I could use a potentiometer to knock down the voltage to a suitable level (after experimenting a bit I could determine a resistor size and use that instead of a pot). <p>Do you think that would work?<p>Craig

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Re: Automatic switching of speaker signals

Post by jollyrgr » Tue Oct 04, 2005 4:35 pm

Use a VOX. You can build your own from one of the numerous schematics all over the place or from a kit. Ramsey has one for $10. Go here:<p>http://www.ramseyelectronics.com/cgi-bi ... on&key=VS1<p>Connect a relay to the output of the VOX. Connect the CD player speaker out to the NORMALLY CLOSED terminals of a Double Pole Double Throw (DPDT) relay. Connect the audio from the Marine radio to the NORMALLY OPEN contacts of the relay. Place a 10K and 1K resistor in series and place this series combination across the speaker lines from the marine radio. Connect the MIC IN of the VOX to the two leads of the 1K resistor. Hopefully when the marine radio receives a signal, the VOX will "hear" it and switch the relay over to the marine radio signal. You might have to play with the signal source as this circuit was designed for a MIC in and NOT a speaker level signal. The resistor combination above will pad down the speaker level signal using a voltage divider that should give enough of a drop down for the mic in. Plus it will load the speaker only minimally. Put an an electrolytic capacitor (100uF ???) in parallel with the relay coil to extend the "wait" time for the drop back to CD audio. If the schematic were available, I'd be able to show you where to add a smaller capacitor on the OP amp to extend the time. (And even add a variable resistor to make this adjustable.)<p>As many times as I've seen this request, I'm surprised that a commercial product does not exist for this circuit. I built something like this into my aviation intercom for my senior design project. Many observers during my presentation wanted these for their mini vans. My intercom had audio for a CD/Tape player with an override for aircraft radio. Same basic concept.<p>Since then I've seen this request for marine radios, police/security radios, phone ringers, and so on. If I had a "Pacific Rim" builder, maybe I'd have something.....<p>
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Cadence10m
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Re: Automatic switching of speaker signals

Post by Cadence10m » Tue Oct 04, 2005 4:52 pm

Jolly Roger,<p>You've answered my question about knocking the audio output down to a level the VOX circuit can handle. Here's the kit I bought.<p>http://www.hobbyengineering.com/H2244.html<p>It's similar to the link you provided, but it includes the ability to set the delay on switching back to the music signal.<p>I too was surprised that there weren't any commercial products for this market. Especially since it's a very common problem and you can triple the price of anything that made for the "marine" market and people will still buy it ;) There are some products out there for motorcycles (switching helmet speakers between radio, music, GPS voice directions, radar detectors, etc - I saw one that would handle 3 or 4 inputs). But they all deal only with unamplified signals and not signals "downstream" of amplifiers.<p>When I have it all figured out, I'll do my good deed for the day and post the solution on some sailing forums.<p>Thanks.

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Edd
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Re: Automatic switching of speaker signals

Post by Edd » Tue Oct 04, 2005 7:42 pm

Since you have ordered/committed to the specified kit, you should be covered. As per its simplification, I was just thinking along the line of building a unit up from scratch and the possibility of cutting its component count. As for the “hang” time of the circuit , that is relevant to the units audio drive level input and its rectified charge stored in C4 electrolytic capacitor, gradually increasing its value , should give more time if required. <p>73's de Edd
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Re: Automatic switching of speaker signals

Post by Robert Reed » Tue Oct 04, 2005 9:39 pm

Caden---
Wow, you are a man of quick action. Looks like the ciruit you need. But ,if I may make a few suggestions:
The mic is part of the bias circuit; replace this with 10K Res. for split rail bias.
Bridge your speaker into this input thru a 1MF cap.
Change R7,R9 to 10K value.
This will give you a gain range of X1 to X 21, which will better handle speaker audio output level.These are miniimal changes (especially if you have a lot of 10K res.) at construction time . I beleive Edd meant C2 on the subject of delay. This will mean increased delay time, but if it slows down the attack time too much, try increasing R5. Don't know what your op amp inputs are (bipolar or FET) this may limit max. value of R5.

Cadence10m
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Re: Automatic switching of speaker signals

Post by Cadence10m » Wed Oct 05, 2005 10:31 am

Robert,<p>Thanks for all the detail. As an electrically challenged mechanical engineer, I do need a bit of further explanation.<p>The audio out from the radio will have two wires coming from the RCA plug. My reading of your post is that I'd put one of the those wires in series with a 10K resistor and then connect the end of that resistor and the other audio out wire, in parallel, across the capacitor. Finally, each end of the capacitor would go to the connectors that would normally go to the mic.<p>In other words, the 10K resistor is in series to the radio, and the capacitor is in parallel across the radio, and also across the VOX circuit kit (where the mic would normally be). Is that correct?<p>Thanks. Craig

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Re: Automatic switching of speaker signals

Post by Robert Reed » Wed Oct 05, 2005 3:13 pm

Caden--
Two wires from your VHF RCA plug--one 'hot' (audio hi) one ground (audio lo). The kit micophone will be replaced with a 10K resistor. this will connect from the lo end of R8 ( also a 10K) to ground. This junction point was the old 'mic' input, and will now be the new 'speaker' input.Connect the 'hot' lead from the RCA plug to a 1MF capacitor. Actually you could use a 0.47 MF here due to a voice frq, lo end cutoff of 300 Hz. Connect the other end of the capacitor to the new 'speaker' input point. Connect the other lead from the RCA plug to kit ground. You might want to use an ohmeter on the plug first to confirm ground. As to the R5 resisistor mentioned in previous reply, yuor op-amp is a bipolar inut and this will limit the upper value ( 2 meg ?) . Hope this does the trick for you.

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