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Author Topic: automatic source selection  (Read 369 times)
ANDYEICH
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Posts: 1




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« on: February 20, 2008, 04:21:24 PM »

I have a house by a small airport and wish to create an audio system that can play the typical music sources (mp3s/cds, AM/FM radio), but can automatically be overridden whenever there is a transmission on civil aviation frequencies.

Lower priorities include:
1. receive a channel of FRS/GMRS from the local ski resort with the same ability to override/mute the music
2. receive HD radio
3. transmit on aviation and FRS/GMRS

What is the cleanest way to get this functionality and make sure that volumes can be balanced from the
different sources?

I found scanners that will switch to a certain "priority" station when there is a transmission, but not ones that will do the same for auxiliary source(s).

As an alternative, I have an old aircraft radio and have a wiring diagram that shows how to rig it for desk top use.  I could probably pick up an old aircraft audio panel as well, which ought to provide this automatic switching capability, but I doubt the audio characteristics would be good for music.

Any help is much appreciated!
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W5FYI
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« Reply #1 on: February 20, 2008, 05:35:10 PM »

   The simplest circuit I can imagine is a transistor-controlled relay that switches the loudspeaker leads from the "entertainment" side to the aviation/GMRS receivers when their audio becomes active. A real easy way to do this is to run the base of an NPN transistor through a 2K resistor to the squelch circuit of the aviation radio; the collector through a relay to the positive power source, and the emitter to the two common grounds. The relay reed contact goes to the loudspeaker, the N.C. contact to the entertainment radio audio output, and the N.O. contact to the aviation radio. As long as the squelch is silent, the transistor remains off, and music gets through to the speaker. When the aviation channel is active, the transistor turns on and switches the relay to the "critical" audio.

   You can get elaborate, sequencing several relays to different radios, assigning priority to whichever is most important, and you can devise a hang-time circuit to keep the "entertainment" audio off until a certain amount of time elapses after completion of the critical audio. If you have some radio reference material, a good place to start looking for circuits is under "transistors as switches," and "carrier-operated relays" to get some ideas.

Stew, W5FYI
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AA4PB
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« Reply #2 on: February 20, 2008, 06:26:30 PM »

I could probably pick up an old aircraft audio panel as well, which ought to provide this automatic switching capability
-----------------------------------------------------
All the aircraft ICS (audio) boxes I've seen are simple mixers with a manual switch in each circuit. The audio from all circuits that are manually switched on are mixed together and sent to the headset. When the airband audio comes through you would hear both it and the music.

I don't think you are going to find anything ready made to do the job. I once built a circuit just as described by W5FYI to transfer the car radio speakers from the AM/FM radio to the ham transceiver when the squelch opened.
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ONAIR
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« Reply #3 on: February 21, 2008, 01:42:08 AM »

   You might try an audio actuated VOX circuit that will disable one receiver when a transmission comes in on the other one.
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W8JI
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« Reply #4 on: February 21, 2008, 02:40:24 AM »

Better be careful!!!!!!

To be safe with all equipment it would take a fast 4PDT relay or other methods like isolation transformers and a DPDT transfer relay.

To be safe with all equipment the speaker leads that are NOT used would have to be terminated in a low impedance load resistor, and BOTH the speaker leads would have to be switched.

The most troubling part is connecting speaker leads to a common point and using a SPST relay. That's just asking for problems because most devices are DC coupled now. Very bad news to common it with another system.

There is also the potential that an unloaded output line, especially one with some capacitance between wires, can allow the power amp to oscillate and destroy the power amp.  

73 Tom
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WA3SKN
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« Reply #5 on: February 21, 2008, 03:31:21 AM »

You do not want to do this!
The FCC frowns on any transmitter that can be used on "multiple services", such as aviation and FRS.
Keep the tranmitters in separate boxes!
Now the "receive only" can be done, but hardly worth the effort.
73s.

-Mike.
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KT8K
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« Reply #6 on: February 21, 2008, 05:46:16 AM »

Back in the heyday of CB there was a product that would switch your car radio to your CB when the squelch opened up, but that is essentially just the transistor switch circuit.  Building this sounds like a fairly simple and fun project, though.
Good luck & 73 de kt8k - Tim
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K0BG
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« Reply #7 on: February 21, 2008, 06:15:05 AM »

Actually, this is fairly easy to do. For example, most Icom radios have a squelch switch output. This output, properly buffered, can be used to control a low-level audio switch, like a CT-3244.

If you want to switch at high level, you can actually buy units for the exact use you're looking for, like this one: http://www.camcor.com/cgi-bin/cat/id=1138653360&src=fg  You still have to follow the rules (like Tom said above).

Or, if you want to spend the bucks, buy a Bogen PA amp with a priority input port.

Alan, KØBG
www.k0bg.com
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W3LK
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Posts: 5639




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« Reply #8 on: February 21, 2008, 06:47:25 AM »

<< 3. transmit on aviation and FRS/GMRS >>

I'm curious as to why you want to transmit on aviation frequencies from your home. Are you licensed as a Unicom station?

73,

Lon - W3LK
Naugatuck, Connecticut
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