I haven't been able to find a simple off the shelf device that connects to the speaker or external speaker of a VHF radio and is able to mute the stereo when there is incoming traffic on the VHF.
Most stereos have a connection that mutes their output when grounded.
The problem is detection of active audio on the VHF and converting that to a contact closure.
Surely such a device must exist out there somewhere so I don't have to resurrect corroded skills in electronic design/fabrication!
The only one I found via extensive web searching seems to be no longer in production.
This issue comes up all the time with mobile phones in cars - most mobile car kits output via a connection that can be tied to the stereo to mute it.
Can anyone help point me at a 'black box' that I can connect to a low impedance audio circuit on one end and will output via a normally open relay or some similar method on the other end?
How about this?
Model ST-ACR Series
Audio Controlled Relay
http://www.radiodesignlabs.com/pdf/Data_Sheets/st-acr.pdf
Thanks - that looks like it would work although it will require a 12V->24vdc converter.
RoyHB, I solved this problem using a Shure SCM262 Stereo Microphone Mixer ($225 at Amazon). This device has three stereo inputs and two mono mic inputs, and can be set to mute the stereo ins whenever audio comes in on the mic inputs. (By the way, such muting is called "ducking" in the broadcast/DJ world.)
I have the SCM262 set up with my two helm computers as stereo inputs and VHF and SSB and mic ins. It works very well, but I did need some ground loop hum isolators to block noise. Only downside I can report is that the mixer requires AC power and has an internal power supply so it would require some (warranty-destroying) work to convert it to DC.
Sorry - missed that 24V part in the specs. Suggest you search for "audio controlled relay" - the other cemmercial market is bars - where say playing the jukebox mutes the house tunes. Sometimes though the circuit is built in to the mixer panel.