PA / Fog horn speaker output

[prv]

Assuming the signal produced by the radio is a sine wave, or something similar. A single diode would rectify that and make it generally positive. A capacitor would smooth it somewhat into a rough DC voltage. This would be enough to activate a simple relay of the appropriate voltage rating to turn the horn on and off. The only unknown is the voltage... a quick look with an oscilloscope would solve that.

Or is that too easy?

It certainly sounds plausible to me, it's exactly the idea I was getting at in post #3. A mechnical relay is not going to operate fast enough to follow an audio waveform, so you should just get "on" as long as the tone is being sounded. Order the parts and give it a go?

Pete

 

[rogerthebodger]

I have also considered that approach but was looking at using a bridge rectifier but a single diode may do.

If the resulting voltage is quite low you could use a reed relay to drive a larger relay

 

[Alpha22]

It occurred to me that this is essentially converting Morse code audio into a keyed Morse code.... hmm I thought, I bet some cunning radio ham has already done that......I found this today......

http://www.polar-electric.com/Morse/TX_via_Soundcard.pdf

They have..... this is for low level audio. I don't know the spec for the VHF fog horn output, but I bet it is quite a healthy voltage, considering it is a fog horn!! Should be able to do away with the transformer and transistor.