I am hoping someone here can help me. I am trying to come up with a circuit that will pull a pin low upon the presence of audio. Ideally, the circuit will have an input to listen for audio, then pass that audio to an output, and pull a pin to ground upon receipt of the audio. Electronics is not my first language. I've been trying to reverse engineer some VOX circuits and have been hitting roadblocks.

 

http://www.rason.org/Projects/basicvox/basicvox.htm

There are many vox circuits with a google search.

 

Welcome to AAC!

Here's a circuit that will generate a low output when the input is positive:


No simple circuit is going to output both a LOW and the original audio.

 

I am hoping someone here can help me. I am trying to come up with a circuit that will pull a pin low upon the presence of audio. Ideally, the circuit will have an input to listen for audio, then pass that audio to an output, and pull a pin to ground upon receipt of the audio. Electronics is not my first language. I've been trying to reverse engineer some VOX circuits and have been hitting roadblocks.

The first thing that you need to do is turn your qualitative goal ("the presence of audio") into a quantitative one. What constitutes the border between audio being present and audio not being present?

 

Thanks for the replies. I'll try to explain further what what I'm trying to do. Most of the circuits that I am finding are doing amplification and other things, and don't necessarily achieve what I'm after. I am trying to interface a 2 way radio with a paging PA system. 4 different emergency tones are broadcast on this system, as well as voice notifications. I am trying to place a radio inline to listen for audio on this line, and transmit that audio when present. Also, when the radio receives audio via RF, it will output this audio to the PA system. That part is simple. the radio will do VOX from the from accessory port, and I have been trying to reverse engineer a hand mic, but I am having issues. When I try to pipe a 1004hz tone from my Halcyon into the mic directly attached to the mic pins, the radio will transmit for about 5 secs and then unkey. I would like to do this from the rear accessory port, but there is no VOX support back there. I have audio pins to get the audio into the radio, but in order to key up and transmit that audio I have to pull a specific pin to ground to activate PTT. The reason I want the circuit to pass the audio through is so I don't have to tap the PA line to get the audio in, and tap it again to feed into a circuit just to activate PTT. I guess when there is audio on the PA there is a small voltage present, but I haven't measured it. I guess that is my border between audio and no audio. I am doing my best to grasp this, but my lack of electronics background is hindering me. Thanks in advance guys.

 

The first thing that you need to do is turn your qualitative goal ("the presence of audio") into a quantitative one. What constitutes the border between audio being present and audio not being present?

This is exactly the kind of thought that gets me thinking in the direction i need to be. What am I asking my theoretical circuit to "hear", in order to act on it? This is where I start to stutter. I don't normally associate audio with voltage, but i guess it is. I'm trying.

 

I can't make any sense out of post #5. Where did the tone come from and what is it's importance? What is a Halcyon?

Can you please describe the big picture?

 

I can't make any sense out of post #5. Where did the tone come from and what is it's importance? What is a Halcyon?

Can you please describe the big picture?

The Halcyon is just a signal generator (model 704A2) so I can send fixed audio to test with, nothing more.

 

I can't make any sense out of post #5. Where did the tone come from and what is it's importance? What is a Halcyon?

Can you please describe the big picture?

I need to pick up audio from the PA system, and get into my 2 way base radio and have that radio transmit that audio as RF. I have 2 pins in the rear accessory port for audio + and -. No problem there. Now the audio is sitting on those pins, but the radio doesn't care. I need to initiate a PTT (Push-to-talk) for the radio to start transmitting the audio coming into it. In order to activate PTT without standing there and pushing a button, I need to pull another pin in the accessory port to ground. This will "key" the radio and it will start transmitting.

 

We used to use analog audio tones in the 70s and 80s for phone patching, privacy, control and telemetry in commercial two way radio. It was all analog filters at the time......before the digital revolution really took off.

I figured all but the sub-tones on radio have disappeared and went all digital. But I'm not sure what kind of vox action/decision that your looking for.

Do the tones have any bearing?.....or you just want audio?

EDIT.......ok.....but you really don't want that do you? How can you edit the transmission......I assume this is not a broadcast station.

 

We used to use analog audio tones in the 70s and 80s for phone patching, privacy, control and telemetry in commercial two way radio. It was all analog filters at the time......before the digital revolution really took off.

I figured all but the sub-tones on radio have disappeared and went all digital. But I'm not sure what kind of vox action/decision that your looking for.

Do the tones have any bearing?.....or you just want audio?

EDIT.......ok.....but you really don't want that do you? How can you edit the transmission......I assume this is not a broadcast station.

When I speak of tones in this scenario, it is not directly related to the 2way side. It is all digital, no PL tones. The PA is an old Gaitronics party/page system. There is an oscillator that generates 4 different alert tones (siren, warrble, etc). I just need to pass these or any voice alert audio into the 2way which broadcast it all out over all radios. Essentially extending the emergency PA calls with the radio system. I see now when I referenced "tones" it through the conversation off.

 

Avi8er...........when some one gets on the PA and says "hey Joe...your wife's on line two"...........do you want that to be broadcast on the radio?

 

Avi8er...........when some one gets on the PA and says "hey Joe...your wife's on line two"...........do you want that to be broadcast on the radio?

Handsets in the plant haven't been used in years. The radio system eliminates the need for them. The PA is strictly used for emergency announcements and alert tones.

 

How does any body announce any thing over the PA?

 

How does any body announce any thing over the PA?

There is a control panel in the control room with a handsets and 4 buttons corresponding to the emergency alert tones. This is the only point where audio enters the PA.

 

Is that the only input to the PA? A person sits there......and IF he has to announce something....first he picks a tone and then makes a public announcement?

Is this how you do it?

 

Is that the only input to the PA? A person sits there......and IF he has to announce something....first he picks a tone and then makes a public announcement?

Is this how you do it?

They test it twice a week. They pick up the handset and announce that they are going to test the emergency alerts. Then, one at a time they push each alert tones and let it run for about 10 seconds each. There are actually 3 branches (zones) to the system. A, B ,and C. There are merge and isolate buttons on this panel as well, but for whatever reason, the ability to isolate was lost long ago. It stays merged. It's a huge system with probably a thousand or so speakers throughout.

 

As for the merge problem....from the pic.....looks like an old relay latch circuit. But that's a wild guess.

Do you have any kind of aux audio(or mic) input, such as headset or anything else on the radio. I know you said you had a keying pin.

"rear accessory port for audio + and -" Is that port on PA or radio?

 

Have you got any paper work on the PA and radio? If the keying pin was located in the radio acc socket.....a lot of times there is an audio pin there also.

I have several ham radios here. They all have audio inputs with the keying inputs.

 

Love the photo - you have to stay put for some "disasters"???

I don't think this is a difficult problem, but a schematic or system wiring diagram showing the components in place, their interconnections, and boxes for the circuit(s) will speed things along.

Also, you mention 4 control tones; do you have to detect each one individually, or just pick off anything that looks and smells like a control tone?

ak