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Author Topic: Audio Activated Relay and AC Voltage  (Read 3610 times)

Alex Davis

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Audio Activated Relay and AC Voltage
« on: November 18, 2017, 06:25:33 pm »

Hi all,
     I'm new at this stuff so please forgive me where I seem ignorant! I created a small circuit with a relay and timer such that when audio output from my radio scanner is detected, a bell will ring and a light will come on (think waking people up at a fire station for a dispatched emergency).

    The relay is activated by the ~4v AC I get when the radio is active by taking a stereo 3.5mm jack out of the stereo headphone jack on the radio. It's 0v AC when radio is silent.  This plug terminates in a dual RCA connection with the right channel attached to a speaker and the left stripped down with the positive wire linked to my relay. It works great!

    Now the issue: I swapped this radio out for an older model capable of the same functions and- according to the manuals- the same headphone jack (3.5mm stereo 6mW at 32ohm).

    When I try to read voltage off of my audio out wire now this is what I see: when I power the unit on, I get 8vAC and then it slowly falls to zero. After that, I get no voltage reading even when the radio is active, but I do get the sound just fine (since I have the Right RCA jack plugged in to a speaker). I did try to read voltage off the male end of right channel rca that I know delivers sound to the speaker and I get the exact same result.

     I feel like the issue has to do with the grounding in this particular radio, but I'm not knowledgeable enough to figure it out (could also be totally wrong).

     Anyone have any ideas what could cause this phenomenon? As soon as I swap the radios back everything works fine with no changes (including any setting or config on the meter). Any tests you'd like me to run to help?

Thank you all so much in advance!
Alex
« Last Edit: November 18, 2017, 06:32:20 pm by Adavis53 »
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Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #1 on: November 18, 2017, 07:03:37 pm »

In case it helps, I am attaching a picture of the little circuit I have (Red is DC+, Black is DC-, Green is my Audio wire and specifically the positive conductor in the cable). The relay is looking for a change in voltage (from below .5V AC to above .8v AC) in the trigger wire.

With my first radio I go from  0V AC (when radio is silent) to 4V AC (when radio is active). With the second I get the goofy phenomenon described above.


Let me add one question. If the headphone jack on the second radio was a mono jack... would the phenomenon above make sense? One portion of the manual (specs) says it's stereo but another mentions it is mono. I tried a mono cable in the circuit and get no reading whatsoever (no 8v going to 0).

Thanks again for taking the time to read and consider a response!

Alex
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Stephen Swaffer

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Re: Audio Activated Relay and AC Voltage
« Reply #2 on: November 18, 2017, 07:55:08 pm »

That diagram doesn't tell me much.  I assume?? one side of the relay coil goes to black?

It could be that the second radio is not referencing the same ground internally-depending on the actually amplifier circuit running the headphone jack.  Are you using 2 wires from the audio out or just one?
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Steve Swaffer

Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #3 on: November 18, 2017, 08:02:30 pm »

That diagram doesn't tell me much.  I assume?? one side of the relay coil goes to black?

It could be that the second radio is not referencing the same ground internally-depending on the actually amplifier circuit running the headphone jack.  Are you using 2 wires from the audio out or just one?

Hi Stephen- thanks for the reply. I think you're on to something. If the amplifier is different- potentially not using a true ground the way the first radio did, I cannot reference either speaker lead to the ground, or i risk ruining the amplifier right? (even if this was the case, i should get a reading as i pulled the signal to the ground, shouldn't I?)

Sorry for the junk diagram. In the initial setup i only had the (+) speaker wire connected to my trigger wire. the relay is grounded, so current on the audio out (+) wire would pass through the relay to the ground (relay is grounded through 12V power source).

Knowing this- any suggestions?
 * Connect both speaker leads (+) & (-) to the trigger wire?
 * Find another way to determine when radio is active because i cannot "Ground" either speaker wire in this case due to bridged amp?

Thanks again,
AD
« Last Edit: November 18, 2017, 08:09:53 pm by Alex Davis »
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Mike Caldwell

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Re: Audio Activated Relay and AC Voltage
« Reply #4 on: November 18, 2017, 11:54:15 pm »

Not sure what type of relay your using but here are two products made to do exactly what your wanting to do.

I have use the Bogen VAR1 to trigger LED strobe lights in school band rooms when every there is an announcement on the building PA system.

https://www.bogen.com/products/specialelectronics/

and another similar unit from RDL

http://www.rdlnet.com/product.php?page=278

Scott Holtzman

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Re: Audio Activated Relay and AC Voltage
« Reply #5 on: November 19, 2017, 05:04:17 am »

Hi Stephen- thanks for the reply. I think you're on to something. If the amplifier is different- potentially not using a true ground the way the first radio did, I cannot reference either speaker lead to the ground, or i risk ruining the amplifier right? (even if this was the case, i should get a reading as i pulled the signal to the ground, shouldn't I?)

Sorry for the junk diagram. In the initial setup i only had the (+) speaker wire connected to my trigger wire. the relay is grounded, so current on the audio out (+) wire would pass through the relay to the ground (relay is grounded through 12V power source).

Knowing this- any suggestions?
 * Connect both speaker leads (+) & (-) to the trigger wire?
 * Find another way to determine when radio is active because i cannot "Ground" either speaker wire in this case due to bridged amp?

Thanks again,
AD

Hi, you are ultimately trying to reinvent the wheel and the speaker output just isn't what I would use.

A commercial radio will have a carrier operated relay so when the squelch circuit is open the contact on the relay will close.  If you a schematic for the receiver you could add a similar relay to the squelch circuit.

The problem with that is noise can open the squelch and trigger a false alert.  Does the dispatch center send tones out before the alert?  If they don't they still use some type of positive squelch system either a continuous low frequency tone or a very low bit rate data stream.  Decoding this tone squelch is much more reliable.  Commercial tone squelch boards are about $30 and you pick unsquelched, unprocessed audio right off the discriminator.

If we know a little more about the system we can guide you in the right direction. 
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Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #6 on: November 19, 2017, 09:17:44 am »

Thanks to both! I'm going to put a bunch of information/insight below. Hopefully it helps you guys to help me! Many thanks in advance!

Not sure what type of relay your using but here are two products made to do exactly what your wanting to do.

I have use the Bogen VAR1 to trigger LED strobe lights in school band rooms when every there is an announcement on the building PA system.

https://www.bogen.com/products/specialelectronics/

and another similar unit from RDL

http://www.rdlnet.com/product.php?page=278

Thanks! I've seen the Bogen product and was trying to come up with my own solution- partly for cost and partly to get the satisfaction of solving my own problem. Of course- that's not going so well!

The relay I purchased for $20 (link below)- i just need some voltage to operate it, which I thought I could get from the headphone jack. It turns out that I can in one radio, and I cannot in the other- likely due to a difference in amplifiers within the radios. I will end up getting one of these things as a last resort- but if I do... my audio out is only two wires (+/-) on each channel... so how do I connect to +/-/GND ? just leave the GND slot open?

Quote
Hi, you are ultimately trying to reinvent the wheel and the speaker output just isn't what I would use.

A commercial radio will have a carrier operated relay so when the squelch circuit is open the contact on the relay will close.  If you a schematic for the receiver you could add a similar relay to the squelch circuit.

The problem with that is noise can open the squelch and trigger a false alert.  Does the dispatch center send tones out before the alert?  If they don't they still use some type of positive squelch system either a continuous low frequency tone or a very low bit rate data stream.  Decoding this tone squelch is much more reliable.  Commercial tone squelch boards are about $30 and you pick unsquelched, unprocessed audio right off the discriminator.

If we know a little more about the system we can guide you in the right direction.

Thanks! The Radio itself has the tone-out function so that when the county dispatch puts out our two tones it will activate (and ignore all other dispatches for other companies). So it's got some sort of tone decoder built in to it. I was hoping to leverage that as the radio will only have audio output after the radio has detected the tones i'm interested in.

More on my system
The relay I have came with a timer (also necessary, since I only want the bell to ring for 4 seconds even when the dispatch transmission might be one minute long. The idea is that the bell rings, the light flashes, then they both stop after 4 seconds).

Here is a link to the relay: https://www.amazon.com/gp/product/B00PD65UGA/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

The radio that works is a Uniden BCD996P2. The radio where this doesn't work is a Uniden BCT15.

Let me know what else I can provide to help.


Lastly,
Here is a general question as I trouble shoot getting an audio signal to readout on a voltage meter from the BCT15:

If the amplifier was single-ended (the way the BCD996P2 appears to be), I can take either audio channel's (+) wire and connect my meter to either the (-) wire in the audio cable or any other ground source (say- the case the radio is in, or the (-) wire from the DC power source) and get my voltage reading.. 4v AC in the (+) wire and nothing in the grounds would read out 4v when radio is active.

IF the amplifier was single-ended but using a reference point in the (-) wire other than true ground,  I would expect to still get a reading if i connected my meter to the (+) and (-) wires. If one had 4v and the other had 0v I would expect the meter to read 4v of difference (after all, that's what a meter does, right?). Further- I'd still expect to get a reading if I connected the meter to the (+) wire and a true ground source, even though that would be bad for the amplifier (right?) 

IF the amplifier was a BTL-type and putting (inverse) signal on both (+) and (-) wires, I would expect to still get a reading if i connected my meter to the (+) and (-) wires. If one had 4v and the other had -4v I would expect the meter to read 8v of difference (after all, that's what a meter does, right?) Further, I'd still expect to get a reading if I selected the wire with the positive voltage and connected the meter to that wire and a true ground, even though that would be bad for the amplifier (right?)

When I connect my meter to the (+/center) & (-/sheath) wires in my RCA jack, I get no voltage reading even when the radio is transmitting and i'm getting audio through the other channel's RCA jack in to the speaker. If i disconnect the RCA jack that delivers sound to the speaker and put my meter on the center pin (+) and the outer ring (-) I get the same thing- no voltage.

To me, this means that the voltage in the two wires is the same (Paralell?) and thus grounding my meter to a true ground versus one of the two wires should read me the voltage in the wire- but again, I get no reading.

In summary: putting the meter between the following yields the following:
(+) & (-): No AC voltage reading
(+) & Ground: No AC voltage reading
(-) & Ground: No AC voltage reading

How can I have audio signal and no voltage in any of the scenarios described above? I'm essentially putting my meter where the speaker would be so wouldn't I have to have different voltages on either side when the radio is active?

I'm sure it is a measurement problem (user error), I just don't know to get that reading! If I did, I might be able to come up with a solution. Then I'd have to figure out if it's okay for the amplifier in the radio to lose signal to true ground like that through the trigger wire and ultimately out the ground wire on the relay. The radio and relay are grounded to the same power source.

« Last Edit: November 19, 2017, 09:25:35 am by Alex Davis »
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Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #7 on: November 19, 2017, 09:48:17 am »

Let me save us all some time...

The output power on the 996P2 from the headphone jack must be much higher than the BCT15. I never paid attention to volume when getting a readout from the 996P2...worked at high, low, wherever I set it.

For fun, i turned the BCT15 volume up a lot and it now works the same as the 996P2.

Sorry to waste everyone's time on such a petty problem! I'm assuming the BCT15 just has a weaker output signal and needed a higher volume to register the voltage on my meter.

Cheers to all for the assistance!
AD
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Mike Caldwell

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Re: Audio Activated Relay and AC Voltage
« Reply #8 on: November 19, 2017, 12:05:22 pm »

Both of the units I mentioned have adjustable hold times on the relay and adjustable relay activation thresholds.

I am very much a DIY type of guy and have had a soldering iron in my hand since the 6th grade (about 1973) but when something like the Bogen VAR1 is available and worked perfect for my strobe light project why try to reinvent the wheel. I just package it with the strobe light using a project box, I also needed to build eight of them.


Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #9 on: November 19, 2017, 08:57:14 pm »

Both of the units I mentioned have adjustable hold times on the relay and adjustable relay activation thresholds.

I am very much a DIY type of guy and have had a soldering iron in my hand since the 6th grade (about 1973) but when something like the Bogen VAR1 is available and worked perfect for my strobe light project why try to reinvent the wheel. I just package it with the strobe light using a project box, I also needed to build eight of them.

Makes sense-
Thanks again!
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Jonathan Johnson

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Re: Audio Activated Relay and AC Voltage
« Reply #10 on: November 21, 2017, 01:52:18 am »

I don't know what your exact application is, but if this is for an emergency service where lives or property could be placed in serious jeopardy in the event of a failure of your alarm circuit, then you really should be going with a purpose-made commercial solution. An "I wanna see if I can figure it out" solution isn't the right solution.

That's just my two cents.
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Stop confusing the issue with facts and logic!

Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #11 on: November 22, 2017, 08:40:37 am »

I don't know what your exact application is, but if this is for an emergency service where lives or property could be placed in serious jeopardy in the event of a failure of your alarm circuit, then you really should be going with a purpose-made commercial solution. An "I wanna see if I can figure it out" solution isn't the right solution.

That's just my two cents.

Hi Jonathan- Agreed! I am redoing my garage and going with a firehouse theme, and this will go in the garage. It's a neat gimick my wife will hate. We have a commercial station alerting system at our fire houses- big bucks. We also carry voice and digital pagers, so this is simply to have a little project to work on and add to the theme.

AD
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Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #12 on: December 05, 2017, 09:46:13 am »

Hi All- One final question on this project that I thought the experts here could help with.

I have two speakers that an audio amplifier will play the dispatch audio through that are about 15 feet each from the amp. I have 16 gauge speaker wire for this task.

I also have three(two +, one -) 14 gauge low voltage wires for powering the bell& light that i want to run through the same openings in my walls, which means they will be in close proximity (especially at the opening where they are likely to be as close as touching) to each other over a span of several feet.

Any concerns with audio quality? low voltage wire to have 12V DC. Everything is insulated of course.

If it helps, it's a radio voice transmission, so I don't need crystal clear audio, I'm just trying to avoid wiring everything up and ending up with a huzz/hum due to some kind of interference.

Thanks!
Alex
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Rob Spence

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Re: Audio Activated Relay and AC Voltage
« Reply #13 on: December 05, 2017, 10:58:05 am »

No issue I would think.


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Alex Davis

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Re: Audio Activated Relay and AC Voltage
« Reply #14 on: December 05, 2017, 11:22:56 am »

No issue I would think.


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Thanks, Rob!
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