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?
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 systemThe 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=1The 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.