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Audio Activated Relay and AC Voltage
Alex Davis:
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
Alex Davis:
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
Stephen Swaffer:
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?
Alex Davis:
--- Quote from: Stephen Swaffer 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?
--- End quote ---
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
Mike Caldwell:
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
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