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Adding a light in parallel to existing loudspeaker circuit

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gubavac111

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I work on a ship which has Public Announcement (PA) system. PA system consists of a couple of zones. Each zone has an a controller, amplifier and a loudspeakers circuit.
In our case, PA system is only used to make announcements and play alarm, not music.

What I need to do is to add a light indication which will be activated when, for example, zone A is activated (meaning that loudspeakers in zone A are producing sound).

I can definitely do it by using one of the control outputs from the controller, but it would take a lot of cable pulling. It would be much more simple if I could just
connect a light parallel to one of the loudspeakers.

Now... my concerns are:

1) When I look at the drawing of one zone, the output from the amplifier to the loudspeakers is 100V. Am I right to suppose this is 100VAC "high" frequency (by "high" I mean higher than 50/60Hz), depending on the input signal?

2) There is around 50 loudspeakers in a zone where I would like to add the light. Each loudspeaker has a marking 100V, 25W, 101dB. If I add a 220V 20W halogen light (which is purely resistive load) in parallel to the loudspeakers, am I going to disturb i any way the loudspeakers circuit or the amplifier (assuming the amplifier will not get overloaded)?

3) Is it correct to calculate the approximate current through the individual loudspeaker I = P/U = 25W/100VAC = 0.25A?
 
100V line doesn't mean there's 100V on the speaker, it's only specified that was so as to avoid calculating impedances.

IF you fed a sine wave signal at full volume to each speaker, then you'd get pretty well 100V on the wires feeding to each speaker box - obviously this wouldn't help with what you're trying to do. Normal audio use will have widely varying voltage levels, unlikely to approach anything like 100V. Simply connecting a bulb isn't a good idea, as for a start it's unlikely to light up (and at best would only flicker with the audio) but a cold incandescent bulb is a much lower impedance than when it's hot (and nothing like a purely resistive load).
 
Annoucements are audio speech which is not continuously at full blast (100V), the level varies from no sounds between words and some parts of words are louder than other parts. The frequencies also vary.

I agree that an old fashioned incandescent light bulb is closer to a dead short circuit when cold and it will destroy the amplifier. The light bulb must be powered separately from a speaker line and needs a signal detection circuit to activate it.

If a custom electronic circuit is made to light the light bulb when it detects levels on a speaker line louder than almost no sound then its output would be active only during each word. The light would flicker on and off. A timer could be added to stop the flickering but the light might still be turned on by the timer when the announcement has ended.
 
How big/powerful/bright do you want the indicating light to be ? Are LEDs satisfactory ?

SPK(+)-------------------------/\/\/\/\/-------------------2LEDs--------------------------SPK(-)

2Leds are two counterparalleled. Resistor tailored to provide 20mA at loudness peaks.
 
You don't need to run wires everywhere back to the source. You just need a 5 to 12v power supply (wall wart) at each speaker. I recommend 12v

The connections to the speaker just need to be connected to a sample/hold peak detector. The capacitor prevents flicker and will stay on for about 1 to 5 seconds after the speaker stops talking (it will take longer if they were really yelling and approaching the 100vac power to the speaker). The resistors, as configured, will detect even small noises by the microphone input. Make the voltage dividers less extreme to sense only loud noises at the mic.

Note, this solution requires a real 12v supply at each speaker and connect to the two speaker wires to detect the peak & light the LED.. red circled area is the new circuit for each speaker...


7664D717-D234-49CF-9CFF-9C9C9601DE59.jpeg
 
How big/powerful/bright do you want the indicating light to be ? Are LEDs satisfactory ?

SPK(+)-------------------------/\/\/\/\/-------------------2LEDs--------------------------SPK(-)

2Leds are two counterparalleled. Resistor tailored to provide 20mA at loudness peaks.
Since the average levels of speech is much less than the full blast power then your LEDs will not light or will be very dim but light brightly occasionally.
The peak detectot circuit in the next post will work if the opamp is rail-to-rail and is powered.
 
Thank you all guys for your quick replies. From what I've seen, it seems much better not to try my idea, but to run the cable to the controller's control output.
 
If a loudspeaker is receiving an audio signal it should be obvious. Why do you need a light as well?
 
It appears that the light is located (up in the control room?) where there is no loudspeaker coverage. Maybe so that the next person in the control room can make an announcement without interfering with the first person's announcement at a different location.
 
It appears that the light is located (up in the control room?) where there is no loudspeaker coverage. Maybe so that the next person in the control room can make an announcement without interfering with the first person's announcement at a different location.

That's not how I understood it, specifically because he mentioned ' to avoid pulling wires', which he wouldn't need to do in any case if it was in the control room.
 
On one of the other websites he explained that the announcement loudspeakers are in a large area (the dance floor of a cruise ship?). When an announcement is being made (about the ship sinking or is on fire?) then the band sees the light and stops playing so that the people there can hear the announcement.
 
The light should be located somewhere on the dance floor, not in the control room. That is why I was wondering about the possibility to connect the light directly to the speakers. As audioguru says - the band sees the light and stops playing so that the people there can hear the announcement (band uses their own speakers not connected to PA system).
 
Not exactly a record for here on ETO, but finally in post #12 we find out what is the real problem, rather than going round in circles trying to implement a solution which probably will not work anyway.

JimB
 
The light should be located somewhere on the dance floor, not in the control room. That is why I was wondering about the possibility to connect the light directly to the speakers. As audioguru says - the band sees the light and stops playing so that the people there can hear the announcement (band uses their own speakers not connected to PA system).

Coming from the chemical and medical device industries, we look for the most idiot-proof system. One step safer would be to Turn on a red light AND cut power to the Band's amplifiers/microphones.

The next step is to wait for more suggestions as people build on earlier suggestions or rethink the concept al-together.

Good luck to you and your guests.
 
Coming from the chemical and medical device industries, we look for the most idiot-proof system. One step safer would be to Turn on a red light AND cut power to the Band's amplifiers/microphones.

The next step is to wait for more suggestions as people build on earlier suggestions or rethink the concept al-together.

Good luck to you and your guests.
Good point!
 
One thing that might work assuming the amplifiers and line transformers have a good enough high frequency response would be to inject a signal at about 25 Khz into the amplifiers and have a high pass filter driving a rectifier connected to a LED .(With a circuit to limit the LED current.) I am assuming that the OP does not want to provide a power supply at each location where there will be an LED so just using a low level high frequency signal would probably not be acceptable. A variation of the above would be to inject a DC signal onto the speaker lines and have blocking capacitors either on each speaker (Or at the 100 volt line transformer depending on how the system is wired.) The DC would be at a suitable voltage to drive the LEDs

Les.
 
How about use the loudspeaker voltage (rectified) to charge a fat cap to provide a steady-ish voltage for powering a constant-current circuit to drive a LED? I assume PA announcements are made at high volume level, so the voltage should be adequate.
 
Hi Alec,
I was assuming that the light should be on when that group of speakers were selected even though there was no audio present at the time.

Les.
 
My favorite solution is Les Jones with the DC offset and cap-couple the speaker at each node. Well played.
 
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