I would like to build a device that would close a circuit upon the presence of a loud sound on a radio speaker lead. I'm not an electrical engineer, so I really don't know how this would be done. But, it seems that if I had a relay that closed a circuit upon the presentation of certain level of power, that would do the trick. Note that I am not trying to turn off the speaker power, I just want to use the speaker power to trigger the relay. This device needs to be quick.

What kind of relay would do this given that the trigger is the presence of the loud volume in the speaker. Assume that the amplifier is just a small portable radio, not a big stereo.

 

I would like to build a device that would close a circuit upon the presence of a loud sound on a radio speaker lead. I'm not an electrical engineer, so I really don't know how this would be done. But, it seems that if I had a relay that closed a circuit upon the presentation of certain level of power, that would do the trick. Note that I am not trying to turn off the speaker power, I just want to use the speaker power to trigger the relay. This device needs to be quick.

What kind of relay would do this given that the trigger is the presence of the loud volume in the speaker. Assume that the amplifier is just a small portable radio, not a big stereo.

Questions:
Do you want to take the signal electrically from the speaker leads or acoustically from the sound?
Do you want the relay to latch on or stay on for a pre determined time?

 

I want to take the signal electrically from the speaker leads and I want the relay to stay closed for the duration of the "loud signal", then open.

 

Then it is a 'speaker power circuit maker'.

Like a relay across speaker lines. When the voltage reaches a predetermined level by the relay specification and overcomes its spring, it closes during that surge/spike/transient.

 

Yes. That is the general idea. I see that there are many types of relays that can be purchased for short money, but what specifications should I be looking for that indicate that the relay would work in the manner needed?

 

Can be a VOX circuit; an AC relay coil -if matters-; a rectifier-comparator circuit to trigger a DC relay; those decisions would determine the relay specifications. There is more than just the relay. You need to know the voltage level present at the speaker that you want to produce activation, at what frequency, which would imply adding a band pass filter... The voltage present on your selected 'spike/surge/transient' will not be the same at every frequency, the duration of the spike to produce activation, the time you want to keep your relay on after the spike fades... Yes, explore VOX circuits.
A LM3914 type of IC can be tapped at the LED of choice to drive the relay...

 

The "device" that you are seeking could be as simple as a sensitive reed relay, but without knowing the voltage that produces the :loud sound" that you are mentioning any guess at the specifications for that relay are not very useful. Some variation of the mentioned VOX circuit could also work, but that would need a lot more parts and an active circuit added. What is the purpose of such a circuit? It sounds a whole lot like the circuits used to detonate IEDs in another part of the world, is why I am asking. Those were triggered by a cell phone ringing.

 

OK, Misterbill2. Legitimate concern in our times I suppose. No, I'm not a terrorist. I'm a photographer. I want to create a lightning trigger for a camera. You can buy these for about $200 but they all work on an optical signal. I would like to build one that works on a radio signal since lightning emits a strong AM signal. I figured the easiest way to do this would be to harness the speaker power in a portable radio.

 

OK, Misterbill2. Legitimate concern in our times I suppose. No, I'm not a terrorist. I'm a photographer. I want to create a lightning trigger for a camera. You can buy these for about $200 but they all work on an optical signal. I would like to build one that works on a radio signal since lightning emits a strong AM signal. I figured the easiest way to do this would be to harness the speaker power in a portable radio.

A lightning flash does not last for very long (about 30 microseconds). You would miss it in the time it takes to detect with the radio receiver and close a relay.

 

Well, the noise on an AM radio certainly lasts longer than that. I say find a relay that will close on the noise and try it.

 

Even if the AM radio responds quickly enough, there's still the time it takes for a relay to physically close. Depending on how tight timing really is, might be worth considering solid state options: transistor (BJT or MOSFET) or pre made low voltage SSR.

If you can tap into the speaker wires, you could probably make a fairly simple circuit with a comparator, transistor, trim pot, and just a few resistors and caps.

A solid state solution, wired directly, would be much faster than anything that relies on waiting for speakers or mechanical relays to physically move.

 

Well, the noise on an AM radio certainly lasts longer than that.

True, but by the time the camera is triggered the visible lighting is likely long gone.

 

Well, the noise on an AM radio certainly lasts longer than that. I say find a relay that will close on the noise and try it.

Yes, the noise burst does last a lot longer because the noise burst bounces around between the clouds and the ground. It would take a very fast relay, they do come as fast as 5 milliseconds. Those very fast ones are reed types, and so using an AM radio tuned to a quiet spot is one approach, not at all original with me. Another way, an approach that we have used for crash recording for a major auto OEM is to have a fast source going to a circular buffer on a computer. It works by stopping the recording an appropriate number of milliseconds after the trigger signal arrives,Circular bufferes are fine, but windows updates often cause problem with their working correctly.

 

Can you build something like this?

http://www.solorb.com/elect/lightning/index.html

 

That circuit is probably not fast enough to catch the start of a lightning flash. Triggering in lower frequency RF is the most likely way to catch the start, unless one uses a continual recording video camera feeding a circular buffer. The problem in triggering off of a bolt of lightning starting at T=0 is that anything sensing that event will be delivering a response at T=0+Td, where Td= Time delay. But it may be possible to trigger on the pre-strike radio emissions. At least, that is worth investigating.
Every optical trigger scheme is delayed by the response time of the photo-detector device. Some are very fast, some are not.

 

The solorb devices are optical. I would like to do this with radio frequency, so a fast relay would be the ticket.

The camera I have has a feature called "pre-burst". It only works in 4K mode (8MP). How it works is that it constantly buffers about 50 frames a second until the shutter is depressed. It then saves the 25 frames before the shutter and the 25 frames after the shutter. (This is a somewhat simplified explanation.) I've used this for action shots, like kids jumping in a pool, or whales breaching, where your eye, finger and camera are just not fast enough to catch the perfect "height of action" photo. Though I'd prefer to use the full resolution of the camera, the pre-burst would nicely work around any relay delay issues.

If I go searching for a fast relay, what trigger specifications would I be looking for that would match the electrical output of a loud speaker signal?

 

I would use a comparator circuit (such as an LM339/393) driving a MOSFET switch to trigger the camera.
That would respond within microseconds of receiving the static burst.
Do you know what the camera needs as a trigger?

 

I would use a comparator circuit (such as an LM339/393) driving a MOSFET switch to trigger the camera.
That would respond within microseconds of receiving the static burst.
Do you know what the camera needs as a trigger?

Yes. It just needs a closure between the #1 and #3 leads on a three lead 2.5mm barrel connector based on a remote shutter cable that I use with a wireless remote trigger. A better description of the mechanism is available at http://www.robotroom.com/Macro-Photography-2.html . That article gets into the half-press, pre-focus mechanism. I don't need all that. I would probably use manual pre-focus to infinity and just need to trigger the shutter.

 

The solorb devices are optical. I would like to do this with radio frequency, so a fast relay would be the ticket.

The camera I have has a feature called "pre-burst". It only works in 4K mode (8MP). How it works is that it constantly buffers about 50 frames a second until the shutter is depressed. It then saves the 25 frames before the shutter and the 25 frames after the shutter. (This is a somewhat simplified explanation.) I've used this for action shots, like kids jumping in a pool, or whales breaching, where your eye, finger and camera are just not fast enough to catch the perfect "height of action" photo. Though I'd prefer to use the full resolution of the camera, the pre-burst would nicely work around any relay delay issues.

If I go searching for a fast relay, what trigger specifications would I be looking for that would match the electrical output of a loud speaker signal?

That "pre-burst" sounds very similar to the circular buffer that we used. Perfect. Depending on the camera input to trigger that capture it may work out to use an LED with a diode to protect it from excess reverse bias, and then a photo-transistor as the switch to trigger the camera. That could be faster than any relay, and probably cost a lot less. You would wire the LEDs across the speaker connection in inverse parallel, and you could then check that portion just by watching, and turning up the volume setting until it triggers the way you want, during a lightning storm. Then mount the photo-transistor looking right at the LED. That is, presuming that you can find one compatible with whatever it takes to trigger the camera. Fast, cheap, and easy, and best of all, simple.

 

Continuing with MB2's thought, how about connecting the speaker leads to the input of a optoisolator, such as as a 4N25.
It has an open collector output, which should be able to directly trigger the camera.
Something like below:
(LED D1 gives a visual indication to help adjust the volume setting).