All-electric simulated machine gun?

[rwanttaja]

I did some digging and what might work here is a "color organ" or "light organ" circuit for synchronizing lights with music, though you want one specifically for LEDs and 12v not for incandescent lighting and the home stereo system. Most are multi-channel -- here's an example including a link to instructions and a circuit diagram -- but it sounds like Ron or others would have no trouble understanding the circuit diagrams and creating a simplified, single-channel version for the machine gun flash.

If someone is looking for a project, it might be worth making the multichannel example in the link and then testing it to decide which frequency response works best for this application. The input would come directly from the speaker feed, which would solve any synchronization issues.

I agree the flash should be triggered by the bang....and considering the size of the bang we hope to issue, triggering shouldn't be any problem. The main trouble is, the flash is basically binary...it's on or it's off. We're talking about wave-shaping the bang, but the flash is going to be pretty simple in comparison.

The organ circuit takes the music input, and runs it through low-pass, band-pass, and high-pass filters, with each filter turning on a bank of particularly-colored LEDs when a threshold is reached. The organ circuit has the right idea...but we'd end up needing multiple high-intensity LEDs to try to implement it. It's going to be tough enough to have one LED bank bright enough to be visible 500 feet away on a sunny day; packing in three or more is going to look really funny. Plus, the closer it gets, the more obvious the flashes are coming from slightly different positions.

What would be nice is if we could input a waveform to the LED and have it track it by *brightness*. An initial high-intensity, dropping off over the next quarter-second or so. Doable (especially if we end up with a microprocessor anyway), but it is getting a bit complex.

If brightness weren't an issue, I'd install a big LED at the muzzle and a trail of additional ones leading forward. Do the big flash, then sequentially zip through the others real fast.

The next step would be to rig up a cheap way to feed the speaker with a looped recording of a single rifle shot as has been discussed. A simple loop would work, but for best sound, I think it would be great if the shots could be overlayed like this, with the next shot firing over the continuing reverberation of the previous one, if you see what I mean:

BANG - r e v e r b e r a t i o n
__________BANG - r e v e r b e r a t i o n
____________________BANG - r e v e r b e r a t i o n

I'm not sure how much reverb would be present; again, not only are the listeners going to be hundreds of feet from the gun, but the gun itself is going to be well off the ground. I found the following spectrum of a gunshot online:

I'm guessing the initial sound pulse is the shot itself, probably the "slap" of the air compressed by the passage of the bullet (and possibly a supersonic shock wave). Note the incredibly short time duration, and the high amplitude compared to the rest of the waveform. Following up 8 milliseconds later is a reflected shock wave (probably an environmental effect we can ignore).

About 15 ms after the first shock is the muzzle blast...probably the "reverb" effect we're looking for. Note, though, how much lower the amplitude is.

It's my contention that the muzzle-blast effect would be severely attenuated by the time it would reach an observer 500 feet away. Hence, we should be primarily interested in reproducing the initial "pop."

Remember that the pop is massive. Hearing protection would be a must if your sound system actually reproduces the sound of the gun, at least in close proximity.

This reproduction has its problems (like most, I guess :-). If you want to SOUND like a gun firing, you need to move the same amount of air at the same speed, within the same short span of time, with amplitudes so high that you need earplugs. If we say that spike is on the order of ~0.2 ms wide, that needs a speaker with decent a frequency response of 5000 hz. So we have two conflicting requirements... we need to move a LOT of air (we need a big woofer!) at a fairly high frequency (we need a tweeter, which generally don't push large masses of air).

Most of us have probably been present when a gun is fired. And we all realize that they don't SOUND like the guns we hear in movies and TV. Hollywood can't reproduce the pop, which is the major sound component. I live a quarter-mile from a police gun range, and when it's active, all I hear is a series of pops.

But...and this part makes me giggle, just a little bit... maybe we DON'T want to be real. Most people are accustomed to Hollywood gun sounds, and that's what they'll expect. Let's give them "Bang chugaluggalugga Bang chugaluggalugga" or whatever sounds like Sylvester Stallone.

Seems to me that microprocessor should be able mix two overlapping waves of the same sound so the reverb effect on the first shot can continue through the second. We'd probably want this anyway, for the folks who might install two guns.

Ron "This is a lot more fun than reading accident reports" Wanttaja