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Shaun

Sun Dec 23 2007, 06:51AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Let me start out by saying that, yes, this is for setting off pyrotechnic mixtures. A friend of mine makes amateur movies and he will give me $50 or so to hook him up with some FX. The actual charges would be at most a gram or so of BP or flash powder, for things like flashes of light, loud bangs, puffs of smoke, and maybe blowing chunks of dirt out of the ground for gunshots, etc.

I only agreed under the condition that I keep whatever devices I make

.
I have the materials to make the pyrotechnic compounds; that part is no problem since I was into that before HV stuff.

Now for the electronics part:

I have already built and tested a remote igniter which is basically an old R/C airplane with the wiring ripped out and the motor replaced with a model rocket igniter. It worked, but it was bulky and slow to ignite since the battery was only like 6 volts (and NiMH).

What I want to build is a a simple transmitter/receiver pair that can receive a signal and use that to gate a medium-size TO-220 SCR which will put a 9V battery in series with a model rocket igniter (or 2 9-volts in series depending on performance). As for distance, at least 20ft or so. The thing is, I've never worked with transmitting or receiving ICs or any circuit of that type.

I'm trying to keep it (the receive unit especially) small and cheap, for 2 reasons:
-chance of being destroyed
-need more than one
Something like a few SMT ICs/components, battery, SCR, igniter, all mounted in a film canister

For the transmit unit, I'd be willing to put more time/money into it. Ideally I'd like have 1 transmitter that could control 4 or 5 receive units, activating them individually.

I'm not asking for this to be designed for me, I'm asking for part numbers, advice from personal experience, or maybe a link. Google only wants to show complex voice transmitters, which are far more than what I need.

Any help or suggestions would be greatly appreciated.
Thanks,
Shaun

EDIT: I'm not sure what forum this should be in, seems a mix of General Electronics and EM radiation with a pinch of Chemistry.

J. Aaron Holmes

Sun Dec 23 2007, 07:40AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Does it have to be wireless? I can understand wanting to build RC stuff, but for this application it really seems like some serious extra credit! I mean, just roll out some wire to wherever you're burying the charges and make sure the guys playing the soldiers aren't going to trip over them or anything

I'll bet you anything that's what Hollywood does 99.9% of the time. On top of the added expense and complexity, wireless explosives don't sound like the kind of occupational hazard any actor would want to be around.

Cheers,
Aaron, N7OE

DrZoidberg

Sun Dec 23 2007, 11:37AM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

What range do you need? 100ft ? 1000ft?
There are tiny RC cars,
like this one

That one costs 10$
Although I've seen them for as cheap as 2$. Maybe try ebay.

The electronics in those cars is very small. Also the remote control has 4 buttons and so it could control 4 different charges.
Just buy 4 of these cars, take out the electronics from the cars and connect them to 4 different igniters. But each one you connect in a different way so that one triggers the igniter when you press up, the next triggers when you press down and so on.

Of course this is not safe at all. Don't use it to blow up anything dangerous.

Bored Chemist

Sun Dec 23 2007, 12:34PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Those cars scoot off in random directions whenever someone opens their remote controlled garrage door.
Do you want your pyros doing the same?
Use wired control unless you can put a lot of encoding on the signal.

DrZoidberg

Sun Dec 23 2007, 02:10PM

Registered Member #350 Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106

As long as he only ignites harmless stuff with it and no no one uses their garage door opener, it might work.

If you want to build the receiver yourself I think you should use a microcontroller. Because you want the number of components to be as small as possible so you can build it very quickly and easily.
With a microcontroller you can make the circuit very simple because you put all the complexity into the software. And you have to write the software only once.
One possibility is to use ultrasound. The circuit for that is simpler then for a radio transmitter. Also since the frequency ( 30 to 50 kHz) is much smaller then the clock frequency of the uC, you can demodulate the signal with the uC very easily and should only need a very small number of components in the receiver.
I think there are ICs meant to be used for IR communication. You just connect the uC and an IR LED to it. You could take that and replace the LED with a piezo speaker.

BTW. Instead of a rocket igniter you could also use a broken light bulb. Take a 3V light bulb, break the glass without ripping the wire inside. Then put it into the flash powder and connect it to 6V.

Mates

Sun Dec 23 2007, 02:40PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

One of the possibilities is one of the GSM mobile phone driven relays. (Very small ones connected to a diode are part of cheap toys-like accessories for the mobile phones, than you can have bigger much more sophisticated like here

). It has an advantage that the signal is very well coded and all you need is some old second-hand mobile phone you can get for a kiss. You place it near the relay and than you can trigger whatever you want from a distance you want after you call the right number. I hope you are not a terrorist!

Wolfram

Sun Dec 23 2007, 03:02PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

That's a lot more secure, but not perfect. Someone might dial the wrong number.

Steve Conner

Sun Dec 23 2007, 03:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Also, the toy ones are just RF detectors, and will go off whenever the phone transmits at all, which it does every few minutes to tell the network that it's online. No amount of black powder is safe if it goes off in your face.

I tried home-made pyros when I was at school too. I got away with only first-degree burns.

uzzors2k

Sun Dec 23 2007, 04:04PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Would you mind using infrared? The range might not be the best, but with an overkill amount of powerful IR LEDS you should be able to get far enough away

I just finished an 8 channel transmitter/ receiver based on two PIC16F88's. I haven't gotten around to testing it in a RC car yet, but I will soon. Breadboard tests have been promising, and since I made my own IR protocol it shouldn't have problems with interference. Of course I can't guarantee that some TV remote might trigger some bits, but what are the chances of someone pointing a remote at your pyrotechnic setup in the first place?

Shaun

Sun Dec 23 2007, 05:06PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Thanks for all the replys, guys.

I like Dr. Zoidberg's idea of using the ultrasonic elements; in going to start looking up some of those ICs. Also, it seems like it would not be prone to interference (how much ambient ultrasonic noise could their be? I can see it now: "Do not operate this device around bats"). That is of course a plus in what I plan to use this for.
But what could recieve the ultrasonic bit? Just a normal electret microphone?
And before I go buying any parts, how much power would one need to pump out of the ultrasonic speaker in order to make it effective at 20ft+?

Oh, and about the wired version, Aaron, I understand how easily that would work, but I can see other uses for this type of device in the future since I keep it after filming.

As for infrared itself, would that be screwed up by sunlight? I had an old lasertag set when I was little that wouldn't work or worse, would falsely trigger, in bright sunlight.

EDIT: About the mobile phone device Mates suggested, that definitely cannot be done. This thing needs to be able to be triggered on cue, as fast as possible. I can't be calling a phone number and waiting for a connection when these things need to be going off on after another. Thats why I plan on using an SCR to do the actual firing.

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