remote
Harmless
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Registered: 16-9-2006
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Perusing wireless intercoms for remote Ignition?
Hello all,
please check this:
http://cgi.ebay.com/62-WIRELESS-HIGH-QUALITY-INTERCOM-SYSTEM...
This wireless intercom has a fine reach of 200 m.
I'd like advice on how to turn this into a safe remote station for remote rocket ignitions and so on. Here, one intercom would be connected to the
electric fuse, using a relay to directly feed 12V battery power into the fuse, which ignites a small incendary mix. The other intercom, which'd be up
to 200 m away, would contain a safety switch, and an ignition switch.
A number of modifications will be necessary to achieve this:
First, the transformer will have to be removed, and it has to be converted to use battery power. That shouldn't be a problem, provided it is easy to
see where the AC current is fed into.
The bigger problem is what type of signal to transmit (a pulse, i.e. a sharp sound, or continuous) and how, at the receiving station, this signal
(which targets the loudspeaker), is used to switch on a relay, which in turn then connects to the electrical fuse.
Ideally the sender would simply send a pulse, rather than having to make a physical noise (for safety reasons), which would allow for removal of the
microphone function of the sender station. So I'd have to feed a weak DC current into the microphone cables instead. How would I go about that? Then,
at the receiving station, how can I use this DC current (which is targeted for the speaker) to turn on a relay, i.e. for a set length of time, or
indefinitely?
Advice, thoughts, ideas, better devices?
[Edited on 17-9-2006 by remote]
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not_important
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Most simple devices like this will not reproduce a DC signal, at best you'll get two spikes whos width will depend on the bandpass of the gear. You'd
do better sending a tone, using something simple like 555 to generate a square wave of a few hundred hertz frequence. It won't be a perfect square
wave when it comes out the speaker drive of the receiver. There you would likely rectify and 'integrate' the tone, just a diode and capacitor, to get
a DC control signal.
If you have a little electronics skills, a garage door opener might be better. They send a coded signal, exactly what the coding is depends on what
generation device it is, and generate DC control output. One problem is that the receiving side is generally designed to run from the house mains,
you'd need to measure the working DC voltage and come up with a battery supply to match.
Places like the following sell kits that often include remote control functions.
http://www.ramseyelectronics.com/
http://www.electronickits.com/
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YT2095
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alternatively, you could use the O/P of the dvice to the base of a transistor to activate a relay.
if you have a relay with a double set of contacts you use the one set to make the relay self latching, as in once it`s pulsed the contacts close, that
then allows current to pas and hold the transistor ON.
you then use the Other set of contacts to engange your battery to electric-match.
simply disconect the power when your finished and it will reset again. 
[Edited on 17-9-2006 by YT2095]
\"In a world full of wonders mankind has managed to invent boredom\" - Death
Twinkies don\'t have a shelf life. They have a half-life! -Caine (a friend of mine)
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unionised
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Normally, a lightning strike has to be quite close to the wiring for an explosive's detonator to set it off prematurely. If you use a cheap radio
receiver then it becomes much easier.
OK, I know that thunderstorms don't always trigger remote garage doors but I have heard of the ignition system of a motorbike generating enough EMI to
do it. Are you prepared to take that risk while you are wiring up the rockets?
A simple boring igniter circuit will do the job with less chance of an unintended firing.
It is, of course, perfecftly possibe to get this idea to work properly and safely; but it's much easier to get a couple of hundred yards of cable.
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Twospoons
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I second your comments, unionised. If you want a 'safe' radio trigger you must use a digital coded transmission, use at least a 128 bit code with a
32 bit CRC check. I'd still be wary of it though, and would put a long wire between rocket and receiver.
I've built many radio telemetry systems in my career, so believe me when I tell you that random crap comes out of radio receivers all the time -
especially the cheap ones used by doorbells and intercoms!
[Edited on 17-9-2006 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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remote
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Thank you for all the suggestions.
I am not an electronics expert, although I know some basics.
I looked up some principles on making DC current of a given frequency, but they involve a coil of a defined number of loops (windings around the
centre, not sure if 'loops' is the correct term'), and a capacitor; and when stimulated, its frequency is defined by f = 1/2Pi[Root(L*C)], where Pi is
Pi  , Root the quadratic root, L the number of coils (it's in Henry, but I am not
sure what the definition of this unit is) and C the capacitance of the capacitor. Sounds simple enough, but probably it'd need a fair deal of
optimising/experimenting until a decent frequency of i.e. 500 Hz could be achieved. Not impossible, but I think I may use a maplin or Radioshack
frequency kit, which essentially generates just whatever frequency I desire. So, imagine, I feed this into the microphone port (using an i.e. 500 ohm
resistor to reduce the input power, but the precise ohm value required for a sensible signal will have to be tested... I guess using a variable
resistor will do the job). The microphone port is likely the speaker itself, as a speaker can (and is often used as such) function as both a speaker
and microphone (good old Maxwell established this eh ). I've seen this in the past
on intercoms (years ago), so I am certain that microphone and speaker are the very same thing.
The tone generated is then transmitted, 200 m further, and received. Then I rectify it using a quadruple set of diodes (or even a single one, thereby
accepting a twofold reduction in power). Now this is where YT2095's advice comes in: The initial pulse is fed into a low power relay, which switches
it to ON. The current flowing through this relay goes straight into feeding the input side of the relay itself? Please tell if I understand you
(in)correctly. I wonder what sort of relays to buy for this, and what sort of resistors to place inbetween. Essentially, now this first relay is on
permanently. This of course I can use to feed the *high* power secondary relay, which then brings the fuse to ignition. Sounds simple enough I guess.
So the mission is to buy a tone generator, and one low and one high power relay. I should probably use a sensitive low power relay for the initial
pulse, since it's only low voltage current that (albeit rectified) feeds directly into the speaker (the speaker being replaced by the input side of
the relay). The second relay will be equally sensitive on the input side, and 12V but high Amp on the output side (which feeds the fuse).
@ Notimportant - I'd rather not use IC's, I didnt have much good experience with them. They die too quickly, particularly if your input is not
optimal. I am not enough of an electronics expert to know how to feed IC's properly without killing them. Transistors and big capacitors is more my
thing. But as I said, I'll probably just get a tone generator from one of the above suppliers.
@ unionised & Twospoons - I appreciate your concerns. The plan is to connect the receiving station to the ignition station with a distance of at
least 5 m (using a cable to bridge this distance). Obviously it is all set up first, and only when everything is ready, the fuse is physically
connected. I'm not worried about thunderstorms or nuclear EMS's or whatever, as any experimenting is planned under benign conditions, and as I'll
have at least 500 m of distance to the nearest settlements that could emit a random signal.
Also, the system will be tested extensively before it is put to use, including how sensitive it is to outside influences.
I'm not a big fan of 200 m cables simply because of practicality. Where I'm from sometimes a quick exit needs to be made (sadly), because the noise
might disturb nearby people. So it has to be portable, simple and safe, with a quick removal time post ignition. Thus the idea of using intercoms.
Originally timers were used, but they are/were inherently unsafe, because you can't stop them if i.e. 20 sec before ignition someone happens to
randomly walk that particular part of the meadow/forest/whatever.
Making a coded signal...mmmh... I'd be up for it, but again I fear that would be beyond my capability.
Are there circuits available that i.e. switch on only once a certain input frequency is received?
Again, further ideas and/or inputs of any sort are welcome
Thanks, mad scientists
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Twospoons
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*sigh* Even under the most benign conditions your radio will produce crap / noise / static that can trigger a simple rectifier circuit such as you
envisage.
I really suggest you have a look at the UHF radio control kits here . They have good range, they're cheap, they're digital, they'll save you a lot of time, grief and eyebrows. I've bought stuff from these
guys before, and they are good to deal with.
Helicopter: "helico" -> spiral, "pter" -> with wings
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unionised
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"I'll have at least 500 m of distance to the nearest settlements that could emit a random signal"
Last time I checked they were still geting signals from Pioneer- that's a shade over 500m.
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