a little RF help, please?

I want to find a way to detect when a very small solenoid is energized for about 10 msec. By "small" I'm talking about 1.5 watts, and turned on with 6 volts, and physically very small, like 3/16 inch diameter. I want to detect it from about 2 feet away. So, it's going to be emitting an electro-magnetic signal, but to save me a lot of trial and error, if someone can get me pointed in the right direction, I'd appreciate it. As soon as I post this I'm just going to see if I can detect it using a piece of wire as an antenna, then try a coil of wire, etc., and just watch the oscilloscope.

Thanks

Reply to
almo
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The magnetic field is going to be very difficult to detect, as magnetic loops go down as the cube of the distance. It's likely to be weaker than the Earth's magnetic field. To evaluate this, hold a compass two feet from the solenoid as you energize it. Oops, 10msec isnt going to be enough time for the compass needle to react.

The rf radiation is going to be really really miniscule and very low frequency, as the current through a solenoid is going to change quite slowly. A wire hooked up to an audio amplifier or scope just might pick up something. But a Harley driving by is likely to swamp out the signal.

Reply to
Ancient_Hacker

Given your stated conditions, I believe no solution is possible.

You need to re-pose the problem from a more elemental level. Why are you activating the solenoid? Why do you need to detect it?

Luhan

Reply to
Luhan

I suspect you cannot do it by detecting the magnetic field. 2 feet from a solenoid will see the field close to the noise. What about putting an IR LED/resistor across the solenoid? It would then be easy to detect the IR pulse a few feet away.

Barry Lennox

Reply to
Barry Lennox

Thanks everyone. I think there is no practical solution to the problem as it was presented. The situation was to detect the solenid being energized but not having access to the system containing the solenoid, so "non-invasive" would have been the key word. Of course, in theory (where everything works) I would use an "ideal" amplifier, and with some fourier analysis I could pick out "my" little signal from all the larger ambient signals. Also, I would normally just say what it is, and why I'm trying to do this, but this is something I'll patent and it'll help pay the rent, although not using the RF from 2 feet away. I'm debating on starting my own company right now, versus working for some big company again and just getting "assignments," and then yearly performance reviews by people who couldn't design their way out of a wet paper bag.

But I've found that it never hurts to ask. These google groups, and usenet, put me in touch with everybody in the world who knows something about anything. Strangely, where I worked, (I quit recently) being caught browsing the user groups might be considered as just screwing off. But, whenever I need to do something without re-inventing the wheel, these groups are the first place to go. Maybe somebody doesn't know, but they know a guy who has a friend, etc.

In fact...I do have another RF question, a real world type question, about my Sirius Radio antenna, which is not working. So, I'll post that one now.

Reply to
almo

Well, let's not give up just yet. As I understand it, your solenoid and wiring and power supply are completely contained within a noncoductive two foot sphere. Is that about it?

Now you are going to hit that solenoid with a pulse 10 milliseconds wide and the solenoid is going to move, but not so as to mechanically disturb the sphere. The problem with detecting RF, is that RF by definition is a constantly varying magnetic field, and your magnetic field, after the initial burst of magnetization, is a relatively constant field, not generating any RF.

But before we go away and say we can't do it, is my scenario exactly correct?

Jim

Reply to
RST Engineering (jw)

Here's a long shot. Do it the way shoplifting detectors do it. When the solenoid is energized it's core is probably magnetically saturated or close to it. You may be able to detect this by applying a powerful alternating magnetic field that drives it in and out of saturation, producing a signal at a harmonic of the driving frequency that could be detected with a sensitive receiver. Shouldn't require more than a thousand watts or so with coils the size of barrel hoops :-(

-- Joe Legris

Reply to
J.A. Legris

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