Inductance Switch

[jsmoker]

hey all,

I'm working on a project that is to be a non invasive add-on to an existing circuit.  We want to activate when ever the existing circuit is activated.  We've heard of a switch which is based on inductance, i.e. when the existing circuit is turned on, the current is detected by a coil and turned on our circuit.  Does anyone know where I can get something like this.  preferably something that can clip on to a wire.  I apologize if this message is only semi-readable.  I'm a bit sick and not thinking straight.

-JSmoker

[jsmoker]

Hmm, it seems they're called "Split Core Current Switch"  but I still can't find anything for a circuit (ie something that can detect less than an amp of current).  Any help is appreciated.

-JSmoker

[paulstreats]

I would think that youd need a coil inside inside the unit aswell as on the outside in order to run an electromagnectic field high enough unless youre talking of the unit using >100v itself. also these type of things generally only work with some kind of split signal such as ac or square waves 

[Admin]

define 'non invasive'

there are many ways to do this . . . depends on your limitations.

one way i'd do this is find a pin that makes a significant voltage change only when that circuit turns on. then attach that pin to an amplifier using clips. there are circuits out there that can make the amplified output either a 0 or 1, then attach that to the gate of a MOSFET that controls your external circuit power. make sure grounds are common.

you could also attach that output pin to the ADC of a microcontroller, and once the ADC reaches a preprogrammed threshold it can turn your external stuff on.

[jsmoker]

Preferably, non-invasive, would mean not tapping into the existing circuit.  Our design is an add-on to a Firefighters SCBA (breathing apparatus). It's just collecting data when ever the gear is turned on.  In the statement, we want to put that our equipment doesn't interfere with the tried and true electronics of the SCBA so there's no danger to the firefighter of adding the data recorder on.  I mean sure, we could say, "hey, this trigger switch only drains .0002 amps from the existing electronics," but I think the idea that we want to convey to the "fire-chiefs" etc. is that our circuit isn't even electrically connected so there's not chance of shorts or battery loss, etc. due to our project.

-JSmoker

PS
This circuit has to be in ultra low consumption mode when not in use cause though it may be turned on only for a couple hours (draining approximately 20-30mA)  at a time, it could be sitting idle for a week or so.

[Admin]

I mean sure, we could say, "hey, this trigger switch only drains .0002 amps from the existing electronics,"

The problem is that to sense something, you interfere with it. Its impossible to sense anything without interfering with it. So the reason I asked is, what is acceptable interference? If you decide .0002 amps is acceptable, then you can design for that.

For example, an inductive sensor requires inductive resistance, which changes current flow in the device being sensed.

How about wiring your sensor (perhaps a current sensor IC?) in series with the SCBA batteries? Chances are the voltage regulator and noise suppression capacitors would remove any and all significant interference. If it senses a current flow, then the SCBA is obviously turned on.

[snow]

You can detect current flow with Hall sensor. Like this one: http://www.allegromicro.com/en/Products/Part_Numbers/0712/index.asp