Detecting current in 12v system

[HeyThevenin]

I'm looking to detect when a device in a 12 volt systems is on, so that I can do something with that information -- like feeding it into an and gate. The device has it's own on/off switch inside it, so I can't simply detect at the regular on/off switch.

I was thinking about putting a small resistor in series with the device and then detecting the voltage across it. (Maybe there's a way to use an op amp to detect voltage this way, I'm not sure. My memory of op amps is a bit hazy.)

I was also thinking that I could measure the voltage across the device's power leads and compare it with the 12v supply. Maybe I could use a simple gate and some pull down resistors that would read the voltage drop as a zero instead of a one.

The device would be drawing 1-3 amps typically. It could be a light or anything else. The power supply would be either a car battery or a car battery on a charger. That would make the latter method a lot tougher, I'd think. There wouldn't be a lot of internal resistance in the power source.

Is there an elegant way of detecting whether something is on?

Regards

 

[vk6kro]

You could have a look at this thread from last December:

https://www.physicsforums.com/showthread.php?t=364951

A few different ways of doing this were discussed, but the final conclusion was that this method was adopted:

https://www.physicsforums.com/showpost.php?p=2512853&postcount=23

You will need to adapt the calculations to suit your application.

 

[pantaz]

A few options...
http://www.linear.com/ad/current_sense.jsp
http://www.maxim-ic.com/app-notes/index.mvp/id/746
http://focus.ti.com/analog/docs/microsite.tsp?sectionId=560&tabId=2180&micrositeId=7

 

[HeyThevenin]

Thanks for the suggestions! Excellent forum.

Regards