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Detecting current in 12v system

  1. Feb 19, 2010 #1
    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?

    Last edited: Feb 19, 2010
  2. jcsd
  3. Feb 19, 2010 #2


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  4. Feb 19, 2010 #3
    Last edited by a moderator: May 4, 2017
  5. Feb 19, 2010 #4
    Thanks for the suggestions! Excellent forum.

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