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measure the current

Discussion in 'Electronic Design' started by dryew, Apr 17, 2005.

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  1. dryew

    dryew Guest


    I need to build a circuit that delivers "logic one" (VCC) if the
    current passing through a wire is higher than 1mA or "logic zero" (GND)
    if the current is lower than 1mA. This "circuit" should draw as less
    current as possible (lower than 1mA).

    I have no ideea how to do that and any help would very be appreciated.

  2. Joerg

    Joerg Guest

    Hello Yew,
    Need more data here. Such as required tolerance, whether it's a mass
    product or a one-up lab thing etc.

    For an experiment a Reed contact with lots of windings around it comes
    to mind. Then there are comparators etc.

    Regards, Joerg
  3. Jim Thompson

    Jim Thompson Guest


    for a circuit that was designed to show current-flow direction, with
    zero as the turn-over point.

    Should take only minor modifications to do what you want.

    In your particular case, only guessing, something much simpler would
    probably do.

    ...Jim Thompson
  4. dryew

    dryew Guest

    I have a commercial device that goes to sleep when it's not needed and
    draws a few uA and when is running draws around 4mA.
    I want to figure out with my microcontroller when this device wakes up.
    Reading the current that passes through is the only way to figure out
    if the device is awake or not.

    I thought about using the analog comparator in the microcontroller but
    the only problem is that I can measure with it Volts and not Amps.

  5. dryew

    dryew Guest

    I'm having problems understanding this circuit as low level electronics
    is not (yet) my thing. If you have time, please tell me what should I
    modify to get the minimal circuit (as the space on my pcb is very
    small) ... If not, I'll do my best to figure this out.

    Thank you.
  6. Luhan Monat

    Luhan Monat Guest

    You can do this with two resisitors and one PNP transistor if the power
    source is the same for both. Emitter to power source, Base to device
    drawing the current, Collector outputs voltage when current is drawn, 1k
    resistor from collector to ground.

    Second resistor connected between Base and Emitter. For 1 ma and .6
    volts to turn on transistor, R=E/I = 0.6/.001 = 600 ohms.
  7. Luhan Monat

    Luhan Monat Guest


    | -------|
    | |
    | |
    | .-.
    | | | 600 Ohms
    | | |
    | '-'
    | |
    |- ----|----------- Sense 1ma
    Logic Out -----|
    | |
    | | 1k

    (created by AACircuit v1.28.5 beta 02/06/05
  8. Joerg

    Joerg Guest

    Hello Yew,

    As Jim mentioned you can do it electronically. But if you aren't
    familiar with chips like the TL431 (there is now a lower power variant,
    the TLV431) then it needs to be simpler.

    You can do it with a comparator, even one in a uC, but you have to
    understand its range limits and tolerance. You could use it to sense a
    voltage drop across a resistor if the voltages on both sides are in the
    "compliance range" of that built-in comparator. Safety is another
    matter. I don't know whether the commercial device is connected to mains
    but if it does then safety rules become an issue.

    Regards, Joerg
  9. Hi dryew,
    There may be another otion in that special case.

    Most micros turn off their oscillators when they go to sleep mode.
    You can simply monitor if the oscillator is stopped (steady hi or low
    signal) or running to derive a sleep/non-sleep signal easily.

  10. dryew

    dryew Guest

    Thank you very much. I'm going to test this today. You are right, the
    VCC is the same from both.

    I see that you used .6volts in the ohm law. Is this the tension on the
    VCC? If yes, I guess that I should use 3.3volts (as my power source) so
    the 2nd resistor should be 3.3kohms. Right?

  11. OBones

    OBones Guest

    Nope, that would be, I guess, the voltage drop accross the transistor
  12. Rich Grise

    Rich Grise Guest

    Do you have a spare output on the micro?

    Just write code similar to a "watchdog" - when it's running, periodically
    output something. When that pulse isn't there, your micro is asleep.

    Added circuitry: Zero.
    Parts cost: Zero.
    Firmware Development: TBD. ;-)

    Good Luck!
  13. dryew

    dryew Guest

    I can't write anything for the device I want to monitor because I don't
    have access to it's software. I can only write code for my

  14. Jim Thompson

    Jim Thompson Guest


    Newsgroups: alt.binaries.schematics.electronic
    Subject: Measure the Current - from S.E.D - CurrentSense1.pdf
    Message-ID: <>

    for a simple-minded way to do it.

    If precision were desired I'd probably do it with a regulator with
    current sensing, to keep the load current from changing the output

    ...Jim Thompson
  15. dryew

    dryew Guest


    I tested the circuit in the scheme and it works.

    The problems is that the voltage drops with 0.6volts for my device. Is
    there any way to fix this?
  16. Jim Thompson

    Jim Thompson Guest

    And whose circuit did you try?

    ...Jim Thompson
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