LED to indicate current draw & other project questions

Discussion in 'The Projects Forum' started by Lambda, Sep 18, 2011.

  1. Lambda

    Thread Starter New Member

    Feb 28, 2011
    Hi there. I'm currently working on a project that is a charger for a USB device. One thing I'd like to include is an LED that lights when the connected device is drawing current. (the device is too small to include an ammeter, as much as I'd love to)
    My first thought was to include a shunt resistor, and the potential difference across it would trigger a transistor. But that didn't work in the simulator. Wiring the LED in series with the load wouldn't work, this device should handle 500ma.

    I've also been wondering about labels. How do you go about labeling projects that demand a more "pro" look? I'd usually just use embossed labels, but looks count on this device. In the past, I've tried clear shipping labels, printed and cut to shape, but even the clear ones are meant for paper and have a matte finish, and they look bad on metal.
  2. John P

    AAC Fanatic!

    Oct 14, 2008
    If you can tolerate the voltage drop caused by an LED, how about putting the LED in series with a resistor, and have those two components in parallel with several diodes (I would guess 4) in series? The idea would be that the voltage drop across the LED+resistor would increase with increasing current, whereas the drop across the diodes would be more like a constant voltage, so at some point the circuit would settle with a limiting current through the LED and any increase in current would go through the diodes.

    As for labels, something like this?
  3. wayneh


    Sep 9, 2010
    Detecting a current could be done with a comparator. It can be powered directly from your 5V USB supply. Set up a voltage reference using a pair of resistors and put that on the inverting pin of the comparator, and put the voltage on the shunt resistor on the non-inverting pin. The shunt should be on the return to ground from the device, so that a small voltage above zero indicates current through the load. The comparator output can then sink enough current (~5mA) to light an LED when the output is "off", when the reference voltage exceeds the shunt voltage, indicating no current. Don't forget the current limiting resistor for the LED.

    There used to be a labeling technique where you would rub the lettering off of a backing, transferring only the text onto the object being labelled. I haven't used it for 30 years, but it worked pretty well back then.
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
    I remember looking for that stuff a few years back where I used to get it (art supply, crafts places) but was unable to find it anymore. I thought that maybe the cyber age killed the demand, but couldn't figure out how. I may even have searched the net for it without any luck. It was good stuff!
  5. SgtWookie


    Jul 17, 2007
    It was called "dry transfer" - you can still find them if you look hard enough.

    Using a laserprinter on overhead transparencies works very well.
  6. wayneh


    Sep 9, 2010
    A comparator circuit similar (but not identical) to what I suggested is posted here.
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    I like the laser printer on overhead transparency method myself. Particularly if you can mirror the image and print it in reverse; then on the other side, the print shows normally and is protected by the transparency.
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    I've done this with excellent results on a color laser printer as well. Use the 3M 77 spray adhesive to the back of the film once it is printed on, then stick it on.

    Making windows for LCD displays, as well as finding nice looking switches is a bit more difficult.