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  1. BobK

    Resistor help

    If you get 4.2-4.5V on each LED and 8.2V across the resistor that is a total of at least 16.6V. Are you sure you are measuring correctly? Are these LEDs lit up WAY bright? It is possible your "9V" wall wart is putting out much more than 9V when not loaded very highly, but 16.6V in one case...
  2. BobK

    transistor and speaker help

    I see no circuit here. Bob
  3. BobK

    Resistor help

    What do you get when you measure your 9V supply? Across the resistor? Bob
  4. BobK

    Arduinos combined to be a gift?

    Unless, of course, your wife is into embedded programming. Bob
  5. BobK

    Terminal voltage sensing

    AFAIK the constant voltage method of charge termination works only for lead / acid and Lithium batteries. Bob
  6. BobK

    Maintaining DC voltage across capacitor with signal?

    It would help if you told us what you are trying to do, instead of asking questions about what you think is the solution. Bob
  7. BobK

    Terminal voltage sensing

    Use a trickle charger at no more than 0.05C and you will be safe. Bob
  8. BobK

    Setting up LED

    Wow, that is one hell of a flashlight! A Li Ion battery like the 18650 will start at 4.2V and go down to about 3.2V as it discharges, so it is great for a 3.1V LED. A buck regulator regulating current would power this from a single 18650 for maybe 30 minutes. You can get buck regulators...
  9. BobK

    FLEDs needed

    Ja, Blinken Lichten sind immer gut! Bob
  10. BobK

    USB Chargers

    I suspect that the charge speed up is based on the fact that various devices look for certain voltages on the data lines to determine how much current they draw. A charger could, conceivably, try these various voltage combinations, while monitoring the charging current and then choose the...
  11. BobK

    Resistance range to output voltage range

    As you have determined, you cannot get this relationship with just a voltage divider. Normally, one would use a constant current source to convert a resistance into a voltage linearly. But your two points indicate that that would not work either. At the high end: I = V / R = 4.5 / 149 =...
  12. BobK

    sampling communication lines without interfering the signal

    If the high signal is > 3V you should have no problem reading them on your Arduino without level translation. Bob
  13. BobK

    Component help

    Steve, I unspoke my thoughts due to the fact that I could not find a workable solution. 1.6uF caps can be had, but they are very expensive. Anyway, my concern was that the cap is used as a ballast for a transformerless power supply, and the value is likely to be critical (unlike most...
  14. BobK

    Setting up LED

    That LED is pretty extreme for a flashlight, with an output almost as high as a 100W incandescent bulb. Bob
  15. BobK

    N channel mosfet confusion

    The gate charge for MOSFETS is measured in nanocoulombs. Which means that nanoAmps will charge it in one second. I don't think you need to worry about the current. Bob
  16. BobK

    TRIAC Switching Circuit

    Kris, Yes, I almost said that you can, in this case, use that configuration for switching, but I promoted the common emitter configuration because it is more generally the correct way to do a transistor switch. The problem, of course with using an emitter follower, is that it cannot be...
  17. BobK

    TRIAC Switching Circuit

    Hi HGB, and welcome to the forum. The first problem I see is that you have the transistor switch wired incorrectly. The LED and the resistor should be connected between +5V and the collector, and the emitter should be connected to ground. After that, the calculation for base current is as...
  18. BobK

    Noob question about my resistor problem

    Like this:
  19. BobK

    Noob question about my resistor problem

    You would be better off to put 2 of the LEDs in series, that way you use only 2/3 the current you would by having 3 in parallel. And your battery would last 50% longer. Bob
  20. BobK

    LM2903N differential op-amp

    The LM2903 is a comparator. It will output a high voltage if the + input is greater than the - input and a low voltage if reversed. Any op amp can be used as a differential amplifier. It does, however require a specific circuit, like this...
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