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  1. BobK

    Simple PWM LED Driver than randomly dims/brightens?

    That is interesting. I wanted to try to get the same effect, and I used a linear shift register random number generator to set the duty cycle of PWM in a PIC microcontroller. To my surprise, it looked just like the flickering candle LEDs. Now I know why! Bob
  2. BobK

    Understanding resistance more.

    Why would you ever want to? Power dissipated in a resistor turns to heat and is wasted power (unless what you really want is heat.) And you seem to be under a misconception. A resistor does not "convert" current into voltage in the sense that you are using the term. It basically wastes power...
  3. BobK

    Want to know more applications of step-up DC to DC converter

    Photo flash comes to mind. These create a voltage of about 300V to set off the flash tube from a 1.5V battery. For smaller voltages, an LED flashlight. A white LED needs about 3.3V. If you run off two 1.5V batteries with a boost converter, you can keep the LED going full speed even after...
  4. BobK

    If the source voltage is the same as LED voltage, is a resistor necessary?

    Depends on the voltage source. If it is greater the twice the forward voltage + a couple of volts, you should have two LEDS in series with a single resistor. Higher still and you can put 3 or more LEDs in series. For example, you have a 12V supply and red LEDs with 2.0V forward voltage. You...
  5. BobK

    A few beginners questions

    I suspect you are not measuring correctly. When you measure the current, did you put the ammeter in series with the battery and the resistor? i.e. like this; battery + -> ammeter + -> ammeter - -> resistor -> battery minus. Or did you just put the ammeter across the battery terminals, i.e...
  6. BobK

    Smoke detector mystery

    We know the batteries last at least 6 months in normal use, so at 450mAH that would be an average current draw of 100uA! That is why I don't believe voltage drop in the wire could be responsible. I calculate 258 mOhms for 16 feet of 22 ga write, so at the average current, only a voltage drop...
  7. BobK

    zener circuit

    Or for more safety, you could place a beefier schottky diode from the pin (anode) to Vdd, which would parallel the internal diode. Bob
  8. BobK

    Smoke detector mystery

    Yeah, ripple current would seem to be the only thing that would heat the cap that way. Bob
  9. BobK

    Smoke detector mystery

    I have a smoke detector that is in a precarious place, on a sloping ceiling high above a flight of stairs. I cannot easily use a ladder to reach it because of the stairs, so the way I get to it to change the battery is by placing a sheet of plywood across the stair railing and a step ladder on...
  10. BobK

    Battery advice needed

    Hmm. Then I suspect it requires much more current than 300mA. C cells are designed to deliver over 1A continuously. Can you measure the resistance of the electromagnet coil? NiCad batteries and LiIon are better at delivering large currents. If you go the 18650 route, watch out, there are a...
  11. BobK

    a circuit that will indicate when a contact is broken?

    This circuit brings up something that has always bugged me about the Darlington configuration. As Kris said, it gives you the product of the current gain of the two transistors, so that is great. But, the second transistor cannot saturate because it's collector (shared with that of the first)...
  12. BobK

    Battery advice needed

    The 4 C cells should have had no problem delivering 330mA. Are you sure they were fresh and wired correctly? They should have more current capability than a CR123. I would think even AA cells would be sufficient. How long a pulse is required? Bob
  13. BobK

    Getting 5V from 6V source

    At 5mA, probably 2 diodes. Bob
  14. BobK

    LED driver without IC and Transformer

    Please use Google. Bob
  15. BobK

    Highest sensor read question

    And quite easy to tweak the behavior, unlike and analog circuit. Bob
  16. BobK

    Request help interpretting LED max Vf spec in spec sheet

    No, it means the voltage at 1500 mA will be somewhere between 3.03 and 4.95V. That is an outrageously wide range, though. Bob
  17. BobK

    Highest sensor read question

    Sounds like a job for a microcontroller to me. Bob
  18. BobK

    How to choose a micro controller?

    Many PICs also have low voltage programming. Bob
  19. BobK

    Blowing a 3A fuse with an AA battery?

    Mine has two fuses, one for the 10A input and the other for the mA input. I have blown the mA one several times.:oops: Bob
  20. BobK

    Best Speaker To Use As Microphone

    In the link I provided, the guy used an audio output transformer to up the voltage (and impedance) from a speaker. I believe the old cheap walkie-talkies that used a little 2in speaker as both mic and speaker also used that method. Bob
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