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  1. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    It is not so much the effect of heat on the resistor, but the fact the more current means more voltage across the resistor and therefore less across the LED. Bob
  2. BobK

    555 Buzzer

    So control your astable 555 with a monostable 555 circuit. You can use the reset input on the astable to turn it on and off. Bob
  3. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    No, thermal runaway will not always happen. I have used (as an emergency light) a high power LED rated at 3.3-3.6V driven directly by a 3V battery, and you will not get in trouble with that. But you will also not get nearly the full capability of the LED. If you are trying to run it at max...
  4. BobK

    Self-made transistor RAM

    It is just a bistable multivibrator. http://www.electronics-tutorials.ws/waveforms/bistable.html Bob
  5. BobK

    Self-made transistor RAM

    Actually you can build a static RAM cell with only 2 transistors. It is simply a discrete SR flip flop. Bob
  6. BobK

    ATMega8 loses program, 328 doesn't

    The bootloader would definitely write to flash, so, if the program is starting in the bootloader, it could be clobbering something right away. Bob
  7. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    The resistor is what provides the safety if the volages are a little off or the LEDs overheat (which causes them to draw more current). So, yes, you still need a resistor. But if you put 4 + a resistor and use 12V you will not get sufficient voltage across the LEDs. So you would use 3 in...
  8. BobK

    My first circuit

    Sorry dave, but you Ninja'd me. I was composing mine when you posted. Bob
  9. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    You got it! 470 is a standard value, so you would normally use 470, which also gives you a little safety margin. Bob
  10. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    The graph shows that the current varies greatly with small changes in voltage. So if you double the current though the LED from its operating point, the voltage might go from 2.0V to 2.1V, or something like that. This is why we don't try to drive LEDs with a voltage, but instead use a resistor...
  11. BobK

    My first circuit

    Welcome to the forum. The hard truth is no, it will not work. First of all, you have the anodes connect to the - of the supply, when they need to on the +. Grrrr Bob I want him to work that out for himself per my comment ;) Here is a tutorial on how to power LED...
  12. BobK

    ATMega8 loses program, 328 doesn't

    Is the program writing to flash? It could be going out of bounds and destroying itself. Bob
  13. BobK

    SMPS design

    You know you can buy one for less that you could ever build it? http://www.ebay.com/itm/150W-DC-DC-10-32V-to-12-35V-Converter-Boost-Charger-Step-Up-Booster-Power-Supply-/390615667209?pt=LH_DefaultDomain_0&hash=item5af2821209 Bob
  14. BobK

    Voltage Divider/EL Wire Circuit Help

    The power going the the EL wire is in the range of 100V. I now see that perhaps the 10mA / foot refers to the low voltage supply, not the high voltage supply. If the Wiki article is right, a 10K pot will only be able reduce the current by a little more than 1/2, that is why I recommended a...
  15. BobK

    LEDs (and diodes?) have CONSTANT voltage?

    No. A diode or LED has a current that is exponential in the voltage. Look at the IV curve for an LED. The Vf of an LED is the voltage at which it will pass the rated current. You can always place more voltage across it, but you will probably destroy it. The resistor is sized to drop exactly...
  16. BobK

    Recordings=actions

    The Clapper Bob
  17. BobK

    Voltage Divider/EL Wire Circuit Help

    I hope you are wrong about the 10mA / foot. I think you must be, because that would be 1 W / foot at 100V and 20 feet would take 20 Watts. As I understand it EL wire uses much less power than that. According to Wikipedia, the EL wire acts like a capacitance of 1nF / foot, and is driven by...
  18. BobK

    I don't "get" capacitor usage if it blocks current

    Perhaps this will help you understand your original question better. In the circuit on the right, does current flow through the resistor? The answer depends on the voltage of the two batteries. If the are the different, current flows, if they are the same none flows. Now look at the...
  19. BobK

    Electronics work situation bad

    move to Hong Kong? bob
  20. BobK

    Can anyone identify the correct value of this resistor

    I vote for 180K. Bob
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