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  1. BobK

    Need help with Warning LED indicator for redundant power supply.

    Harald has already given you the answer. Bob
  2. BobK

    PWM solenoid driver

    You might consider a bistable solenoid valve: http://www.staiger.de/en/need-to-know-staiger-valve/function-principles-staiger-valve/156-funktionsprinzipien-2-way-solenoid-valve-bi-stable-function-nc This type of valve needs a pulse in one direction to open and pulse in the other to close...
  3. BobK

    high pulse delay circuit with transistor

    You won't get a snappy on off with that circuit. Here is a circuit with a '555 that should do the trick. http://www.electronics-project-design.com/timedelaycircuit.html Bob
  4. BobK

    Two phones into one headset.

    You could try running each though a 33Ω resistor then combing the other end of the resistor to go to the headphones. There will be a reduction in volume, but it might be acceptable with both phones turned up high enough. Bob
  5. BobK

    Microcontroller controlled by pushbutton for LED flashlight

    Looking good! The pullup should only use current while the button is pressed, so is it really a worry? Bob
  6. BobK

    Peltier Cell Help

    Why would you need cooling to 37C if the ambient is less than this? Is something else heating the surface? Bob
  7. BobK

    Voltage Reference Supply Help

    What is the 2.5V reference? Are you talking about a single device? Which one? How is it connected? Bob
  8. BobK

    Peltier Cell Help

    Why don't you describe a little better what you are doing. What is it you are trying to keep at a constant temp? A small enclosed space, a surface or what? What is the range of the ambient temp? Are the Peltiers used for heating, cooling or both? Bob
  9. BobK

    Microcontroller controlled by pushbutton for LED flashlight

    Welcome to the wonderful world of SMT! I was also afraid of it at first, but now that I have some experience I love it. A lot fewer holes to drill, and smaller boards. With drag soldering, I can actually solder a 44pin PIC faster than a 40 pin through hole one. Bob
  10. BobK

    Peltier Cell Help

    What is your intended use? If you are just trying to maintain a temperature they might not need to be run continuously, and more insulation is cheaper than more battery. Bob
  11. BobK

    Mini Speaker/Amp project

    Yes, I would beware of them. I bought some cheap ones, supposedly 3000mAH. Two of the 4 were basically DOA, the others ran down in less than 2 hours at 100mA draw, so more like 200mAH. Bob
  12. BobK

    How to control 700 solenoids?

    MOSFETs are fine for switching solenoids, you just need a diode put in reverse polarity across the coil to suppress the high voltage spike when the solenoid turns off. How many of these solenoids can be on at the same time? If it is 700 you will need 2800W of power for them which is more than...
  13. BobK

    Mini Speaker/Amp project

    You could run that off any of the 5V power banks that have become popular for recharging cell phones. Like this, for example: http://www.ebay.com/itm/5600mAh-5V-USB-Power-Bank-Case-Kit-18650-Battery-Charger-DIY-Box-For-Cell-Phone-/261736951501?pt=LH_DefaultDomain_0&var=&hash=item3cf0bceecd...
  14. BobK

    Binary to BCD Converter

    In other words, back to the days when any small change in behavior required a hardware revision. :eek: Bob
  15. BobK

    Peltier Cell Help

    Quick answer: a large one. Bob
  16. BobK

    Can anyone identify this transistor?

    You don't say what kind of reading you are taking. Is this with the diode test range? If so, try the leads the other way as well. A PNP transistor will need the negative lead on the base. Bob
  17. BobK

    Measuring speed at impact between two objects....

    It is not just the electronics that make this project difficult. It is the physics as well. In a closed system (no info coming in from outside) you cannot sense velocity or direction. The only thing you can sense is acceleration. Now, if you know your position and velocity at the start, you...
  18. BobK

    Momentary Buzzer

    Throw all the parts in a blender and run at medium for 1 minute. Bob
  19. BobK

    UPS shows odd graph and frequency..

    That is what they call a "modified sine wave". It is a square wave with the same peak and same RMS voltage as a sine wave. The frequency is being calculated wrong by the scope, probably due to the overshoots. If you notice that the time base is 8ms per division, and the waveform repeats in a...
  20. BobK

    Looking for a task management system that uses fobs...

    By the way, the search I used was "security rounds tracking", which brought up the link above and other commercial products to do this. Bob
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