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  1. BobK

    From batteries to Wall power supply

    If there are only two wires coming out of the battery pack, it is only a single voltage, most likely 4.5V if there are 3 cells. If there are more than 2 wires coming out, your teacher might be right, it is might be supplying more than 1 voltage. Bob
  2. BobK

    Christmas Gift for my 15 year old electronics enthusiast

    I wish I had had a parent like you! I spent all my lawn mowing and snow shoveling money at Radio Shack when I was 15. Bob
  3. BobK

    Resistive Load and Battery Life

    What you are missing is that lower resistance means more current, given the same voltage. Since the equation for power has the current squared, the increase in current override the lower resistance. P = I^2 * R Drop R by a factor of 2 with the same current, and the power is now P = I^2...
  4. BobK

    how determine type unknown connection?

    I presume you are measuring with a multimeter. The signals are changing rapidly and the multimeter cannot follow them. You will get something like the average voltage over the time the multimeter is sampling. If you want to really see what is going on, you need an oscilloscope or logic...
  5. BobK

    How to Select a Charger?

    Here is an example of a charger that should work. http://www.all-battery.com/smartuniversalchargerfornimhbatterypackRed.aspx Bob
  6. BobK

    how determine type unknown connection?

    Okay then, It does sound like RS232. It sounds like you are successfully exchanging data then, is this correct? Bob
  7. BobK

    how determine type unknown connection?

    That sounds like TTL levels then. You would need something like a MAX232 to convert them to RS232. Bob
  8. BobK

    How to Select a Charger?

    Changers need to be matched to the battery. You could get a universal charger that allows you to configure it for the battery. Bob
  9. BobK

    how determine type unknown connection?

    RS232 signaling voltage are specified as +-3 to +-15, so 3.5V is a valid signal level. Bob
  10. BobK

    Modify circuit to add switch and timer

    Harald, That circuit might be confusing to a beginner since, though it does use a 555 as a monostable, it is using it as a frequency meter, which is not what the OP is trying to do. Here is a link to a better tutorial of the 555 including monostable mode...
  11. BobK

    NPN BJT Transistor Biasing problem

    I would typically use 1/10 of the collector resistor, and bypass it with a capacitor that matches the impedance of the resistor at a frequency of about 1/2 the low end cutoff frequency. The collector resistor would be calculated to drop 1/2 of Vcc at your 10mA bias level. Edit: Oh, and...
  12. BobK

    LED strobe with minimal tailing

    The turn off should be very sharp, unless you have used a capacitor on the output. The fall time for the output pin is typical 100ns. max 300ns, which I doubt you can see. Please show us the circuit. Bob
  13. BobK

    Transformer problems

    So just cut off the center taps. :) Bob
  14. BobK

    10X20 bar led, to pushbutton

    Do you mean that you want to physically push down on the 10x20 LED bar to activate a switch? If so, a lever type microswitch might be appropriate with some mechanical guides from the LED bar to keep it straight. Edit: can you show us some pictures of the LED bar? bob
  15. BobK

    Impact of surrounding logic - causes

    I don't know anything about FPGA, so take this with a grain of salt. I think a ring oscillator would be special case. It's timing is dependent on the propagation delays of the inverters and it most likely operates above the max frequency specified for the FPGA. Normal circuitry would not do...
  16. BobK

    Hot Air Station - Finally arrived!

    That is what the tweezes are for! I put tweezers on the part and pull gently up. When the solder melts it come up nice and clean. bob
  17. BobK

    Fireflies blinking lights

    With a PIC, adding a simple photoresistor and some additional programming could make it work only at night and sleep during the day to conserve energy. You could even make it operate for only several hours after dark quite easily. The possibilities are unlimited. Bob
  18. BobK

    Battery question

    That is not correct. You can charge a battery at 1A then pull 10A out, if the battery is capable of 10A. Closer to the truth is: "energy out = energy in - losses" And energy = power * time. Bob
  19. BobK

    Inverter and Array of Batteries

    I would charge all of the batteries to the float level before connecting them together. They should then be within 100mV or so. Bob
  20. BobK

    Hot Air Station - Finally arrived!

    Yeah, I got my first one a couple of months ago and soon wondered how I got along without it. Desoldering SMT components is a breeze (pun intended). Bob
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