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  1. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    No problem. It was proof enough to me that there was 1 less equation than needed to constrain the problem to a signle solution, but with less experience, one might not see that. Bob
  2. BobK

    Newbie Project

    There are many tutorials available on the web. Find one that has you make a lot of circuits starting with very simple and progressing in complexity. You will want to get: A solderless breadboard. A multitmeter. A bunch of parts, Sparkfun sells a kit of common parts that would be useful...
  3. BobK

    Capacitance for microcontroller timing crystal?

    My experience is with PICs, which don't seem to have this internal capacitance. So I would go with what your datasheet recommends. Bob
  4. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    I am glad you are finally convinced. I started out to do a proof on the 2x2 network, but I got distracted. Glad to know someone did it for me! In that network (two parallel legs with 2 resistors in series and one bridge resistor), you get 4 equation for currents, in 5 unknowns. Set the value...
  5. BobK

    DIY altoids USB charger won't charge sufficently

    To reinforce what C.C. is saying. If you look at the datasheet I posted earlier for the Energizer alkaline AA cell (which is going to be as good as you will get), it is listed at 2000maH at 50ma discharge rate, 1000maH at 1A discharge rate. With 3 cells the discarge rate while charging will be...
  6. BobK

    Transistor, AC amplification

    Harald, Sorry, you are right. The text wrapped in the middle of the expression, which made it hard to read. Bob
  7. BobK

    Can you help me develop my invention?

    Let's think about this. You need a device that is 1 cubic cm. and can operate as a receiver 14 hours a day for 2 months on a single battery, which needs to be a small button cell based on the size. Have you ever seen such a device? I think it would be great to have an FM radio that small that...
  8. BobK

    Transistor, AC amplification

    The equation is not parenthesized incorrectly, it should be: (12K / (12K + 150K)) * 24V This is just the equation for 2 series resistors across a voltage source, i.e. a voltage divider. The voltage at the juncion is the ratio of the lower resistor to the sum of the two resistors times the...
  9. BobK

    DIY altoids USB charger won't charge sufficently

    If you want to know what it will really take to get a full charge, we need to know the rating of your cell battery. You have not given us that. I suspect that 2 AA does not even have the energy to give a full charge under ANY circumstances. My 10 year old dinosaur flip-phone has a battery...
  10. BobK

    DIY altoids USB charger won't charge sufficently

    Have a look a this: http://data.energizer.com/PDFs/l91.pdf At 1A discharge rate, the capacity of an AA cell is only 1000maH. You are going to need a 2A discharge rate to raise it up to 5V at 1A, and the capacity will go down, to maybe 750maH. 750maH at 3V is 2250mWH. Your cell battery...
  11. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    I have already shown you that there is not unique solution in the simplest case, 2 parallel resistors. Show me the circuit and values that you think has a unique solution. Bob
  12. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    You see how you can do it in my simplied case, right? That proves that in some cases it can be done. Just because you don't see how to do it when all the resistors are in a narrow range, does not mean you cannot. Try an example with a a 2 x 2 version of your net and see if you can set the...
  13. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    Okay, let's take your example. Set all of the vertically oriented resistors to 100M so essentially no current flows through them and assume all the others are much smaller. Now you have 3 3-way voltage dividers and you know all the intermediate voltages. You can now caculate the ratios of the...
  14. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    The fact that you cannot write down enough equations to determine the resistances proves it. I think you need to know n-1 resistors in each n way branch point to have it fully specified, but that is only a gut feeling. Bob
  15. BobK

    Simple 'Blink' Circuit / PCB

    You could try to fnd a cheap toy that blinks lights alternately and take out the board from it. Bob
  16. BobK

    Capacitance for microcontroller timing crystal?

    Multiply the load capacitance by 2 (beause the two are in series) then subtract a little for stray capacitance. I..e for the first one, load capacitance 12.5 x 2 = 25pF use 22pF for the second, load capacitance 18 x 2 = 36pF use 33pF Bob
  17. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    No it is not. Without knowing the total equivalent reistance, you can control one or the other but not both. The reason you cannot solve the problem is that is it underspecified. Take the simplest network you can think of. Two parallel reistors, R1 and R2 between 10V and ground. Here...
  18. BobK

    Remote car entry as remote trigger

    Look up wireless remote control on Ebay. There are many options for < $20. These have a key fob like tranmitter and a receiver board with 1 or more relays. Bob
  19. BobK

    Finding the resistor values in a mesh from the voltages at the nodes

    Given only the voltage on the nodes and the total current, there is no guarantee that the problem has a solution. Where are the voltage coming from? Bob
  20. BobK

    Variable power supply LED indicator

    A double pole switch comes to mind. Run the LED off the input to the regulator. But why would you only switch the output anyway? Bob
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