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  1. L

    Question about resistor networks in the Electronics Demystified book

    The first thing you are doing wrong is treating the 10+20+30 ohm resistors as though they are in parallel instead of in series.
  2. L

    Beer Pong Table. Please help

    An alternative approach that may be worth experimenting with is a sensor based on the theremin, a musical instrument controlled by how close one's hand is to the sensing antennae which changes the oscillation frequency of several hundred kilohertz slightly as the hand changes the capacitance in...
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    Capacitor help

    Capacitance is defined as the charge stored per volt: C=Q/V=dQ/dV=I*T/dV. Let's apply this to a single LED with I=20mA. I would assume you don't want the voltage to vary by more than 0.1V over the 15 hr period. The total charge Q that the LED will consume over the 15 hr (54000 sec) period is...
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    Explanation

    Don't miss the classic Pease "What's All This Stuff, Anyhow?" article explaining all you really need to know about PID controllers. http://electronicdesign.com/article/analog-and-mixed-signal/what-s-all-this-p-i-d-stuff-anyhow-6131
  5. L

    talking toys

    I googled the words: talking toys voice synthesis. What I got back was more than you would ever want to know about how talking toys work. I especially liked the Wikipedia entry for the Texas Instruments Speak & Spell since it reminded me of the time I worked with Gene Frantz before he went to...
  6. L

    Transformer help

    So you got the answer with less than 0.14% measurement error. I would call that amazingly accurate!
  7. L

    Transformer help

    So you measured the voltage and current on the primary side of the 208V:120V single phase transformer. And you measured the resistance load on the secondary side of the transformer. When you reflect that load resistor through the turns ratio of the transformer, does it agree with your measured...
  8. L

    Reverse the voltage?

    What is necessary for your application is a simple inverting amplifier with gain=(-1) and symmetry around (+2.5) volts, i.e., 2.5v IN = 2.5v OUT. CDRIVE has posted the schematic for such a circuit where Rin=Rf for unity gain and a voltage divider puts +2.5v reference voltage on the '+' input of...
  9. L

    Op-amp help please!!!

    @KrisBlueNZ: Your post sounds just like the one I wanted to be able to write.
  10. L

    Calculate current in shunt resistor

    From the wording of the question I can't tell whether the second equation should be (Im=740 mA) or (Im+Ish=740 mA).
  11. L

    Photo sensor

    All you need is a single S-R flip-flop or even just a pair of cross-coupled NAND or NOR gates. One side would be controlled by the light sensor while the other side goes to the reset switch. The choice of NAND or NOR will be governed by the HI/LO logic polarity of the light sensor output. The...
  12. L

    Calculate current in shunt resistor

    You have one equation and two unkown variables. What is the second equation?
  13. L

    Op-amp help please!!!

    @BobK: "Okay, please describe to me why this circuit does not work." Why do you believe that a voltage follower would not work?
  14. L

    Op-amp help please!!!

    @BobK: The point is that all op amps can be faked out with a virtual ground but if the op amp is in a circuit where high current will be expected to flow through the virtual ground, then the virtual ground must be a a low-impedance point. One way to make it a low impedance point is for it to...
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    Op-amp help please!!!

    @Harald: Note that for the single-supply case I never claimed the negative rail had negative voltage with respect to power ground. In fact I said the negative rail was power ground. @BobK: "All opamps work off a single supply." This is true, but not all op amp circuits will work correctly...
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    Op-amp help please!!!

    First, realize that all voltage-input operational amplifiers are inherently dual-supply - they expect to have a positive supply rail, a negative supply rail, and a ground halfway between the positive and negative rails. This is true even for op amps advertised as working with a single power...
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    Photo sensor

    Is there a minimum time duration the circuit needs to be activated once light is detected? What should happen if the reset is done while light is still detected? Should a reset force the circuit off. Should a light-on reset be remembered for the next light-on transition, or should a reset...
  18. L

    Gear help

    These small motors don't have much torque. This one is rated at only 10 g.cm and would normally be geared down rather than up. I doubt the motor will be able to turn the belt pulley even with no gears attached. You would need a bigger motor.
  19. L

    measuring current ripple with oscilloscope

    The plan sounds good in theory; however, it may be fraught with practical pitfalls that could subvert the final answer. Getting the .csv file seems straightforward. But how accurately do you know the value of C? What calibration method will be used to establish an accurate voltage scale...
  20. L

    Capacitor help

    When charge (coulombs) flows into a reservoir (capacitor) at a certain rate (amperes) the pressure (voltage) rises according to the capacity (farads) of the reservoir. A flow of electricity of one ampere for one second transfers a charge of one coulomb. A capacitance of one farad will store a...
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