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  1. L

    AC resistance of a diode

    From my textbook on analysis of the p-n junction: The static resistance of a diode is defined as the ratio V/I of the operating point voltage and current. The static resistance varies widely with the V and I and is not a useful parameter. For small-signal operation the dynamic, or incremental...
  2. L

    Need a design for a project

    Here is a starting point for the logic. Your job is to condition the signals to drive the logic.
  3. L

    help with simple circuit, willing to pay for full design!

    My Texas Instruments datasheet for the CD74HC221 Monostable Multivibrator shows each of the timing resistors connected to Vcc. Now if your datasheet shows the same thing, could you explain why you would choose to connect your R1a to a pull-up resistor and your R1b to a pull-down resistor...
  4. L

    help with simple circuit, willing to pay for full design!

    Also, I'm guessing not. Each monostable requires its own timing capacitor and resistor. Would suggest getting the CD74HC221 datasheet and studying that. There are other CMOS monostables available that might be made to work here, but that is an implementation issue.
  5. L

    help with simple circuit, willing to pay for full design!

    As I said, it is not the full circuit. You still have to add in the timing resistor and capacitor for each monostable. I was more interested in just the logic design. This is what the design process yielded. To find out whether it works exactly the way you want, either breadboard it or do a...
  6. L

    help with simple circuit, willing to pay for full design!

    Here is a concept design using just 4 chips. One dual monostable multivibrator with reset (CD74HC221), one quad NOR gate (CD4001), two power MOSFET pairs (IRF7389). Caveats: I have not checked and cross-checked the design nor built it on a breadboard; the Karnaugh maps are on scraps of paper...
  7. L

    LED circuit help?

    How will the light tube be powered? The circuits for 220 VAC power will be different than for 6 VDC battery power. Will the LEDs have 50 milliwatt power dissipation or 3 watts? 300 watts of LED light will have superb brightness. Will the light tube provide electrical isolation and protection...
  8. L

    Op-amp Oscillating in Spice

    One of the reasons for PNP transistors is to reflect NPN circuits through a polarity mirror. See attached image. If Vref in the NPN circuit has a range of 0 to 5 volts, then Vref in the PNP circuit would have a corresponding range of +V to (+V - 5) volts.
  9. L

    light sensor advice

    Except for applications requiring sub-microsecond response times, I tend to think of a phototransistor as just a photodiode with gain. You can use both with the same type of transimpedance amplifier. It should be easier to find a photodiode with a larger aperture whereas a phototransistor will...
  10. L

    Help me with this tachometer

    Obviously the circuit will work since it comes from a working commercial product. But there are a few things to be considered that are not spelled out. There are 3 series-connected capacitors involved but only the 22 pF is specified. There is the capacitive pickup which could be just some...
  11. L

    Op-amp Oscillating in Spice

    The first problem I see is that U2 is an opamp in open-loop configuration. You have U2 in a linear feedback circuit but U2 is not operating in a linear mode. So there is over 120 dB of gain in the overall feedback loop. This could cause feedback loop instability problems leading to...
  12. L

    Light voltage sensor

    Your basic questions should be answered by Fairchild Application Note AN-3005, Design Fundamentals for Phototransistor Circuits. http://www.fairchildsemi.com/an/AN/AN-3005.pdf
  13. L

    Colpitts Crystal Oscillator

    Here is some of what the book "Crystal Oscillator Circuits" ISBN 0-471-87401-9 by Robert J. Matthys has to say about the Colpitts oscillator. Physically, this parallel-resonant circuit is very simple, but analytically, it is very complex. The amplifier is an emitter follower with a gain of...
  14. L

    Fading/dying Blue LEDs

    I went to the website http://www.superbrightleds.com/ and looked up the specs for a typical blue LED, their 5mm LED RL5-B2445. Maximum forward current is 30 mA at 25 degrees Centigrade ambient temperature. From the attached voltage/current curve, the forward voltage will be 4.1 volts at 30 mA...
  15. L

    DC motor control with fixed ground

    I had never heard of plug braking so I looked up the Alltrax document explaining what is plug braking. http://www.alltraxinc.com/files/Doc120-044-A_TN008-What-Is-Plug-Braking.pdf That document has the following passage. "Plug braking commonly found on electric forklifts, walk behind pallet...
  16. L

    Need help with circuit

    I suspect this circuit will not work with a battery as the power source. What is needed is a power supply that will provide parasitic capacitance back to the power mains through the step-down transformer. When you touch the base plate your body becomes a virtual capacitor to ground which...
  17. L

    Synchronising two wave forms.

    I explained how to sync the inputs, and you said you understood. Is there anything else?
  18. L

    100k Micro pot

    I sometimes use a spoon when soldering. Back in the 1960's I got a quart of Kester resin and still have a pint left. The Kester plastic bottle only lasted for 20 years and I transferred the remaining resin to a glass jar. I sometimes put a drop of resin into a spoon along with several drops...
  19. L

    Synchronising two wave forms.

    Manchester encoding is done to combine a data signal and clock signal into a single stream. But you have two data signals and a clock signal? And you want to sync the two data signals without varying the frequency of the clock, so no phase-locked loop. In that case the only thing to do would...
  20. L

    Wire Kit for Battery

    Diode D2 is only there to provide reverse polarity protection. What are the chances that you will insert the batteries in the wrong direction? If you were only using 2 cells, then it might be necessary to eliminate the voltage drop across D2. But with 3 cells there should be enough margin to...
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