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  1. audioguru2

    opamps

    Opamps don't work properly without negative feedback. Look at the datasheet for the opamp. Without a negative feedback resistor, the voltage gain is 1,000,000 at DC and at low frequencies, then the output would be saturated against a supply rail due to the opamp amplifying its own input offset...
  2. audioguru2

    Help me to understand the MC3359P IC

    Hi Walid, Is it a wireless home phone? I never looked inside one before and I have never seen a schematic of a narrowband dual conversion FM receiver circuit before. I am sorry I can't help you. Look in its sevice manual.
  3. audioguru2

    Help me to understand the MC3359P IC

    I don't know how you are going to design a narrowband dual conversion FM radio without a schematic. It needs a double-sided pcb with a ground plane over most of its surface.
  4. audioguru2

    help in using google

    In Google I selected Images then searched for LM555 circuits and got a whole page of many schematics.
  5. audioguru2

    Any math'a maexperts out there? Need help rearranging this formula...

    There are many programs on the web that calculates parts values for a 555 oscillator. Maybe your program should allow you to select a standard capacitor value then it has 5% resistor values that are correct for it.
  6. audioguru2

    12V Fan controller

    The max output of the LM317 is about 14.5V to 15V when it has a 17V supply. If the LM317 has a 220 ohm resistor and a 2k pot then its max output voltage will be about 12.6V.
  7. audioguru2

    Any math'a maexperts out there? Need help rearranging this formula...

    The datasheet for the LM555 has a graph to select parts for a certain frequency. Changing the resistance of only one of the two frequency-determining resistors affects the duty-cycle of the output.
  8. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Arman, Did you substitute any parts? The circuit is easy to fix if you list voltage measurements and have it turned on without a load and with the voltage and current pots turned up to max. Connect the negative wire of a DC voltmeter to the negative wire of C1. Measure the voltages: 1) The...
  9. audioguru2

    12V Fan controller

    You don't need to cut down the voltage with a resistor. The LM317 will reduce the output voltage to whatever the pot is set to.
  10. audioguru2

    12V Fan controller

    Don't use the ICL7660 voltage doubler IC. Its absolute max supply voltage is only 10.5V and its output current is typically only 20mA but might be able to go as high as 50mA. A fan for its load draws much more current.
  11. audioguru2

    12V Fan controller

    It is difficult to stepup voltage. Usually a power supply voltage is selected to be high enough for the application.
  12. audioguru2

    12 volt light fader

    Hi Daisy, Most opamps need a supply voltage about 1.5V hgher than their max output voltage. So if your supply voltage is 12V then an opap's max output voltage to your meter is only 10.5V. Merry Christmas.
  13. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    The output current of a 4017 is only about 2mA when it has a 5V supply so no base resistor won't hurt anything. The circuit will still work fine if the transistor has a 10k base resistor.
  14. audioguru2

    12V Fan controller

    Hi Monkey, Welcome to our forum. ;D I guess you forgot to look at the datasheet for the LM317. 1) Your power supply already has a filter capacitor so your LM317 desn't need another one. It needs just a small 0.1uF ceramic disc capacitor at its input to keep it from oscillating. 2) The LM317 is a...
  15. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    It looks like you are going to connect the collector of your transistor to a button on a PC's mouse. Then it will already have a pullup resistor so a collector resistor won't be needed. A resistor in series with the base isn't needed but use a 10k resistor anyway in case you want to measure the...
  16. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    So your transistor is not turning on a powerful motor nor a lightblb. It is the low current output of a pc's mouse? I never looked, but I would think that a mouse has a logic output that is either high or is low. Your transistor will make an output that is only low, never high. Connect a 1k...
  17. audioguru2

    12 volt light fader

    Hi Daisy, You forgot to say how much current your 12V light uses. Here is a slow LED fader cricuit that can have an offset voltage and an additional power transistor added to drive a light bulb. Scroll to "Fading Red Eyes": http://ourworld.compuserve.com/homepages/Bill_Bowden/page5.htm
  18. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    You need to know how much current is used by "the device" that will have one wire grounded by the transistor, so you can use a suitable transistor. The 4017 has a minimum output current of only 2mA when it has a 5V supply. A 2N3904 transistor will turn on pretty well with only 2mA of base...
  19. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    Did you select a transistor that has a high enough current rating so it can pass the amount of current for the device? Which transistor? How much max current? Can the "ship" drive the 1k resistor in series with the base of the transistor with at least 3.5V/1k= 3.5mA? Is the current gain of the...
  20. audioguru2

    NPN transistor circuit - Is this right? I doubt it...

    Without the current-limiting resistor in series with the LED then the LED, the transistor and maybe even the 5V power supply will all blow up. A logic high at the input produces a logic low at the output. Nothing is negative in that circuit. The input uses a low current and the transistor's...
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