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Posts posted by walid

  1. hi guru
    we all know that resistors are used to limit current to some value
    can a 0.5 ohm resistor limit current and to what level
    secondly: why it so big 10 watt, what if i use a 0.5 watt regular resitors

    no no, i'll ask my question another way:
    what exactly i want to know that: what should i think when i see say 0.5 ohm 10 watt resistor in any board

    power = I*I*R
    10 watt = I*I*0.5
    I max= 4.5amper
    so i shoud expect that this R can safely pass a 4 A of current

    is it true guru

    thank u guru

  2. hi
    in many many circuits u faced a wattage resistance
    for example in a PC monitor i see a 1 ohm 5 watt and in a low cost inverter in our project section you can see a 6 0.5 ohm 10 watt
    also in many car batt charger you find one directly before the batt

    why this resistance
    can i replace it with a thick long wire
  3. Hi
    In the Fig shown below is the input part of a switching mode ps
    as most of us know the 220VAC is connected directly to the rectifer (4 diodes) through a filtering network
    the rectifier o/p is about 310VDC
    in some SMPS there is a one main filter capacitor 400v 220uF or so
    in another there are two main filter capacitors 400v 220uF or so (C53 and C54 )in the sch below)
    then this 310VDC is connected to the collector of Q16 and a snubars diode (damping diode)

    My questions are:
    1) Why using 2 caps (C53 and C54 ) instecd of only one
    2) why using two transistors Q16 and 15 instead of one
    3) if the voltage at the collector of Q16 is 310VDC what it should be at the Emitter of the same transistor
    4) can u summarize the operation of this part of the circuit

    thank u very much


  4. Try to forget this MOD
    I have one and its datasheet, i know its pins and wait your answer to costruct the circuits above

    i want to get logic 1 or 0 or any thing else at pin 3 of the nand gate when someone pass, is this possibe or not, or you have a better idea
    please tell me


  5. Hi guru

    ok, lets agree that Radio Shack doesn't have the specs for the MOD anymore so nobody knows what it does.

    i need you help in designing a gate guard using IR MOD as RX and IR diode as Tx
    i baught  the RX mod's

    i search the net but no circuit satisfied me

    i want your help guru so please help me

  6. Hi

    from this link:  http://home.maine.rr.com/randylinscott/aug98.htm

    Iread the following:

    The MOD accepts a 40khz IR signal that is modulated at 4 khz. When a signal is received the MOD will go low. The sensitivity of the MOD is set by different values for R1 and C1.

    from that I understand that the modulating sig is 4khz and the modulated (carrier) sig is 40khz

    Look at the fig below:
    the freq of the transmitter sig is calculated from:
    f = 1/[0.693 x 0.05 uf x ( 2.2k + 2 x 1K)] =6.871kHz niether 40 nor 4 khz

    R1*C1 = 0.4 sec

    from where he had that values of freq


  7. Hi

    my friend asked my help in his design
    he wants some sensors placed some where in a gate that counts the visitors come in or out. when the number inside the room is say ten, the gate must closed and prevent more people from inteing

    i tell him that he may use a controller like PIC16F84A and use two pair of IR TX and RX placed like to horizontal parallel lines one pair connected to say A1 port as i/p and the other pair to another i/p say A2
    if a person cut ir connected to A1 then A2, then, this person is assumed to be entering the gate
    if A2 then A1 he is out
    the cotroller sums or substract till ten and close the gate for some period of tme

    My questions:

    (1) is this idea is the best for this progect or not?
    (2) if it is OK can you help me in designing these sensors, the Tx is an IR diod connected to pin 3 of 555 IC. and the infrared receiver module .....

    i need you help may be some links or any thing else

  8. hi

    Some simple FM transmitters have the antenna connected to a tap on the coil. Then things getting near the antenna don't affect the tuned frequency as much.

    Are u mean by "things getting near the antenna " my hands, and why when connected to the tap of the coil the osc be more stable
  9. Hi

    Thanks Audioguru,

    Since the topic is tunning(radio), I recently found an old audio/radio system for TV. The Volume control knob said :o motorized volume. I took it apart, and I think I saw motor. I'm not sure how this thing works? ???

    the motor acts simply as your hand
    i have a remote controlled cd, when i press the + or - volume i can see the volume nob rotate
    it is connected by a motor rotate in two direction as the polarity applied on its terminals

    For the Varactor diodes, i can add to what my big friend guru said that its capacitance is inversely propotional to the applied voltage; whev v high , c is low so the tunned freq is high

    good luck
  10. yes it is very small like that

    I'm pleasure, it is the first time i can help a friend in this great community

    good luck truf

    always I get all my project's parts from my old junk so i check them especially the transistors, you can say that i check more than 5000 transistors in that way and always the difference be that small.

  11. I understand his question in another way

    may be he ask like this

    if he has a transistor on the table and he wand to identify its terminals and type

    use your digital multimeter diode function, first assume your trans is npn type, then put the red wire in one leg and the black on another leg .... repeat this untill you have a reading with the red on one lege and the black one time on the 2nd and then on the 3d
    then the leg in contact with red is base and the others are C and E the bigger reading is E and the smaller is C

    if u can't get any reading then assume it is pnp and repeat the above with the black first to one leg assumed base....

    may be u hardly understand me but i want to help

  12. Hi guru


    I understand u but i intend to use this inside a building of 4 floors, i want to talk while move from place to place, may be inside the lifter ........

    when i first test it with that problem mentioned above my son talk near mic inside a room while i'm going away from the building, i hear him at about 150 meter and still hear the transmitter noise at more than 300 meter, there are many things in between.

    I want to spend any time to succeed, but i'm afraid from that after spending this time and still have those problems of stability; its freq moves away
    and the human body affect and the clearness of the received voice

    In my work I have pair of wireless (about 200 meter) with LCD and 8 channels and some functions like outoscan..... i remove its cover to take a look, one second and i quickly close it, it contains many SMD ICs and tiny parts, i feel not OK, disappointed, in fact it is one of my big dreams to build one pair with one function TX and Rx voice with good fidelity and range.

    Now  I ask u some questions:
    1) you said the antenna must be about 80 cm long, is it important the diameter of this wire and if it have a plastic cover or not; that is can i use a 1.5 mm wire like that used in the electrical power connections

    2) to reduce human body effect why not to enclose the oscillator stage inside a metal can like tunner used in TVs

    thank you audioguru

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