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Posts posted by walid

  1. Hi
    I built a circuit that when a person pass some red led illuminate
    This signal is connected to a pic to count.
    The problem is that when that person pass, the led is
    Intermittent lighting glows irregular and weaken and boosted in a very short time
    The pic see that several persons How can stabilize it at 5 volts
    Thank you very much

  2. At the outset extend deep thanks to Mr. AUDIOGURU for his effort and interest and wonderful answers.
    In truth, I thought the circuit was working in a different way, but now the picture clarified somewhat.
    Still I have some questions:
    (1)I am confident that your true 100%, but I can not imagine. Note the following figure: 
    gate thy.jpg
    please comment.

    Pushbutton P1 is connected to the positive supply. When it is pushed, it applies a positive voltage to the base of Q2 through D3. Then Q2 turns on

    I know that: to turn on the npn transistor, its collector must be connected to +ve batt (through a resistor) and its emitter to ground, then when applying a voltage to its gate it passing current from C to E.
    In our Q2 its collector didn't connected to any Vcc, so how to be ON.
    I don't know why C2 is there.

    Are concluded from this talk that it could be removed from the circuit without any difference?
    thank you guru
  3. Hello,

    Have a look at the amplifier circuit shown

    I want to know your views about it..... if u find any mistake in it, do point it.....

    one stage to be mosfet was the requirement, so i have used the mosfet as voltage amplifier and then connected it to the power amplifier......

    Thanx for any help

  4. Hi
    I built this circuit on a test board
    the sensing leads are simply two thin copper wires
    i connect these two leads together, this equivalent to a very low resistive solution.
    when operate the circuit, the red led (D3) is on and the other green and yellow are OFF
    when trying to adjust the variable resistor R4, it has no effect at all
    i check the connections many times, they are OK.
    I understand the three comparators operation
    and know that the two red leds D1 & D2 are a voltage references as the writer said
    also i know that the first op amp (IC1A) is function like this:
    and this:
    I use a 10.5V power supply
    I measure pin1 voltage = 9.1V where pin5 = 4.3v
    i try to make pin1 voltage lower than pin5 but can't
    please help me in fixing the broblem

  5. Hi guru
    First, thank you very much

    The two transistors form a thyristor

    A thyristor has 3 terminals; A1, A2 and G. Can you please mark these terminals on the schematic (Q1 & Q2).
    I think they are as shown in the following fig.thyq1q2-001
    In the above Fig, and if my idea is true, this thyristor is triggered when a 0 volt signal is applied to its gate.
    when a high or a VCC signal is applied, it is turned OFF.
    please guru correct me.

    The push button turns on Q2

    there are two push button, P1 and P2, I think you mean P1 which when we pushing it we triger or RESET the counters to begin counting.
    pin3 of 4060 o/p is zero for 30 minute, and so the thyristor is conducting. at the end of the 30 minutes, pin3 go HIGH so turn off the thyristor.

    In fact I try to understand the mechanism of turning this thyristor ON and OFF but can't. I used SWCAD III but still unclear. please guru explain it for me.

    The push button turns on Q2 which turns on Q1 They are connected together so both stay turned on. Applying a positive pulse to the emitter of Q2 turns it off then Q1 no longer has anything to keep it on so it also turns off

    Before pushing P1, the lower terminal of C3 was float.
    When pushing P1, this applying a negative pulse to the emitter of Q2 turns it ON which turns on Q1.
    After 30 minutes, the o/p of pin3 is changed to HIGH this applying a positive pulse to the emitter of Q2 turns it OFF which turns OFF Q1.
    OK, I can keep this in my mind, but i'm sure that i didn't understand how this be done. I'm waiting hope GURU explain it.

    the last point: Is C2 (=100n) is a part of the thyristor equivalent circuit.

    thank you GURU very very much.
  6. Hi

    Three days and there was no answer or response, this means that what I said is true and the original writer of the article was wrong.
    I have another question:
    We know that Pin 3 of 4060 turns from Low to the High after half an hour.
    This High signal is applied to the emitter of Q2 through C3
    The subcircuit composed of Q1 and Q2 and related components is responsible for connecting or disconnecting the operating voltage from the rest of the electronic circuit.
    Can you please explain the mechanism of how these two transistors do their job.
    thank you very much.

  7. Hi
    The following circuit is function as a sleep timer:
    from: http://www.redcircuits.com/Page4.htm
    I edited it and put the parts values and some other information
    sleep timer.jpg
    The writer wrote the following about it:

    {After turn-on by P1 pushbutton, the LED illuminates for around 25 minutes, but then it starts to blink for two minutes, stops blinking for two minutes and blinks for another two just before switching the lamp off, thus signaling that the on-time is ending. If the user want to prolong the reading, he/she can earn another half-hour of light by pushing on P1. Turning-off the lamp at user's ease is obtained by pushing on P2.}

    I calculate the clock pule duration T = RC/0.455 = (1M*100n)/ 0.455 = 0.22 sec
    Now the clk o/p (pin 9) is ON - OFF at rate of 0.22 sec
    And according to the values at the above schematic (the colored one), I think that the LED is illuminate from 0 sec up to 15 minutes, then it blinking for anther 15 minutes before the lamp off.
    thank you in advance

  8. Hi
    I answer myself

    look at the followig figure:
    At the +ve half of the main AC voltage (311 V peak) the upper zener acts as a diode and the lower zD2 acts as a zener regulating the voltage to 16.7v
    At the -ve half of the main AC voltage (-311 V peak) the lower zener acts as a diode and the upper zD1 acts as a zener regulating the voltage to -16.7v

    In my software there is no 16V zener, there is a 15 v zener so I use it to simulate this part of the circuit


  9. Hi
    look at the figure shown below:
    you noticed that the two nand gates have the same inputs, so they willl have the same outputs
    the led is always on till all the i/ps are High
    my question:
    if the two nand gates have not the same inputs, and the o/p of one of them is high and the other is low, will the LED ON or OFF?

  10. Hi guru

    (1) the word "at the moment" is the problem
    if we agree that the voltage reaches 311 volt 100 times per a second
    that is every 1/100 sec the voltage value is 311v
    in the human sense it is every time, every any bit of time..........

    (2) in this circuit AlimStr.GIF
    there is no resistor to limit the capacitor's charging current, why there is no problem?

    (3)Also related to the above circuit; if i increase C1 to 900nF to get about 60mA, when there is no load, zener eats all this current
    if it is, for discussion, 1/2 watt it will be burned, so if i don't want to use a 2watt zener, and i want to use a 300 ohm resistor in series with the 1/2 watt zener, would this protect the zener?
    How much this resistor decrease the current?
    thank you very much

  11. hi

    please look at the following circuit, it is a transformerless power supply:


    this is a 12 volt and can supply a max current = 15 mA
    I calc this current as:
    Xc = 1/2 pi f C = 14.47 K ohm
    I = (220 -12 - 1.4)/Xc = 15 mA
    the 1.4 volt is the drop voltage on two diodes
    Now, please tell me What are the disadvantages of this circuit, and
    why the zener diode is Often burned
    what if i connect 100 to 300 ohm resistor in series with that diode.

    the last question: at the ollowing circuit, why using R3 =220 ohm
    its value did not affect the the current at all
    guru said before that: "The circuit needs this resistor to limit the current in C1 if the circuit was connected to the mains at the moment it is at its peak of 311V"
    I did not fully understand the intent, especially, I know that the freq of electricity = 50 that the voltage reaches 311 fifty times per second, how can this be?

    thank you very much.

  12. Hi guru

    Never mind the battery voltage. This circuit doesn't work.
    It is an example of "how to make a transistor amplifier wrong".

    No, the circuit worked
    i built it 3 times for me and for some frinds
    it was very very sensitive to voice
    it picks a very very low voice
    at night i put it and hear persons speak with eachother, Believe me.
    the only change i do, it was by a chance, is that it operate very well with one not new 1.5v battery
    with this batt u will Surprised great ability to capture votes
  13. Hi sarma

    the circuit should work from a supply down upto 1.8V (2*0.9V) where after the batteries are to be replaced.

    What is the 0.9V?

    perhpas you have to maintain the overall gain of the device in the battery supply range.


    Hi guru

    The mic and its load/powering resistor is about 2k ohms. The 1st transistor has negative feedback through its base bias resistor so its bias voltage will drop a little when the resistor value is reduced and its its voltage gain will drop a lot. Its input becomes attenuated.

    I read somewhere that the Q1 stage voltage gain = approx. R2/R1,
    with R2 = 680K Av= 680/3.3 = 206, where at R2=10, Av = 3 only??????

    Q3 has base bias current directly from the positive supply.

    No it is biased through R7 = 100K

    So it is saturated all the time causing severe distortion and low output level. It causes Q4 to also be saturated.

    I forgot, Q2 is also saturated.

    please explain it more.

    Nearly every circuit from that site has severe errors.

    I enjoy seartching the errors

    The circuit is not a hearing aid, it is a fuzz circuit for an electric geetar.
    Have you ever seen a hearing aid without a volume control? New ones have an automatic volume control.

    I'm sure it is a hearing aid without a volume control, later I'll attatched it.

    Finally, you use a very good simulator, can you please tell the its name to try get it.
  14. Hi guru

    Hi Walid,
    The transistor is not straining so hard without the LED. With the LED, its load was 60mA for the relay and about 25mA for the LED if it had a current-limiting resistor. The total collector current would have been 85mA so the 8mA base current is fine. Without the LED then you can increase the value of R4 a little for a base current of 6mA. But 2mA doesn't matter.

    The transistor was poorly chosen as a BC547 with an absolute max allowed collector current of only 100mA. Its current gain drops above only 30mA. I would have used a relay that uses less current or a higher current BC327 or 2N4401 transistor. Their saturation voltage is much lower so the base current can also be much lower if you want.

    BC547 max allowed collector current is 500mA and not 100mA
    BC327 is pnp tansistor and not npn
    2N4401 is 1000mA but the same power of 625mW
    here is a table of some suggested transistors, i see all are suitable.
    thank you guru
  15. hi guru

    (1) Is all of an op-amps suitable for use as a comparator or is that only certain types?
    An opamp has frequency compensation so it can work with negative feedback without oscillating. The frequency compensation makes an opamp slow to switch its output.
    A comparator doesn't have frequency compensation so its output switches very quickly. Therfore a comparator cannot have negative feedback.
    There are many slow applications where an opamp can be used as a comparator. 

    It is very good answer.
    I read somewhere that op-amps designed to be a comparator has an o/p stage like open collector transistor or a darlington pair. is this true?

    AND what is the frequency compensation ? No thanks I found it in: http://en.wikipedia.org/wiki/Frequency_compensation
    thank you guru
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