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Clap Switch using 555 timer in monostable mode(Values of components))

  1. Nov 20, 2011 #1
    Hello Experts!

    Here is my simple clap switch schematic. First let me know is it correct or not?

    I have used 555 timer in monostable mode here this means LED will remain on for the particular period of time defined by R3 and C1 i.e. T=1.1(R1)(C1).
    Let I use red LED here. So I think 1.7V is its voltage drop and 5mA of current to turn it on. I have used 6VCC biasing.

    I want to know 2 things.
    1)Is this circuit is correct, means any component is less or you think it should be here which is not?
    2)What should be the values of components here? and how will I know which transistor is to be used here?

    Thanks in advance.

    Attached Files:

  2. jcsd
  3. Nov 20, 2011 #2


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    You would have to have a resistor to V+ from the trigger input.

    I doubt if you would have enough gain to trigger the 555 from a microphone, though. You need another stage and a conventional method of driving the first amplifier after the microphone.

    Go to Google.com
    Select "image" mode.
    Type in "microphone amplifier"
    There are dozens of them there.

    R4 should be about 680 ohms.
    Assume 5 volts out and 1.7 volts drop across the LED so there is 3.3 volts across the resistor.
    R = 3.3 volts / 0.005 amps = 660 ohms.
  4. Nov 20, 2011 #3
    I have added another resistor between trigger and VCC. Shown in schematic.

    As you said add another stage. So I have added another BJT to amplify gain. Shown in schematic.

    Kindly don't say me to search any site. Because I alone want to design my own clap switch. And I need your help.

    I have got it. But what about other components? What should be their values?

    Attached Files:

  5. Nov 20, 2011 #4


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    Because I alone want to design my own clap switch. And I need your help

    Seriously, have a look at how other people have done it. If I design it for you, how is that different to finding a circuit on Google?
  6. Nov 20, 2011 #5
    You are right. But I know how to design simple circuits. I have just one serious problem that is I don't know what value of component should I use at the particular place.
    If I search it on Google then I will get my circuit complete, but I will lose the chance to get a concept of choosing right values of components.

    I hope you got my point now. Thanks.
  7. Nov 20, 2011 #6


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    Selecting the component values is part of the design process.

    Direct coupling of stages as you have done is only possible with very low supply voltages, so your circuit will not work with any resistor values you could choose.


    You could use two stages like the NPN one above.
    Power the microphone with a 4.7 K resistor to V+ and ground the other side of it. Take the output between the resistor and the microphone.

    You could use another transistor from the trigger input of the 555 to ground with a 22K resistor to V+
    Collector to pin 2 of the 555, emitter to ground and 22K to the base of the transistor from the collector of the previous stage.

    You may have to adjust the resistor shown as 47K. You do this by measuring the collector voltage. It should be about half the supply voltage. If it is 9 V or nearly zero volts then you adjust the 47 K to be smaller or larger respectively.
    This is necessary because transistors vary in gain.
  8. Nov 21, 2011 #7
    Yes I know and therefore I want to request you that you kindly tell me how you choose the values of components.

    What do you mean by low? I have chosen 6VCC here. Is it low or high? If is it low then I will surely add coupling capacitors.

    Yes I have used two stages to amplify gain. But how would you know that I should use two stages to amplify gain? How did you come into know that clapping will not produce as gain as required to derive 555 timer(used in monostable mode)?

    How did you know that there must be a resistor of 4.7k? And I must take the output in between +ve of microphone and the resistor of 4.7k while in first image I had directly connected +ve of microphone with VCC and place a resistor from -ve of microphone to the ground and taking output in between them?

    Same question here that how did you know that another transistor is to be needed here?

    You mean voltage at the collector of transistor is approximately half of the supply voltage?
    Here I used 4.7k and providing VCC=6v. So 4.7k is enough?

    Please answer all of my questions.

    Thanks a lot vk6kro

    Attached Files:

  9. Nov 21, 2011 #8


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    The circuits in post #6 are classics and you should use the NPN one.

    The circuit you are persisting with is useless except in amplifiers using about 1.5 volts as in some older hearing aid amplifiers. Even then, they were extremely difficult to use and gave poor gain per stage,

    A microphone gives about 10 mV out and a 555 will take about 6 volts to trigger. So, this takes a gain of (6 volts / 0.01 volts) or about 600. You are unlikely to get this gain from a single stage, so you probably need two stages, at least. You can get rid of gain by removing emitter bypass capacitors.

    Electret microphones usually use a 4.7 K resistor. There is even a 4.7 K resistor built in to the microphone input of computer sound cards.

    Learning transistor theory and design or any other electronics is best learned in class. If you can, try to get into a class where this is covered.
  10. Nov 21, 2011 #9
    Now I have done as you are directing to me. Look at the image.

    I have got what are you saying & added emitter bypass capacitors so that I could get gain.

    I have got this idea as well.

    I have much idea about theory but I am trying to design this circuit first time so I just need exact way to do it. Therefore I ask little things to you. Until I have studied analog electronics.

    Now is the circuit according to your saying? However just a little bit.

    Attached Files:

  11. Nov 21, 2011 #10


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    There should be a capacitor from the bottom of R1 to the base of Q1. Maybe 0.1μF.

    There should be a resistor (maybe 100K) from the base of Q1 to the bottom of R2.
    This is to give base current to Q1.

    There should be a resistor (maybe 100K) from the base of Q2 to the bottom of R3.
    This is to give base current to Q2.

    There should be a resistor (maybe 22K) from the right of C2 to the top of R4.

    The 22K resistors are too big. Make them 5.6K.

    The emitter resistors are there to make the circuit more stable with varying temperature. The capacitors across these resistors are there to avoid loss of gain due to the resistors.
  12. Nov 22, 2011 #11
    I know why we are using capacitor here. But why it is 0.1μF, don't know?

    I know that capacitor blocks DC. Does it block current as well? If it is not then why we use resistor of 100k here and that is such a high value? Since we are providing just 6VCC.

    Is this resistor is to provide current to pin2 of 555 timer?

    I know this reason. But the rest answers are required to me.

    I have attached an image as it is modified as you said.

    Attached Files:

  13. Nov 22, 2011 #12


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    Because the transistor gain can be so variable, it is necessary to know the current gain of the transistor to get accurate values for bias and collector resistors.

    If you don't know the type of transistor or the likely gain, you can produce the kind of "ballpark" design we have now.

    Firstly, you remove obvious circuit flaws where the circuit will not work.

    Then you arrive at a reasonable configuration and make reasonable guesses about the component values.

    If you have a good stock of components, you could then build it and adjust it to work properly.

    As I don't have time to build every circuit discussed here, I like to put the first guess circuit into the simulator LTSpice and see what that predicts. Previously, I would have calculated a result.

    I did that with your circuit and I can now modify some values as folows:

    Remove R6, R7, C5 and C6. They work but are not really needed with a low supply voltage.
    Change R2 and R3 to 3.3 K.
    Change R8 and R9 to 330 K

    This gives a gain per stage of about 33 with a 2N2222 transistor or about 1089 overall.
    This should be enough.

    The 100 K resistors (now 330 K) are there to provide base current for the transistors. In this case, the current is about 5 μA.
    The collector voltages are about 2.4 volts.

    Having this type of bias makes the amplifiers self adjusting to some extent at the expense of some gain and a low input impedance. For your purpose, this doesn't matter.
  14. Nov 23, 2011 #13
    I have fully understand you. Thanks it was a great and much appreciated help.
    I have one question that when I clap then how much volt peak to peak is produced and what could be frequency of signal which would be generated by clapping?

    Here is my schematic.

    Attached Files:

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