how to make transistor switching networks

Discussion in 'Homework Help' started by lezahyoj, Feb 7, 2011.

  1. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    I am just a beginner, can you help me?

    I need to make a simple circuit that the goal is if I am going to turn the switch "off" the LED should turn "on" and vice versa. But the problem is I do not have any idea on how am I going to start it and what are the values of resistors and transistors am I going to use.

    I need to connect one resistor each for the collector, base and emitter of my transistor, a battery which will be my source, a "push" switch and an LED where I will see the output.

    If you had already done a project like this, please send me the schematic diagram with the values of resistors, battery, LED and transistor that I need to use.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's a bit hard to be helpful when we don't have any parameters. What is being switched? Why do you need to use transistors? What voltages are involved?
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
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    In this circuit: If S1_NO is off (open) LED1 is on. If S1_NO is pressed on (closed) then LED1 is off. Since you didn't specify, S1_NO is a normally open push button switch.

    Since this sounds like a class assignment I've not provided any component values.
     
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  4. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    what I really need is a transistor inverter with a switch. The goal is:
    • When the input is high (+Vs) the output is low (0V).
    • When the input is low (0V) the output is high (+Vs).

    The output is represented by an LED which is supposed to "light" if I turn the switch "off" and if I'm going to turn the switch "on", the LED must be "off".

    There are no parameters nor resistor values are given on our project.

    Any info will be a great help. Thank you.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    From this statement, this must be a school assignment.

    Let me give you some links into our Ebook for useful information.

    Transistors - http://www.allaboutcircuits.com/vol_3/chpt_4/5.html

    LED's - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

    You will need to select some voltage to work with. Anything between 6 and 12 volts is usually convenient to produce. Using the LED information, you can figure the current limiting resistor you will need. The transistor will need at least 1/10th the LED current into the base to switch. You should be able to see where to place the switch.
     
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  6. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    Thanks for the big help, but what I really need are the component values. My project is to create a prototype and our instructor want us to learn it on our own, we may ask someone for help but we need to build it ourselves. I really have no idea on how am I going to start it. I have here a 9V battery which is supposed to be my input and a 12V LED. I need to know the resistor values and the name of the transistor I should use. I also want to know if the input and output of the circuit you gave me was a square wave.

    Thank you. :p
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you already have e look at the pages provided by beenthere in post #5 ?
    With the information from that page you can calculate the values.

    Just show us your effort.

    Bertus
     
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  8. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    Actually, this is really a school project.
    My project is to create a prototype and our instructor want us to learn it on our own, we may ask someone for help but we need to build it ourselves. I really have no idea on how am I going to start it. I have here a 9V battery which is supposed to be my input and a 12V LED. I need to know the resistor values and the name of the transistor I should use. My input and output should be a square wave.

    Thank you. [​IMG]
     
  9. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    Do you have any information about the transistor I need to use if only β is given? I have already learned to compute for that. And what is the difference of hFE from β?
     
  10. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    Is it okay if I'm going to use a toggle switch?
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    What is the LED? Does it have a built-in limiting resistor so it lights properly with 12 volts?

    How will you get it to work with 9 volts?

    As far as the resistors, the material I linked to will let you calculate them from the operating voltage. As far as transistors go, here's another link - http://www.kpsec.freeuk.com/components/tran.htm This has more information on transistors.

    Pay attention to step 2 in the testing part. Use an NPN type off the list. One in TO-92 is good If you have problems finding one listed, try a 2N2222.
     
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  12. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    I am supposed to build a simple circuit the would make a square wave input & output. If I am going to turn my S1 "on" the LED should turn "off" and if I am going to turn my S1 "off" the LED must turn "on".
    My V1 is a 9V battery. My problem is that I don't know what NPN transistor I need to use. I know how to compute for the values of my R1, R2 & R3 if β of the transistor and Icsat are given. I also want to know if it is okay to remove the R2 in the circuit. I also need to know what kind of switch I need to use.

    The circuit attached is given to me by a senior member and he is not replying yet.

    Can anyone help me?[​IMG]
     
  13. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    You can use any kind of a small-signal BJT for example 2N2222 or BC548.
    And you can remove R3 form your circuit.
     
  14. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    I merged the threads as they are a continuation of each other.
    Did you read the pages beenthere gave in post #5 ?

    What have you done yourself upto now?

    Bertus
     
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  15. lezahyoj

    Thread Starter New Member

    Feb 7, 2011
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    so far, i've already computed for the resistor values and I assumed that my β=100 and Icsat=10mA. I removed R2 from the circuit which leaves R1 and R3

    I got Rc=900Ω and Rb=83kΩ from the formula Icsat=Vcc/Rc since that Vce=0, Ib=Ic/β and Rb=(Vcc-Vbe)/Ib. Is this correct?

    from the link,
    is hfe = β? and Icmax=Icsat?

    is 2n2222 the same as pn2222?

    Thanks for the help.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    You removed R2 from the circuit then you cannot use the switch. The input to R3 should be +9V when high and be 0V when low.

    You are not using the transistor as a linear amplifier so hFE (DC beta) is not used. The datasheet for all transistors specifies a certain minimum collector to emitter voltage for hFE which is 10V for the 2N2222. Instead you want your transistor to be a saturated switch so you must look at the saturated spec's in the datasheet where it says to use a base current that is 1/10th the collector current. Then R2 is about 8.3k so use 8.2k which is the nearest standard resistor value.

    The collector current is Vcc/Rc only if the LED is shorted. So you must subtract the LED voltage from Vcc to calculate Rc. A red LED is about 1.7V to 2.0V. Then Rc is about 720 ohms. Use 750 ohms which is the nearest standard resistor value.

    The datasheet for a 2N2222 and PN2222 show that the difference is the physical case. The metal case is allowed to operate at a higher max temperature than the plastic case.
     
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  17. CDRIVE

    Senior Member

    Jul 1, 2008
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    The only reason I included R3 was if the switch was a remote distance from Q1 it would insure that it would turn off when the switch was closed. It's not really necessary when the switch is local to the transistor.
     
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