Parallel Loads switched by a single transistor

Discussion in 'General Electronics Chat' started by cevval, Dec 28, 2012.

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  1. cevval

    Thread Starter Member

    Dec 6, 2011
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    Happy new year to everbody, first of all.

    Herebelow I have posted a part of my draft circuit diagram.
    My questions to experts are as flollows: Please, see the attached diagram.
    1) Is it a good way to switch on and off different loads by a single transistor. Or a separate transistor for each different load is necessary?
    2) You will see that transistor base is grounded through a 10k resistor. I learned this from some where from internet. Is this absolutelly necessary or can I omit it?
    3) You will also see that across terminals of the buzzer (magnetic type) a 1.0k resistor is connected. I learned this from a manufacturer's application hints. Is this absolutelly necessary or can I omit it?

    Thanks for all in advance.
     
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  2. ScottWang

    Moderator

    Aug 23, 2012
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    1) You should use separate transistor for each different load is better.

    2) The 10K is designed to turn off the transistor more quickly or when the input terminal is a switch, the Rbe can be turn off the transistor and you can't let B of Q3 into a floating status.

    If you want to use Rbe, then you may reduce it to R25(10K)/5, but normally when you used 10K is OK, you may try to take it away.

    3) If the Buz is one kind of the Inductance then you should add the Diode, 1K is designed to reduce the oscillation of Buz, if you take the 1K away and then the Buz is still working fine, then you can take 1K away.
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What is IC1.3?
    What are the power supply voltages connected to it?
     
  4. cevval

    Thread Starter Member

    Dec 6, 2011
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    The supply voltages to loads as follows:
    1)To buzzer and led on PCB: approx. 12-14 volts.
    2) External led: 12-16 volt, maybe. Just to supply ground when the base input is on.
    3) Base voltage: 9V
    Thanks.
     
  5. cevval

    Thread Starter Member

    Dec 6, 2011
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    Sorry, I missed your first question: IC.3 is an opamp.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Each type of op amp is different.
    What is the part number?
    What are the op amp's supply voltages?
     
  7. cevval

    Thread Starter Member

    Dec 6, 2011
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    Opamp no: LM224 or LM2902
    Supply volt: +9 VDC
    Thanks.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Sometimes you can play a game like this, but what you have to do is consider the behavior of the circuit when the transistor is turned off. In this case, the top part of the circuit still has power and ground connections. Ignoring the remote LED option, consider that you have the following path:

    +12V -> R27 -> LED1 -> D11||R26 (plus possibly the buzzer) -> VR8 (wiper) -> GND
     
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  9. Audioguru

    New Member

    Dec 20, 2007
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    With a 9V supply, the typical output high voltage from an LM224 is about +7.8V. The base-emitter voltage of the 2N2222 is about 0.7V so the 10k base resistor has a voltage of 7.1V and a current of 0.71mA.
    Then the transistor saturates well when its collector current is only 7.1mA. The beeper will not be loud and the LED will not be bright.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Good observation.
    A diode could eliminate the backfeed problem. See attachment.

    Cevval, what does this mean?
     
  11. cevval

    Thread Starter Member

    Dec 6, 2011
    30
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    I revised and improved the schematic and attached here below according
    to your recommendations. What I mean is: Remote led is on the car
    dashboard cluster for giving alert already for hand brake and brake fluid
    low level. Now, I am designing a circuit as an hobiest, of course with the
    help of those guys at internet i.e. like you, which is about car speed limit
    alerting when driving on highway. This circuit should give visiual and sound
    alerts when the legal speed limits exceeded.
    So I intend to use the existing led on dashboard which is for hand brake
    and brake fluid level monitoring. When I believe that the circuit design
    safe enough and then I will make and home-made circuit and after testing
    of course, put it in a box and install under the dashboard of the car.
    See below attachment for more detail for your question.

    Regards,

     
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  12. cevval

    Thread Starter Member

    Dec 6, 2011
    30
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    I revised and improved the schematic and attached here below according
    to your recommendations. What I mean is: Remote led is on the car
    dashboard cluster for giving alert already for hand brake and brake fluid
    low level. Now, I am designing a circuit as an hobiest, of course with the
    help of those guys at internet i.e. like you, which is about car speed limit
    alerting when driving on highway. This circuit should give visiual and sound
    alerts when the legal speed limits exceeded.​
    So I intend to use the existing led on dashboard which is for hand brake
    and brake fluid level monitoring. When I believe that the circuit design
    safe enough and then I will make and home-made circuit and after testing
    of course, put it in a box and install under the dashboard of the car.​
    See below attachment for more detail for your question.​
    Regards,

     
  13. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
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    Tinkering with automotive electrics/electronics is not allowed here. This is explained in a sticky somewhere here.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I am closing this thread as it violates AAC policy and/or safety issues.

    Quote:
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    Bertus
     
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