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HELP : Simple Buzzer and LED design does not work

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abicash

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Hello

I am trying this simple design to turn on a buzzer on certain condition and an LED on another.
I have attached the schematic. R1 is LOAD
What i intend is
LOAD PRESENT-BUZZER OFF-LED ON
LOAD ABSENT-BUZZER ON-LED OFF
CURRENT FLOW THROUGH LOAD-BUZZER ON

What is the basic mistake i am doing since when LOAD is absent, buzzer doesn't sound.LED glows okay.

All help appreciated
 

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  • Safety.PDF
    14.5 KB · Views: 63

Q2 base is floating and need a resister to ground to turn off. Why do you have D3 and D3?

Enjoy your design work!
 

Hi HTA
Thanks for replying

Grounding it with a resistor has no effect.
At D3 cathode , an external voltage arrives, so D3 isolates that voltage with this ckt.

Can you suggest something?
 

Hi Venkadesh

Find attached pls
 

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  • safety.rar
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abicash said:
Grounding it with a resistor has no effect.
Click to expand...

Grounding Q2 base will make sure that it is off when a load is present, otherwise it will be floating. Your circuit should work, try checking your buzzer's voltage/current requirement. Better yet, replace the buzzer with a known working LED and a LED load resistor in place of the buzzer and the 2k resistor. If it works then your buzzer could be damaged or it load resistor value is incorrect.
 

The red LED might need 2.0V. The BC547 that drives it might saturate with Vce at 0.1V. Then the 330 ohm resistor has (12V - 2.1V)/330 ohms = 30mA. The datasheet for the BC547 shows that it saturates well with a base current of 1/20th its collectore current so the base current should be 1.5mA.

We do not know the resistance of the buzzer but we will assume it to be low. The 470k base resistor has a current of only (12V- 0.7V)/470k ohms = 24uA which is 63 times too low!

The 130k resistor might also have its value much too high.

The rule is that hFE (beta) is used for a linear amplifier that does not saturate, never for a transistor used as a switch.

- - - Updated - - -

Venkadesh_M said:
Hi is this what you need??
Click to expand...
Please post a normal schematic as a PNG file.
 

Audioguru said:
Please post a normal schematic as a PNG file.
Click to expand...

Here It is...........

untitled.JPG
 

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