DARK SENSORS BY TRANSISTORS

BC 547

BC 547 Is a bipolar junction transistor. the series from BC 546 to 550 are used in Audio frequency small signal amplifier stages and DC coupling. They are complement to BC 556 to 560 that is BC 546 is NPN and BC 556 is PNP
The collector current of BC 547 is 100 milli amperes with the maximum power dissipation of 500 mW.The collector to emitter voltage varies from 0.07 to 0.25 with Ic = 10 milli amperes and Ib = 0.5 milli amperes. Vbe saturation value
ranges from 0.7 to 0.9 corresponding to base current of 0.5 milli amperes. The hfe or beeta value depends upon the class of transistor used. for class A that is BC 547 it is 180 for BC547B it is 290 and for BC547C it is 520.

CIRCUIT DIAGRAM

COMPONENTS REQUIRED

• 100K variable resistor
• BC 547
• LED
• 1K RESISTOR
• BATTERY
• LDR

WORKING

This circuit works as a dark detector. whenever light intensity is very low , the LED starts glowing. circuit is designed around BC 547 along with LDR. To understand the function of LDR read my previous page on  ldr basics

CALCULATIONS

To calculate the resistance required to make a dark sensor we should have complete knowledge of LDR and transistor

now we need to glow led which starts glowing when 5 milli ampere current is passed through it

so    Ic = 5 mA

        b VALUE(BETA) = 200

WE know

      Ic = b X Ib

       Ib = 0.025 mA

MAKING THE THEVENIN EQUIVALENT TO CALCULATE THE RESISTANCE

APPLYING KVL AT THE INPUT SIDE

25/(5+R) = 5R/(5+R)X 0.025 + 0.7

R = 26 k 

WORKING

This circuit works as a dark detector. whenever light intensity is very low , the LED starts glowing. circuit is designed around BC 547 along with LDR.