Flip Flop LED Flasher Using BC547 Transistors

Flip Flop LED Flasher Using BC547 Transistors

This post presents circuit for blinking two LEDs alternatively using two BC547 transistors. This circuit is commonly called a flip flop LED flasher. Generally used in alarm systems to produce flashing lights effect.

Components

No.ComponentQuantity
1.Power supply (3V - 5V) 1
2.BC547 transistor2
3.330Ω resistor2
4.10kΩ resistor2
5.100μF electrolytic capacitor2
6.LED2

Circuit diagram

Flip Flop LED Flasher Using BC547 Transistors (Circuit Diagram)

The blinking rate of the LEDs can be increased or decreased by changing the values of R2, R3 resistors, and C1, C2 capacitors. The frequency of a flashing LEDs can be calculated by given formula:

Formula to calculate the frequency of a flashing LEDs

If R2, R3 resistors (R2=R3=R) and C1, C2 capacitors (C1=C2=C) have identical values then frequency can be calculated as follows:

Formula to calculate the frequency of a flashing LEDs when R2=R3 and C1=C2

The values of resistors are specified in ohms (Ω), values of capacitors are specified in farads (F).

By using values given in the circuit diagram (R2=R3=10kΩ and C1=C2=100μF), the frequency is about 0.72Hz and a time period is 1.386 seconds.

Example to calculate the frequency of a flashing LEDs

Designed circuit

Circuit is designed on a breadboard.

Flip Flop LED Flasher Using BC547 Transistors (Designed Circuit)

The 5 Comments Found

  1. Avatar
    Tahir Bangash Reply

    halloo I Hope you are doing well I just wanted to conform that how to set the values of Resister R3 R2 or the capacitor c1 c2 to set the blinked frequency to 4 hz or set the time period to 0.25s

    • Avatar
      lindevs Reply

      Hi, Tahir Bangash
      I updated the post description. I provided formulas and calculation example. You can choose 18kΩ R2, R3 resistors and 10μF C1, C2 capacitors. The frequency approximately will be 4Hz and a time period will be about 0.25 seconds.
      f = 1 / (1.386 * 18000 * 0.00001) ≈ 1 / 0.25 ≈ 4Hz

  2. Avatar
    Nini Reply

    Hi guys,
    I am a beginner in electronics, and wanted to make this circuit. But, after making it it flashed twice and stopped. I tried it again but nothing happened. Can someone with better knowledge of this kind of stuff please help?

    Materials used:
    Breadboard Power Supply
    BC547 transistor
    440Ω resistors
    10kΩ resistors
    100μF electrolytic capacitors
    Green LEDs
    Breadboard
    Jumper Wires

    • Avatar
      lindevs Reply

      Hi
      Perhaps your voltage is too low. What your power supply voltage? Try to use 9V battery or change current limiting 440Ω resistors R1 and R4 to lower value such 330Ω.

      • Avatar
        Nini Reply

        Well if it already flashed twice, so that might not be it. Also I don't have any 330Ω resistors and the output voltage is at 5v. But of course you might be right.

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