 # 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.

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

Circuit is designed on a breadboard. 1. 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

• 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. 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:
BC547 transistor
440Ω resistors
10kΩ resistors
100μF electrolytic capacitors
Green LEDs
Jumper Wires

• 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.

• 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.

3. Thank you!!!! I finally got this!

4. Hi, what values need to be changed if I used a 9v power supply?

• Hi,
To use a 9V power supply, you should change current limiting resistors R1 and R4 to higher value, such 470Ω.

• 