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# Flashing LED Circuit

This is a simple flashing led circuit with 2 leds and 2 NPN transistors.
It illustrates the behavior of transistors and capacitors and if you use an oscilloscope it will be very easy to determine what happens in this astable multivibrator circuit. It’s state is constantly changing and this change affect the flow of current and voltage and the effect will be visible with the two leds.

The speed of the led flasher may be adjusted with potentiometer P1. Being an astable multivibrator, the circuit has no stable state but oscillates continuously between the two states back and forth. The two transistors T1 and T2 turn and lock each other by turn. The smaller the capacitor value is and the smaller the resistance, the appropriate LED goes out faster, for the benefit of other, who then immediately turns on.

• The activation time of T2 is t, a = 0.7 x R1 x C1, the switch-off t off = 0.7 x R2 x C2.
• The switch from T1 is t, a = 0.7 x R2 x C2, the off t off = 0.7 x R1 x C1.

The transistors do not necessarily have to be BC547B, you may use BC238 or similar small-signal transistors. It is recommended to always use the equivalent transistors. If one of the transistors is defective, wrong or have a malfunction, so does this to the full functionality of this circuit. One LED lights up and the other is dimmed.

The “two flashing led” circuit is designed for 9 Volts but it works at lower voltages too. In this design we used red leds but by changing the series resistors R1 and R4 you can also use different LED colors.

## Two flashing LED circuit diagram

Components values
R1 – 470 Ohm
R2 – 470 Ohm
R3 – 3.9 kOhm
R4 – 3.9 kOhm
P1 – 50 kOhm
C1 – 47 μF / 16V
C2 – 47 μF / 16V
T1 – BC 547 B
T2 – BC 547 B
D1 – LED Standard , 5 mm , red
D2 – LED Standard , 5 mm , red

##### Related Tutorials

• andrius

What could be wrong if both leds stay on? Could not get potentiometer so used 50k resistor

• Subham Pramanik

Can you plz explain the operation of the circuit…also kindly mention how do we need to choose component values…

• N3M0

Thinking of trying this circuit with a cosplay project, but I only need it to pulse 3 times. How would I do that?

• OH8EFI

Heh, trying this with Crocodile Clips (Version 3.5)… First two flashes takes place and then it lags and goes near to crashing 😀 But hey, cool and simple design!

• Two-Face

Could’nt collect a potentiometer….What can i do ? Can i use two constant resistors ?

• Dan Meam

why delete my 1o comment.. now the second make no sense xD

• Dan Meam

by the way… the “L1” inductor its just an switch i made.. like one 0Volt Relay that make the camera take the picture everytime the led flash’s.. i think this can be used in many ways..

• clifton

thanks for any help looking for the circuit. when you look close i see r1 c1 ,single led . bubble button.

• clifton