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
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