This circuit allows two LEDs to blink alternately using two transistors, capacitors, resistors, and diodes. When the battery is connected, the capacitors begin to charge through the diodes and base resistors. One transistor will turn on before the other, pulling its LED cathode to ground and lighting it. The opposite capacitor then discharges through its diode, turning on the other transistor and lighting the second LED. The capacitors alternate discharging through the diodes, causing the transistors to alternate turning on and off, making the LEDs blink one after the other. Changing the capacitor values changes the blinking timing sequence. Potential applications include railroad crossing signals, bike safety blinkers, and Christmas decorations.

1. Low Frequency Two LED Light
Blinker Oscillator
1
E L E C T R O N I C D E V I C E S
A N D C I R C U I T S
E C S e m e s t e r - 3

2. Contents:
1. Introduction
2. Components used
3. Function of components
4. Circuit Diagram
5. How to connect the circuit
6. How it works
7. Result
8. Applications
2

3. 1. Introduction
 The Transistor used along with the capacitor allows the
LEDs to flash as per given timing sequence.
 Hence the LEDs flash only one at a time.
 On changing the value of capacitors the timing
sequence changes.
 This circuit is a type of square wave oscillator.
3

4. 2. Components used
I. 2N2222 NPN Transistors 2
II. Diodes 2
III. 470 ohm resistors 2
IV. 10k resistors 2
V. 100 µF capacitors 2
VI. LED 2
VII. 9V Battery 1
VIII. Connecting wires --
IX. PCB 1
4

5. 3. Function of Components
 2N2222 NPN Transistors are high speed switching
Transistors.
5

6. Different Transistors pins
6

7. Function of Components…(2)
 Diodes used in the circuit allow the flow of current
only in one direction.
 470 ohm resistors decrease the current going to the
LEDs so they do not overload.
 10k ohm resistors decrease the current and maintain
the circuit in stable condition.
 100 µF capacitors get charged when the current is
flowing to provide constant current to transistor.
 LEDs work only in forward bias.
 9V battery is used to supply power to the circuit.
7

8. 4. Circuit Diagram
8

9. 5. How to connect the circuit
 The first thing is the battery connector. It has two wires
one red and one black. Of course the red is positive and
the black is negative. The red connects to two wires,
one goes to the resistor R1 and the other goes to R2.
 Notice that at ground the both emitters of transistors
Q1 and Q2 connect there, as well as both base resistors
R3 and R4. They are all connected to the same common
point or ground.
9

10. How to connect the circuit…(2)
 At this point, we will look at only one half of the
schematic since both halves are identical. On the left
side, the other end of R1 connects to LED diode D1's
anode and D1's cathode connects to the collector of
transistor Q1. Then Q1's emitter goes to ground.
 Now look at Q1's base. It connects to three parts,
R3,D3 and the other is C1. R3 and D3's anode connect
to ground. The other end of C1 connects over to
transistor Q2's collector.
10

11. 6. How it works
 When the battery is connected both capacitors C1 and
C2 begin to charge through the diodes and the base
resistors R3 and R4 This will begin building a positive
voltage on the transistor bases but because not all the
parts will be exactly identical, one transistor will turn
on before the other.
11

12. How it works…(2)
 As soon as one turns on the collector of that transistor
will pull down toward ground completing the circuit
through one of the LED's. That LED will light because
it's cathode is pulled to ground through the transistor
but look closely and you will see that capacitor
connected to that diode will begin to discharge back to
ground. This will place a positive voltage on the
opposite transistors base and turn on that transistor.
12

13. How it works…(3)
 Now that action will cause a positive voltage to be
applied to the other transistor's base turning it back on
again which will cause the second transistor to turn on
again and so on over and over again. Diode D3 and D4
discharge the capacitors C1 and C2 between cycles so
they are ready to charge again on the next cycle.
 The flash rate is determined by the value of R3, R4 and
C1, C2.
13

14. 7. Result
 Due to constant charging and discharging of capacitors,
the LEDs go on constantly blinking.
14

15. 8. Application
 Railroad crossing signal.
 Safety blinker for bicycles, etc.
 Christmas decorations
 Blinkers to locate items in the dark.
15

16. Members Involved
1. RATHOR VIJENDRASINGH 130170111092
2. RUDRA PAVAN 130170111095
3. RATHAVA JAYDEVSINH 130170111091
16
PPT available at: bitly.com/led-blinker