Simple LED Flasher Circuit

Simple LED Flasher Circuit with 9V supply:

  • It is a very cheap and easy to build project.
  • The LEDs will go ON/OFF automatically (Blinking).
  • Add couple of various colour LEDs. You can modify this circuit by replacing LED with a relay. Then you feed the fluctuating output voltage to the relay and use it.
  • The circuit diagram is given below.


  • The principle of the circuit is Relaxation oscillator.
  • At this point it is good to refresh about the concept of Oscillator.
  1. Any electrical circuit which is used to generate AC voltage without applying AC input signal is called an Oscillator.
  2. To generate the AC voltage, the oscillator circuit will receive the energy from a DC source.
  3. If the output of the oscillator is a sine wave function of time, then it is called as Sinusoidal Oscillator.
  4. It is also known as Harmonic Oscillator.
  5. Some of the examples of sinusoidal oscillator is Positive feedback oscillator and negative resistance oscillators.
  6. Based on the type of circuit used, the sine wave oscillators are classified as follows:
    (1) LC tuned Oscillator.
    (2) RC Phase Shift Oscillator.
  7. The Oscillators which generate non sinusoidal waveforms like square, rectangular, sawtooth or triangular waveforms are called as Relaxation Oscillator.
  8. This oscillator generates voltages or currents which vary abruptly one or more times in a cycle of oscillations.
  9. The relaxation oscillator covers a wide range of frequencies.
  10. It is simple in construction, compact and low-cost. The different types are Linear sweep oscillators, Multivibrators and Blocking Oscillators.
  • Remember that if you are using normal commercial LEDs available in the market (generally termed as 3mm LED or 5mm LED), then make sure the current should not exceed the LED rated current.
  • For general purpose LEDs (3mm LED or 5mm LED), the nominal current will be 20mA. It is good practice to use a resistor in series with the LEDs.
  • General Tip:
    Even though you are calculating the LED current properly and use appropriate components, always use a current limiting resistor in series with the LED loads.

Parts required to build the circuit:

Transistor = BC177, BC107
Resistance = 220ohm, 470Kohm,
Capacitor = 50uF
battery = 9V

How to identify the BJT terminals:

Remember that, always  the transistor casing should  have some marking to indicate which leads are connected to the emitter, collector, or base of a transistor.
Generally BC107,177 will come in TO-18 package which looks like below figure.

The terminals can be identified with the help of below figure. Take the BC107 transistor and keep it in a table in such a way that the pins are towards top. You can observe a small notch on the transistor. From the notch the emitter, base and collector pins are available in the clockwise direction.
BJT_Terminal_Identification_TO_18_PackageLead #1 = Emitter
Lead #2 = Base
Lead #3 = Collector

You may also like to do the following Projects:
555 Timer Based LED Blinking Project
Frequency to voltage converter Concept Based Project
Simple LED Flasher Circuit
Electronics Mini Project for Diploma Students – 1

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