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NPN Transistor Astable Multivibrator Circuit

  • Concept of Operation

    The astable multivibrator, also known as a free-running multivibrator, is a circuit that produces a steady square wave without the need for external input. This square wave can be used as the clock source for a digital circuit. For the demonstration, the output is used to drive a pair of LEDs making a nice standalone light flasher project.

    The astable multivibrator example circuit here is built from two cross-coupled transistor switches. The transistors are connected via resistor capacitor circuits, which effectively tie the output of each transistor to the input at the base of the other transistor.

    The circuit has no stable state. When Q2 is high, capacitor C1 is discharging and C2 is charging. Once the charge on C2 is sufficient to bring transistor Q1 high, the process reverses. Q2 goes low, C1 begins charging and C2 begins discharging.

    The frequency of the oscillation is determined by the RC constants of C1 R3 and C2 R2. In our basic example circuit, C1 equals C2 and R3 equals R2 making the output waveform symmetrical and the calculation straightforward:

    Frequency = 1 / (1.38 * R * C)

    Frequency is in Hertz; resistance is in Ohms, and capacitance is in Farads. Plugging in the values for the example circuit yields a frequency of about 0.72 Hertz, which is a comfortable flashing rate for the LEDs:

    0.7246 = 1 / (1.38 * 10000 * 0.0001)

  • Schematic

  • Breadboard Diagram

  • Parts

    • PN2222A - NPN General Purpose Transistor (x2)
    • 100uF Capacitor (x2)
    • 10K Resistor (x2)
    • 1K Resistor (x2)
    • Green 5v LED (x2)

    (All parts for the project are available in our store.)

  • Notes on Implementation

    1. The LEDs used in the project have a built in current limiting resistor eliminating the need for an external resistor to protect the device. This resistor is shown in the schematic as a resistor with the value “Lim” for completeness.
    2. The circuit was built and tested at 4.8 volts DC (4 AA NiMH rechargeable batteries).