LED Flasher Circuit

    Simple alternating LED flasher circuit using transistor flip-flop configuration

    Circuit Description

    This simple LED flasher circuit uses two transistors in a flip-flop configuration to alternately flash two LEDs. The circuit demonstrates basic transistor switching principles and is an excellent starting point for learning about oscillator circuits without using integrated circuits.

    How It Works

    The circuit operates on the principle of a bistable multivibrator (flip-flop). The two transistors are cross-coupled so that when one is ON, the other is OFF. The capacitors cause the state to periodically switch, creating the flashing effect. The rate of flashing is determined by the RC time constant of the resistors and capacitors.

    Circuit Schematic

    +9VGNDQ1BC547Q2BC547LED1LED2R1470ΩR2470ΩR310kΩR410kΩC110μFC210μF

    Components List

    • 2 × BC547 NPN transistors (Q1, Q2)
    • 2 × LEDs, any color (LED1, LED2)
    • 2 × 470Ω resistors (R1, R2)
    • 2 × 10kΩ resistors (R3, R4)
    • 2 × 10μF electrolytic capacitors (C1, C2)
    • 1 × Breadboard
    • Jumper wires
    • 9V battery or power supply

    Installation Instructions

    1. Place both BC547 transistors on the breadboard with enough space between them.
    2. Connect the 470Ω resistors (R1, R2) from the positive rail to the collector of each transistor.
    3. Connect the LEDs with the anode (longer lead) to the collectors of the transistors and the cathode (shorter lead) to the corresponding resistor.
    4. Connect the 10kΩ resistors (R3, R4) from the base of each transistor to ground.
    5. Connect the capacitors cross-coupling the circuit: connect C1 from the collector of Q1 to the base of Q2, and C2 from the collector of Q2 to the base of Q1. Pay attention to the polarity - the positive end should connect to the base.
    6. Connect the emitters of both transistors to ground.
    7. Connect the power supply or battery.
    8. The LEDs should start alternately flashing immediately.

    Troubleshooting Tips

    • If the LEDs don't flash, check your connections and ensure the transistors are properly oriented.
    • To adjust the flashing speed, change the values of the capacitors. Larger values will slow down the flashing, while smaller values will speed it up.
    • If one LED remains on constantly, check the capacitor connections and ensure they are properly oriented.
    • For more brightness, you can reduce the values of R1 and R2, but don't go below 220Ω to avoid damaging the LEDs.

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