The Railroad Crossing Light

This circuit makes two LEDs flash in an alternating pattern. You can use it, for example, for a model railroad crossing, or for eyes on a toy robot.

This is a classic circuit, called an astable multivibrator. The basic idea here is that the two transistors alternate between turning each other on and off. If you want to change the blinking speed, try replacing the two capacitors and/or the resistors R2 and R3 with other values.

Here’s a video of the circuit I built:

The Circuit Diagram

Find the components used in the table below. Click here to buy the complete kit.

Part Value Note
R1, R4 470 Ω Standard Resistor
R2, R3 47 kΩ Standard Resistor
Q1, Q2 BC547 Any general purpose NPN transistor will work
C1, C2 10 μF Polarized Capacitor
L1, L2 Standard output light-emitting diode

Are You Stuck?

Let me know your questions and comments in the comment field below

Comments on The Railroad Crossing Light

  1. DeeDee Gibson says:

    I built this circuit and it worked (the first time!!! – I could hardly believe it!). But the lights blink really, really fast. What resistor and capacitor values should I use to replace the current parts to get a slower, steadier blink rate? Thank you!

  2. Oyvind says:

    Hi DeeDee,

    Congratulations! Well done =)

    You can decrease the blinking speed by increasing the capacitor values. For example, you can try 100µF capacitors instead of 10µF.

    Best,
    Oyvind

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