Experiment 15 - The Astable Multivibrator with 555

With an NE555 you are also able to build an astable multivibrator or blinker. Find out how, and what you need to be aware of.

Consider the circuit shown. You surely recognize the circuit of the Experiment 9, now with a capacitor C1 of larger capacity.

Circuit diagram astable multivibrator with NE555.

When you take the circuit into operation, the LEDs light up alternately. Thereby LED2 lights up about twice as long as LED1.

Circuit astable multivibrator with 555. (Enlarge)

You can calculate the switching times approximately. The time τ₁, in whichLED2 lights up (charge time)...

τ₁ = 0.7 × (R₁ + R₂) × C₁

    = 0.7 × (47 000 Ω + 47 000 Ω) × 0.0001 F = 6.5 sec

The time τ₂, in which LED1 lights up (discharge time)...

τ₂ = 0.7 × R₂ × C₁

    = 0.7 × 47 000 Ω × 0.0001 F = 3.3 sec

The oscillation t is calculated as the sum of τ₁ and τ₂...

t = τ₁ + τ₂

   = (6.5 + 3.3) sec = 9.8 sec

and the frequency f...

f = 1 / t = 1 / (τ₁ + τ₂)

   = 1 / 9.8 sec = 0.1 Hertz

From the formulas you will certainly understand why the LED2 lights up twice as long as LED1. The capacitor is charged via the resistors R1 and R2, but discharged via the resistor R2.

In the next experiments, you play with the switching times ...