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# Experiment 15 - The Astable Multivibrator with 555

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 ... Experiment 16 - Gated aMV

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