# 555 IC — Astable Mode

## Playing with 555 Timer — Part III — Astable #basicTronics 07

Here is the internal layout of a 555 timer in its astable mode.

Summary

`As the capacitor C1 discharges, the voltage on the pin 6 decreases to the point at which pin 2 (connected thereto by external wire) accuses less than one-third of the supply voltage and triggers 555 to the down position. Then C1 charges up to two thirds of voltage supply, trigges it back to the up position and the cycle repeats.`

Initially, the flip-flop grounds capacitor C1.

But now the low voltage on the capacitor C1 is connected from pin 6 to pin 2 through an external wire.

The low voltage tells the chip to trigger itself.
The flip-flop flips to its down position…

…and sends a positive pulse through pin 3, while removing the negative voltage from pin 6.

Now C1 starts charging as usual, except that it is being charged through resistor R1 + R2 in series.
Comparator B monitors the rise in voltage throught pin 6, known as threshold.

When the capacitor reaches ⅔ of supply voltage the flip-flop goes to up position and the output goes low.

The capacitor starts to discharge through resistor R2 via pin 7, the discharge pin.
When the capacitor diminishes to ⅓ of the full voltage, the pull-down on pin 2…

…flips the flip-flop again and the cycle repeats.

Formula for Astable Mode

`Freq (Hz)  = 1440 / ( R1 + 2 x R2 ) x C1`

In experiments we have below: R1 = 1000 ohms, R2 = 10000 ohms and…

For 1 uF cap:

Freq (Hz) = 1440 / (1000 + 2 x 10000) x (1/1000)

Freq (Hz) = 68.571428571428571428571428571429 Hz

For 4.7 uF cap:

Freq (Hz) = 1440 / (1000 + 2 x 10000) x (4.7/1000)

Freq (Hz) = 14.589665653495440729483282674772 Hz

For 10 uF cap:

Freq (Hz) = 1440 / (1000 + 2 x 10000) x (10/1000)

Freq (Hz) = 6.8571428571428571428571428571429 Hz

and for 47 uF cap:

Freq (Hz) = 1440 / (1000 + 2 x 10000) x (47/1000)

Freq (Hz) =1.4589665653495440729483282674772 Hz

note: This last result can not be achieved using the soundcard oscilloscope due to the limits of data collection. It’s already good enough, is not it? … actually, you can measure the period and reverse the result …

Here is the experimental circuit:

You can use either the 10k resistor or the 22k pot in this configuration.

Components You will need for this breadboard:

`1 x NE 555N IC1 x 8-pin socket1 x capacitor 1 uF1 x capacitor 4.7 uF1 x capacitor 10 uF1 x capacitor 47 uF1 x capacitor 0.1 uF (104)1 x capacitor 100 uF1 x resistor 10k1 x resitor  1k1 x resistor 470R1 x potentiometer 22k1 x Piezzo1 x Arduino Uno1 x breadboard1 x LED`

The potentiometer is used to change the frequencies in run-time.

In this task we tested several capacitors and discovered the resulting frequencies.
We use the soundcard oscilloscope from Christian Zeitnitz.

That it for a Astable 555 timer mode!
Bringing your attention to wiring through pins 2 and 6 (yellow wire).

Thank you for watching (reading) the episode!

If you want to discuss it, jump over to my youtube channel!

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Catch you in the next time!

Thank you!

Here the slides! Modify if you will! It is Free!

References & Credits

555 IC-Monostable Mode

555 IC-Bistable Mode

555 IC-Astable Mode

The 555 Timer in Monostable Mode

Proteus — IC 4026 — Reaction Timer Circuit

Proteus — IC 555 Timer — Debounce Circuit

— — — — First published at 27/05/2017 at 11:03 PM — — — —