Joshua 1:9 (KJV)
9 “Have not I commanded thee? Be strong and of a good courage; be not afraid, neither be thou dismayed: for the LORD thy God is with thee whithersoever thou goest.”

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The 555 Timer – Part 3 | Astable Mode

Welcome to Part 3 of the 555 Timer series!

In Part 1, we explored monostable mode, and in Part 2 we looked at bistable mode. Now we’re going to put the NE555 into astable mode.

The word astable means the circuit has no stable state. Instead of remaining HIGH or LOW, the output continually switches back and forth between the two states.

This makes the 555 timer useful for oscillators, clocks, LED flashers, tone generators, pulse generators, and many other circuits.

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How the Astable 555 Works

In astable mode, the timing capacitor repeatedly charges and discharges.

The timing capacitor is connected to both:

• Pin 2 – TRIGGER
• Pin 6 – THRESHOLD

These pins monitor the timing capacitor voltage.

Inside the 555 timer, two comparators use reference levels of approximately:

1/3 VCC

and

2/3 VCC

When the capacitor voltage falls to the lower trigger level, the internal latch is set. Pin 3 goes HIGH, and the discharge transistor connected to Pin 7 turns OFF.

The capacitor then charges through RA and RB.

When the capacitor voltage reaches approximately 2/3 VCC, the threshold comparator resets the internal latch.

Pin 3 goes LOW, and the discharge transistor associated with Pin 7 turns ON.

The capacitor then discharges through RB and the discharge transistor.

Once the capacitor voltage falls back to the lower trigger level, the process begins again.

This continuous charging and discharging produces the repeating waveform at Pin 3.

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Circuit Values Used in the Lab

The capacitor connected between Pin 5 and ground is used to help reduce noise on the control-voltage pin.

10 nF is the same as 0.01 µF.

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LED Output Indicators

I used two LEDs so we could visually see the output switching between HIGH and LOW.

When Pin 3 is HIGH, the green LED turns ON.

When Pin 3 is LOW, the red LED turns ON.

The LEDs are connected in opposite directions relative to the output.

For the red LED:

+5 V → 1 kΩ resistor → Red LED → Pin 3

For the green LED:

Pin 3 → Green LED → 220 Ω resistor → Ground

I used different resistor values to make the apparent brightness of the two LEDs closer to one another.

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Astable Timing Equations

The amount of time the output remains HIGH is:

tHIGH = 0.693 × (RA + RB) × C

The amount of time the output remains LOW is:

tLOW = 0.693 × RB × C

The total period is:

T = tHIGH + tLOW

We can also calculate the total period directly using:

T = 0.693 × (RA + 2RB) × C

Frequency is:

f = 1 ÷ T

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Where Does 0.693 Come From?

The number 0.693 comes from the natural logarithm of 2.

ln(2) = 0.693147…

For our calculations, we simply round this to:

0.693

This number appears because of the exponential charging and discharging behavior of the timing capacitor between the 555 timer’s switching thresholds.

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Our Circuit Values

For our circuit:

RA = 4.5 kΩ = 4,500 Ω

RB = 45 kΩ = 45,000 Ω

C = 10 µF

Before using the capacitor value in our equations, we convert microfarads to farads:

10 µF = 0.000010 F

Now we’re ready to calculate the timing.

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Calculating the HIGH Time

The equation is:

tHIGH = 0.693 × (RA + RB) × C

Substitute our component values:

tHIGH = 0.693 × (4,500 + 45,000) × 0.000010

First:

4,500 + 45,000 = 49,500

Therefore:

tHIGH = 0.693 × 49,500 × 0.000010

Which gives:

tHIGH ≈ 0.343 seconds

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Calculating the LOW Time

The equation is:

tLOW = 0.693 × RB × C

Substitute our values:

tLOW = 0.693 × 45,000 × 0.000010

Which gives:

tLOW ≈ 0.312 seconds

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Calculating the Total Period

The period is the HIGH time plus the LOW time.

T = tHIGH + tLOW

Substitute our calculated values:

T = 0.343 + 0.312

Therefore:

T ≈ 0.655 seconds

We can also verify this using the period equation:

T = 0.693 × (RA + 2RB) × C

Substitute our values:

T = 0.693 × (4,500 + 2 × 45,000) × 0.000010

First:

2 × 45,000 = 90,000

Then:

4,500 + 90,000 = 94,500

Therefore:

T = 0.693 × 94,500 × 0.000010

Which gives approximately:

T ≈ 0.655 seconds

Both methods give us essentially the same result.

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Calculating the Frequency

Frequency is the reciprocal of the period:

f = 1 ÷ T

Our period is approximately 0.655 seconds:

f = 1 ÷ 0.655

Therefore:

f ≈ 1.53 Hz

So theoretically, our 555 timer should oscillate approximately 1.53 times per second.

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Calculated Results

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Time to Go to The Lab! ⚡

With our calculations complete, it’s time to actually build the circuit!

I constructed the astable 555 timer circuit on the breadboard using our 4.5 kΩ and 45 kΩ timing resistors and a 10 µF timing capacitor.

When power was applied, the red and green LEDs began alternating.

Green = Pin 3 HIGH

Red = Pin 3 LOW

The circuit keeps doing this automatically because there is no stable state.

That’s astable operation!

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Looking at the Output With an Oscilloscope

Next, I connected the oscilloscope to Pin 3 so we could actually see the repeating output waveform.

The oscilloscope settings were approximately:

Vertical Scale: 1 V/div

Time Base: 0.2 s/div

Coupling: DC

The oscilloscope showed the repeating square-wave output produced by the 555 timer.

The HIGH output displayed on the scope was approximately 3.5 V in our actual experiment.

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Measuring the HIGH Time

From the oscilloscope, the HIGH portion of the waveform was approximately:

tHIGH ≈ 0.36 seconds

Our calculated value was:

0.343 seconds

That’s pretty close!

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Measuring the LOW Time

The LOW portion of the waveform was approximately:

tLOW ≈ 0.32 seconds

Our calculated value was:

0.312 seconds

Again, that’s very close.

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Measuring the Period

One complete cycle consists of one HIGH time plus one LOW time.

Using our measured values:

T = 0.36 + 0.32

Therefore:

T ≈ 0.68 seconds

We can also estimate the period directly from the oscilloscope.

One complete waveform occupied approximately 3.4 horizontal divisions.

With the scope set to 0.2 seconds per division:

T = 3.4 × 0.2

Therefore:

T ≈ 0.68 seconds

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Calculating the Measured Frequency

Now use our measured period to determine the actual frequency:

f = 1 ÷ T

f = 1 ÷ 0.68

Therefore:

f ≈ 1.47 Hz

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Calculated vs. Measured Results

Our calculated frequency was:

1.53 Hz

Our measured frequency was:

1.47 Hz

That’s a pretty good match!

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Why Aren’t the Measurements Exactly the Same?

When we build a real circuit, our measured values usually won’t match the theoretical calculations perfectly.

There are several reasons for this.

Component tolerances: A resistor marked 45 kΩ may not measure exactly 45 kΩ, and capacitors can have even larger tolerances.

Oscilloscope measurement: Reading the divisions on the scope introduces some measurement uncertainty.

Real-world circuit behavior: The actual 555 timer, power supply, wiring, LEDs, resistors, and other components aren’t mathematically perfect.

This is one of the reasons it’s useful to both calculate a circuit and then build and measure it.

The calculations tell us what we expect.

The lab tells us what the real circuit actually does.

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What We Learned

In astable mode, the 555 timer has no stable state.

Instead, it continuously switches between HIGH and LOW.

The timing capacitor repeatedly charges and discharges between the 555 timer’s internal switching thresholds.

The capacitor charges through RA + RB and discharges through RB and the Pin 7 discharge transistor.

The timing equations allow us to predict the HIGH time, LOW time, total period, and frequency.

For our circuit:

Calculated Frequency = 1.53 Hz

Measured Frequency = 1.47 Hz

Our experiment showed that the real circuit behaved very closely to what our calculations predicted.

Whoo Buddy!! ⚡

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Resources

Texas Instruments NE555

NE555 Datasheet — Texas Instruments

NE555 Product Page — Texas Instruments

Wikimedia Commons

The 555 block diagram shown in the video is from Wikimedia Commons:

NE555 Block Diagram — BlanchardJ / Wikimedia Commons — Public Domain

NE555 Block Diagram — Wikimedia Commons

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Thanks for spending some time with me learning about the 555 timer in bistable mode.

Next time we’ll head back to The Lab and see what happens when we let the 555 switch those states automatically.

Build Circuits With Rich

Whoo Buddy!! ⚡🔧