Psalm 119:105 (KJV)

105 Thy word is a lamp unto my feet, and a light unto my path.

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The 555 Timer – Part 2 | Bistable Mode

Welcome to Part 2 of the 555 Timer series!

In Part 1, we explored monostable mode, where the 555 produces one timed output pulse.

This time we’re looking at bistable mode.

Bistable means the circuit has two stable output states:

HIGH

and

LOW

Unlike monostable operation, we’re not using an RC timing network to determine how long the output stays in either state. Instead, we’re taking advantage of the SR latch inside the 555 timer. Your video introduces this distinction right at the beginning.

Press one button and the output goes HIGH — and stays HIGH.

Press another button and the output goes LOW — and stays LOW.

That’s bistable operation!

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No RC Timing Cycle

One of the first things you’ll notice about our bistable circuit is what’s missing.

There is no timing resistor and timing capacitor charging and discharging to determine an output pulse width.

We do still have a small capacitor connected to Pin 5, the Control Voltage pin, but we’re using that capacitor for noise filtering rather than timing.

In bistable mode, we’re manually controlling the internal latch with two pushbuttons.

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The SR Latch

Inside the functional block diagram of the 555 timer is an SR latch, also called a Set-Reset latch.

The latch provides something very important:

Memory

Once the latch is SET, it remembers that state.

Once it is RESET, it remembers that state.

It doesn’t need a timing capacitor to tell it when to change.

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Understanding the NOR-Gate Model

An SR latch can be modeled using two cross-coupled NOR gates. This is a useful way to understand how an SR latch can remember a state.

Important Note

The cross-coupled NOR-gate circuit is a functional model of the 555’s SR latch. It is not intended to represent the exact transistor-level implementation inside the NE555.

With our functional SR-latch model:

SET → Q becomes HIGH

RESET → Q becomes LOW

Q and Q̅ are complementary outputs. When Q is HIGH, Q̅ is LOW, and vice versa.

For our 555 circuit, we’re mainly interested in the two conditions that SET or RESET the latch.

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The Bistable Circuit

We’re using an NE555 timer powered from 5 volts.

The circuit has two momentary pushbuttons:

S = SET
R = RESET

The SET button controls the Trigger input on Pin 2.

The other button controls the Threshold input on Pin 6, which we’re using to reset the latch in this circuit.

We also use two LEDs so we can immediately see which stable state the output is in.

🟢 Green LED = Output HIGH / SET

🔴 Red LED = Output LOW / RESET

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SET — Pin 2

Pin 2 is the Trigger input.

A 10 kΩ pull-up resistor normally holds Pin 2 HIGH. When we press the SET pushbutton, Pin 2 is momentarily pulled toward ground.

That brings the Trigger voltage below:

⅓ VCC

With a 5 V supply:

⅓ × 5 V ≈ 1.67 V

The lower comparator responds and sets the internal latch. Pin 3 goes HIGH.

The green LED turns ON.

The really interesting part is what happens when we release the button:

The green LED stays ON.

The latch remembers its state.

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RESET — Pin 6

Pin 6 is the Threshold input.

A pull-down resistor normally keeps Pin 6 LOW.

When we press the RESET pushbutton, Pin 6 is momentarily brought toward +5 V, taking it above:

⅔ VCC

With a 5 V supply:

⅔ × 5 V ≈ 3.33 V

The upper comparator resets the internal latch and Pin 3 goes LOW.

The green LED turns OFF and the red LED turns ON.

Release the button and:

The red LED stays ON.

The circuit remembers this state too.

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Why Two LEDs?

The two LEDs make the two stable states easy to see.

Pin 3 HIGH:
🟢 Green ON
🔴 Red OFF

Pin 3 LOW:
🟢 Green OFF
🔴 Red ON

The green LED is arranged so the 555 output sources current when Pin 3 is HIGH.

The red LED is connected from the +5 V side toward Pin 3, allowing the 555 output to sink current when Pin 3 is LOW.

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Let’s Go to The Lab!

On the breadboard, bistable operation becomes very easy to see.

Press SET:

🟢 GREEN — HIGH

Release the button and it stays green.

Now press RESET:

🔴 RED — LOW

Release the button and it stays red.

Press SET again and we’re back to green.

That’s bistable operation:

Two stable states, with the circuit remembering the selected state.

Your corrected demonstration shows exactly this behavior.

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Monostable vs. Bistable

Monostable

One stable state

A trigger temporarily changes the output. After an RC-controlled time interval, the circuit automatically returns to its stable state.

Bistable

Two stable states

SET changes the output to one stable state.

RESET changes it to the other.

There is no RC timing interval automatically forcing it back.

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What’s Next? Astable Mode!

We’ve now covered:

Monostable → One stable state

Bistable → Two stable states

Next comes:

Astable → No stable state!

Instead of manually pressing SET and RESET buttons, we’ll use an RC timing network to make the 555 continually switch between HIGH and LOW.

And that means our LEDs can:

🟢 → 🔴 → 🟢 → 🔴 → 🟢 → 🔴

all by themselves!

That’s where we’re headed in 555 Timer Part 3 – Astable Mode. Your Part 2 video introduces that next step at the end.

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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

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