What Is a 555 Timer?

The 555 is a timer integrated circuit designed to generate precise timing and switching functions.

The basic 555 contains several functional blocks:

• Voltage divider
• Two comparators
• SR latch
• Discharge transistor
• Output stage

External resistors and capacitors determine the timing behavior of the circuit.

555 Timer Pinout

The standard 8-pin 555 timer has the following pins:

555 Timer Pin Diagram

             ┌───────────────┐
       GND  1│               │8 VCC
    TRIGGER 2│     555       │7 DISCHARGE
     OUTPUT 3│               │6 THRESHOLD
      RESET 4│               │5 CONTROL
             └───────────────┘

The exact pin arrangement should always be checked against the datasheet of the particular 555 device being used.

Pin 1 — Ground

Pin 1 is connected to the circuit ground.

It provides the reference point for the internal circuitry and external timing circuit.

Pin 2 — Trigger

The trigger input is used to initiate a timing cycle.

In the standard 555 configuration, the trigger comparator responds when the trigger voltage falls below approximately one-third of the supply voltage.

VTRIGGER < 1/3 VCC
 This causes the internal latch to change state. 
 

Pin 3 — Output

Pin 3 is the main output of the 555.

Depending on the configuration, it can produce:

• High and low logic levels
• Timing pulses
• Square waves
• Oscillations

The output current capability depends on the particular 555 family.

Pin 4 — Reset

The RESET input provides a way to force the output into its reset state.

For normal operation, RESET is commonly held HIGH.

Pulling RESET LOW overrides the normal timing operation.

If the reset function is not required, the pin should normally be connected according to the manufacturer's recommended circuit.

Pin 5 — Control Voltage

The CONTROL pin provides access to the internal reference-voltage network.

It can be used to modify the normal timing thresholds.

In many basic circuits, a small capacitor is connected from the control pin to ground to reduce noise.

Pin 6 — Threshold

The THRESHOLD input monitors the voltage on the timing capacitor.

When the threshold voltage rises above approximately:

 2/3 VCC 

the internal comparator changes state and resets the latch.

Pin 7 — Discharge

The DISCHARGE pin is connected internally to a transistor that can discharge the external timing capacitor.

This pin is particularly important in monostable and astable timing circuits.

Pin 8 — VCC

Pin 8 is connected to the positive supply.

The acceptable supply-voltage range depends on the specific 555 family. Always check the manufacturer's datasheet.

Internal Structure of a 555

The basic internal operation can be represented as:

 VCC │ Voltage Divider │ ┌───────┴───────┐ │ │ 2/3 VCC 1/3 VCC │ │ ▼ ▼ Threshold Trigger Comparator Comparator │ │ └──────┬────────┘ │ SR Latch │ ┌──────┴──────┐ │ │ ▼ ▼ Output Discharge Stage Transistor 

The comparators monitor the trigger and threshold voltages and control the internal latch.

The 1/3 VCC and 2/3 VCC References

The classic bipolar 555 uses an internal voltage divider to establish two reference levels.

 VCC │ │ 2/3 VCC │ │ 1/3 VCC │ │ GND 

These reference levels determine when the trigger and threshold comparators change the state of the internal latch.

How the 555 Starts Timing

Consider a capacitor connected to the timing network.

When the trigger voltage falls below the lower reference level, the internal latch is SET.

The output changes state and the discharge transistor is switched OFF, allowing the timing capacitor to charge.

How the 555 Ends Timing

As the timing capacitor charges, its voltage eventually rises above the upper reference level.

The threshold comparator then resets the internal latch.

The output changes state and the discharge transistor can turn ON, discharging the timing capacitor.

555 Monostable Mode

In monostable mode, the 555 produces one output pulse for each trigger event.

The circuit has one stable state.

 Trigger │ ▼ 555 │ ▼ One output pulse │ ▼ Return to stable state 

This is why monostable operation is often called a one-shot.

555 Monostable Circuit

 VCC │ R │ ├──── THRESHOLD │ C │ GND TRIGGER ───────► Pin 2 RESET ───────── VCC OUTPUT ──────── Pin 3 

The resistor and capacitor determine the pulse duration.

Monostable Timing Formula

For the standard 555 monostable configuration, the approximate output pulse width is:

 t ≈ 1.1 × R × C 

where:

• t = pulse width in seconds
• R = timing resistance in ohms
• C = timing capacitance in farads

Monostable Example

Suppose:

 R = 100 kΩ C = 10 µF 

Then:

 t ≈ 1.1 × 100000 × 0.00001 t ≈ 1.1 seconds 

The output pulse will therefore be approximately one second long, subject to component tolerances and the characteristics of the particular 555.

555 Astable Mode

In astable mode, the 555 continuously switches between HIGH and LOW.

There is no stable state.

 HIGH │ ▼ LOW │ ▼ HIGH │ ▼ LOW │ ▼ ... 

This makes the 555 useful as an oscillator.

555 Astable Circuit

 VCC │ RA │ ├──────── Pin 7 │ RB │ ├──── Pins 2 & 6 │ C │ GND Output = Pin 3 

The capacitor repeatedly charges and discharges between the internal threshold levels.

Astable Frequency

For the standard 555 astable circuit, the approximate frequency is:

 f ≈ 1.44 / ((RA + 2RB) × C) 

where:

• f = frequency in hertz
• RA = resistor A in ohms
• RB = resistor B in ohms
• C = capacitor in farads

Astable High Time

The approximate HIGH time is:

 THIGH ≈ 0.693 × (RA + RB) × C 

Astable Low Time

The approximate LOW time is:

 TLOW ≈ 0.693 × RB × C 

Duty Cycle

The approximate duty cycle of the standard astable circuit is:

 D ≈ (RA + RB) / (RA + 2RB) ``` 
 The standard configuration therefore normally produces a duty cycle above 50%. 
 

555 Bistable Operation

The 555 can also be configured as a bistable circuit.

A bistable circuit has two stable states and can be used as a simple latch or flip-flop.

External circuitry determines when the device changes from one state to the other.

555 as a Square-Wave Oscillator

The astable configuration can generate a continuous square-wave signal.

The frequency can be adjusted by changing the timing resistors or capacitor.

Applications include:

• Clock signals
• LED flashers
• Buzzers
• Pulse generators
• Test signals
• Simple tone generators

555 LED Flasher

An LED can be connected to the output of a 555 astable oscillator through an appropriate current-limiting arrangement.

The output alternates between HIGH and LOW, causing the LED to flash.

The flashing rate is determined by the timing network.

555 Frequency Generator

The 555 can generate frequencies ranging from very low timing pulses to much higher oscillator frequencies, depending on the specific device and component values.

At higher frequencies, the limitations of the particular 555 version, timing components, wiring and supply decoupling become increasingly important.

555 Tone Generator

An astable 555 can generate an audible square-wave signal when its frequency is within the audio range.

For example:

 f ≈ 1 kHz 

can be used to create a simple tone when connected to a suitable audio transducer or amplifier.

555 PWM

The 555 can also be configured to generate pulse-width-modulated signals.

PWM changes the ratio between the HIGH and LOW portions of the waveform.

PWM can be used for:

• LED brightness control
• Motor control
• Simple power control
• Pulse generation

The exact circuit depends on whether frequency, duty-cycle adjustment or both are required.

555 Voltage-Controlled Oscillator

The CONTROL pin can be used to alter the internal timing reference levels. This allows the oscillation frequency to be influenced by an external voltage.

This principle can be used to build simple voltage-controlled oscillator circuits.

555 as a Delay Timer

A monostable 555 can produce a delayed or timed output after receiving a trigger pulse.

The timing period is controlled primarily by the external R-C network.

Applications include:

• Relay delay circuits
• Automatic lights
• Power-on delays
• Alarm timers
• Fan timers

555 Relay Timer

A 555 can control a relay through a suitable transistor or driver stage.

 555 Output │ ▼ Transistor Driver │ ▼ Relay Coil │ ▼ Load 

A flyback diode is normally required across a DC relay coil when it is driven by a transistor.

555 Reset Function

The RESET pin can immediately override the normal timing operation.

This can be useful for:

• Emergency shutdown
• External control
• Enable/disable functions
• Sequencing circuits

555 Control Voltage

The CONTROL pin provides access to the internal reference voltage.

Changing this voltage changes the levels at which the timing comparators operate.

This makes the control pin useful for modulation and specialized timing circuits.

Timing Capacitor

The timing capacitor is one of the most important external components in a 555 circuit.

Its value directly affects:

• Pulse width
• Oscillation frequency
• Timing accuracy

Capacitor tolerance and leakage can therefore affect timing accuracy.

Timing Resistors

The external timing resistors determine the rate at which the capacitor charges and discharges.

Their values should remain within the ranges recommended for the particular 555 device.

Extremely large resistance values can make leakage currents significant.

555 Bipolar vs CMOS

There are two broad technology families of 555 timers:

• Traditional bipolar 555
• CMOS 555

Exact specifications vary considerably between manufacturers and device families, so the datasheet should be checked before substitution.

Common 555 Timer ICs

Commonly encountered devices include:

• NE555
• LM555
• SE555
• ICM7555
• TLC555
• LMC555

These devices can have significantly different electrical characteristics, particularly between bipolar and CMOS families.

555 Supply Voltage

The allowable supply voltage depends on the specific device.

Always check the datasheet before connecting a 555 to a power supply.

CMOS versions are often available for lower-voltage applications, while traditional bipolar versions are commonly used at higher supply voltages.

555 Output Current

The output stage of the classic 555 is relatively strong compared with many logic IC outputs.

However, the maximum source and sink currents depend on the specific device and supply voltage.

Do not assume that every 555 version can safely drive the same load.

Can a 555 Drive a Relay?

A small relay may sometimes be driven through an appropriate transistor driver.

For larger relay coils, motors or other inductive loads, a dedicated driver transistor or MOSFET is recommended.

The 555 should not be exposed to excessive current or inductive voltage transients.

555 Decoupling

A supply bypass capacitor should normally be placed close to the 555's power pins.

A small ceramic capacitor is commonly used for high-frequency decoupling, with additional bulk capacitance where required.

Poor decoupling can cause:

• False triggering
• Output instability
• Timing errors
• Noise
• Oscillation

555 Troubleshooting

When a 555 circuit does not work, begin with the power supply and pin connections.

1. Check VCC.
2. Check ground.
3. Check RESET.
4. Check the trigger voltage.
5. Check the threshold voltage.
6. Check the discharge pin.
7. Check the timing resistor.
8. Check the timing capacitor.
9. Check the output.
10. Check the control-voltage pin.

555 Output Stuck HIGH

Possible causes include:

• Incorrect trigger circuit
• Threshold not reaching the required level
• Timing capacitor not charging correctly
• Discharge circuit problem
• Incorrect RESET connection
• Damaged 555

555 Output Stuck LOW

Possible causes include:

• RESET held LOW
• Trigger permanently active
• Incorrect timing network
• Shorted output
• Damaged IC
• Incorrect wiring

555 Not Oscillating

For an astable circuit, check:

• RA value
• RB value
• Timing capacitor
• Pins 2 and 6
• Pin 7 discharge connection
• RESET pin
• Supply voltage
• Output load

An oscilloscope is useful for observing the capacitor waveform and output signal simultaneously.

555 Timing Is Incorrect

Timing errors can result from:

• Resistor tolerance
• Capacitor tolerance
• Capacitor leakage
• Temperature variation
• Incorrect component values
• Supply variation
• Different 555 characteristics

For precision timing, component tolerance and temperature coefficients should be considered carefully.

Testing a 555 Timer

A multimeter can verify supply voltage and some DC conditions, but a complete 555 functional test generally requires observing the circuit's timing and output behavior.

An oscilloscope is particularly useful.

Useful measurements include:

• VCC
• RESET
• TRIGGER
• THRESHOLD
• CONTROL
• DISCHARGE
• OUTPUT

Testing the Timing Capacitor

A defective timing capacitor can cause the 555 circuit to malfunction.

Possible capacitor faults include:

• Open circuit
• Short circuit
• Excessive leakage
• Incorrect capacitance
• High ESR

Replacing the capacitor with a known-good component of the correct value can help identify timing-network problems.

Common 555 Failure Modes

• Output permanently HIGH
• Output permanently LOW
• No oscillation
• Incorrect frequency
• Excessive supply current
• False triggering
• Damaged output stage
• Internal short circuit

555 Replacement

When replacing a 555 timer, do not assume that every 555 is an identical drop-in replacement.

Check:

• Pinout
• Supply voltage
• Technology type
• Output current
• Timing range
• Input thresholds
• Power consumption
• Frequency requirements
• Package

Can NE555 Replace LM555?

In many standard applications, devices from the same traditional bipolar 555 family may be functionally compatible, but the exact electrical specifications should still be checked.

Never assume interchangeability without comparing the datasheets.

Can CMOS 555 Replace a Bipolar 555?

A CMOS 555 can often perform the same basic timing functions, but it should not automatically be considered electrically identical.

Important differences can include:

• Supply-voltage range
• Output current
• Input thresholds
• Input leakage
• Power consumption
• Frequency capability

The surrounding circuit should be checked before substitution.

555 Timer Selection

When selecting a 555, consider:

• Supply voltage
• Power consumption
• Output current
• Operating frequency
• Timing accuracy
• Input leakage
• Temperature range
• Package
• Single or specialized application requirements

555 Applications

• Time-delay circuits
• One-shot timers
• LED flashers
• Square-wave generators
• Pulse generators
• Frequency generators
• Audio tone generators
• PWM controllers
• Relay timers
• Alarm circuits
• Oscillators
• Simple frequency dividers
• Control circuits
• Switching circuits

Advantages of the 555

• Simple to use
• Low component count
• Widely available
• Inexpensive
• Many configurations possible
• Useful output drive capability
• Available in bipolar and CMOS versions
• Large ecosystem of existing circuits

Limitations of the 555

• Not ideal for precision timing compared with dedicated timer ICs
• Frequency accuracy depends on external components
• Traditional bipolar versions consume more power than CMOS timers
• Output voltage does not necessarily reach the supply rails
• High-frequency operation is limited by the particular device
• Timing capacitors introduce tolerance and leakage errors

555 Timer Calculation Summary

Common 555 Design Mistakes

• Incorrect pin numbering
• Leaving RESET in an undefined state
• Using incorrect timing resistor values
• Using an unsuitable timing capacitor
• Ignoring capacitor leakage
• Ignoring supply decoupling
• Exceeding the output-current rating
• Connecting an inductive load without appropriate protection
• Assuming all 555 variants have identical specifications
• Using the standard formulas outside their practical operating range

Key Points

• The 555 is a versatile timer and oscillator IC.
• The standard device has eight pins.
• Trigger controls the SET action of the internal latch.
• Threshold controls the RESET action.
• Discharge controls the timing capacitor's discharge path.
• RESET can override normal operation.
• The CONTROL pin can modify the timing reference.
• Monostable mode produces one pulse for each trigger.
• Astable mode continuously oscillates.
• Bistable operation can provide two stable states.
• External resistors and capacitors determine timing.
• The classic monostable formula is approximately t = 1.1RC.
• The standard astable frequency is approximately 1.44 / ((RA + 2RB)C).
• CMOS 555 devices generally consume less power than traditional bipolar versions.
• Always check the exact datasheet before replacing a 555.