COMARC - 555 Timer IC

The integrated circuit designed by Hans Camenzind in 1972

one of the most popular and versatile ICs ever produced

The commercial-grade version of the 555 timer with an operating temperature range of 0°C to 70°C

NE-555

The military-grade version of the 555 timer with an operating temperature range of -55°C to 125°C

SE-555

The input pin that sets the output HIGH when its voltage drops below 1/3 Vcc

Trigger (Pin 2)

The input pin that resets the output LOW when its voltage rises above 2/3 Vcc

Threshold (Pin 6)

The output pin connected to an internal NPN transistor that drains the external timing capacitor

Discharge (Pin 7)

The active-low input pin that forces the output LOW regardless of other inputs and should be tied to Vcc if unused

Reset (Pin 4)

The pin that allows external adjustment of the internal reference voltages and is often bypassed with a capacitor

Control Voltage (Pin 5)

The internal memory element that stores the current output state

set by the Trigger comparator and reset by the Threshold comparator

The internal circuit consisting of three 5 kΩ resistors connected in series between Vcc and Ground that creates reference voltages of 1/3 Vcc and 2/3 Vcc

Voltage divider

The internal circuit that drives the Output pin (Pin 3) high or low based on the flip-flop's state

Output Driver

The internal component that compares the Trigger pin voltage against the 1/3 Vcc reference and feeds the Set input of the flip-flop

Comparator 1 (Trigger comparator)

The internal component that compares the Threshold pin voltage against the 2/3 Vcc reference and feeds the Reset input of the flip-flop

Comparator 2 (Threshold comparator)

The 555 configuration with no stable state where the output continuously oscillates producing a square wave without external triggering

Astable Multivibrator

The 555 configuration with one stable state (low) that produces a single output pulse per external trigger

Monostable Multivibrator

The 555 configuration with two stable states (high and low) that behaves like a simple flip-flop or on-off switch

Bi-stable Multivibrator

In astable mode

the resistor that connects +Vcc to the Discharge pin (Pin 7) with a typical value of 1 kΩ

In astable mode

the resistor that connects the Discharge pin (Pin 7) to the joined Threshold/Trigger node with a typical value of 100 kΩ

The timing capacitor in astable mode with a typical value of 1 μF (electrolytic) that connects the Threshold/Trigger node to Ground

C1

The pin that is tied directly to +Vcc in astable mode so the timer is never held in reset

Reset (Pin 4)

The capacitor placed on Pin 5 (Control Voltage) in astable mode to filter the output signal

0.01 μF capacitor

The capacitor placed from Vcc to Ground in astable mode to filter supply voltage "shoot up" (noise)

0.1 μF capacitor

In monostable mode

the timing resistor with a typical value of 1 MΩ that connects +Vcc to the joined Discharge/Threshold node

In monostable mode

the timing capacitor with a typical value of 0.1 μF on Pin 6 that connects the Discharge/Threshold node to Ground

In monostable mode

the component that pulls the Trigger pin (Pin 2) up to +Vcc so it idles HIGH

In monostable mode

the input device used to momentarily pull the Trigger pin LOW (below 1/3 Vcc) to start the pulse

In monostable mode

the additional component placed on the trigger switch to prevent false triggering from contact bounce

In bi-stable mode

the component that pulls the Trigger pin (Pin 2) up to +5V so it idles HIGH

In bi-stable mode

the component that pulls the Reset pin (Pin 4) up to +5V so it idles HIGH (not held in reset)

In bi-stable mode

the switch that momentarily connects Pin 2 (Trigger) to ground to set the output HIGH

In bi-stable mode

the switch that momentarily connects Pin 4 (Reset) to ground to reset the output LOW

The pin that is tied directly to Ground in bi-stable mode (permanently disabled so the timer never resets itself)

Threshold (Pin 6)

The pin that is left unconnected/unused in bi-stable mode because there is no external timing capacitor

Discharge (Pin 7)

In bi-stable mode

the LED and resistor connected from Output (Pin 3) to Ground to visually indicate the output state

The package type of the 555 timer IC with 8 pins (four on each side)

DIP-8 package

The reference voltage at which the Trigger comparator trips to set the output HIGH

1/3 Vcc

The reference voltage at which the Threshold comparator trips to reset the output LOW

2/3 Vcc

The origin of the name "555 timer" derived from the three internal resistors that form the voltage divider

Three 5 kΩ resistors

The process in astable mode where the capacitor charges through both R1 and R2 while the Output is HIGH

Charging phase

The process in astable mode where the capacitor discharges through R2 only through Pin 7 while the Output is LOW

Discharging phase

The proportion of time the astable output stays HIGH versus LOW during one oscillation cycle

Duty Cycle

A 555 configuration used for generating clock pulses

PWM signals

A 555 configuration used for pulse generation

timers

A 555 configuration used for flip-flops

memory circuits

The condition in the resting state of the monostable circuit where the Trigger pin is held HIGH by its pull-up resistor and the Output stays LOW

Idle state

The condition where Threshold and Trigger pins are tied together at the same node as the timing capacitor in astable mode

Self-triggering loop

The critical wiring difference in bi-stable mode where grounding the Threshold pin permanently disables the Threshold comparator

Threshold grounded

The number of stable states in Astable mode

0

The number of stable states in Monostable mode

1

The number of stable states in Bi-stable mode

2

The output behavior of Astable mode where the output continuously oscillates

Continuously oscillates

The output behavior of Monostable mode where the output produces one pulse per trigger and then returns low

One pulse per trigger

The output behavior of Bi-stable mode where the output toggles and holds until switched again

Toggles and holds until switched again

The Threshold pin connection in Astable mode where it is tied to the timing capacitor node with Trigger

Tied to timing capacitor node (with Trigger)

The Threshold pin connection in Monostable mode where it is tied to the timing capacitor node with Discharge

Tied to timing capacitor node (with Discharge)

The Threshold pin connection in Bi-stable mode where it is tied directly to Ground

Tied directly to Ground (disabled)

The typical use for Astable mode

Clock pulses

The typical use for Monostable mode

Delay circuits

The typical use for Bi-stable mode

Flip-flops

The memory tip for counting stable states: A-stable = 0

Mono-stable = 1

In astable mode

the capacitor charges through both R1 and R2

The memory tip that describes astable charging and discharging

"charge long

The component that provides a path to ground to discharge the external timing capacitor when turned on

NPN Discharge transistor

The key difference between astable and monostable regarding Trigger and Threshold pins

In astable they are tied together

in monostable they are separate

The reason the astable output is not a perfectly symmetric 50/50 square wave

Capacitor charges through R1+R2 but discharges through R2 only

The NE-555 variant has a narrower operating temperature range than the SE-555 variant

False

The NE-555 and SE-555 variants have identical internal operation

True

The 555 timer gets its name from the five 5Ω resistors inside the IC

False

The Trigger pin (Pin 2) sets the output HIGH when its voltage drops below 1/3 Vcc

True

The Threshold pin (Pin 6) sets the output HIGH when its voltage rises above 2/3 Vcc

False

The Reset pin (Pin 4) is active-high and should be grounded if unused

False

The Control Voltage pin (Pin 5) allows external adjustment of the internal reference voltages

True

The 555 timer's internal voltage divider uses four 5kΩ resistors

False

The internal SR Flip-Flop is set by the Threshold comparator and reset by the Trigger comparator

False

The Discharge pin (Pin 7) is connected to an internal NPN transistor that provides a path to ground

True

The astable multivibrator has no stable state and continuously oscillates

True

The monostable multivibrator has two stable states and toggles between them

False

The bi-stable multivibrator behaves like a simple flip-flop or on-off switch

True

In astable mode

the Reset pin (Pin 4) is tied to Ground

In astable mode

a 0.01 μF capacitor is placed on Pin 5 (Control Voltage) to filter the output signal

In astable mode

a 0.1 μF capacitor is placed from Vcc to Ground to filter supply voltage "shoot up"

In monostable mode

the timing capacitor charges through R1 and discharges through R2

In monostable mode

the Trigger pin is held LOW by its pull-down resistor in the resting state

In monostable mode

pressing the push button pulls the Trigger pin below 1/3 Vcc to start the pulse

In bi-stable mode

the Threshold pin is tied directly to Ground

In bi-stable mode

the Discharge pin (Pin 7) is used as the timing capacitor connection

In bi-stable mode

pressing the Set switch (Trigger) makes the Output go LOW

In bi-stable mode

pressing the Reset switch (Pin 4) forces the Output LOW

The astable mode is used for delay circuits and pulse generation

False

The monostable mode is used for flip-flops and memory circuits

False

The bi-stable mode is used for on-off switches and memory circuits

True

All three 555 modes use the same internal comparators and flip-flop — only external pin wiring changes

True

In astable mode

the capacitor charges through R2 only and discharges through R1 + R2

The bi-stable mode operates as a push-button light switch: one button turns the LED ON (Set/Trigger)

the other turns it OFF (Reset)

In monostable mode

the Trigger and Threshold pins are tied together at the same timing node

In astable mode

the output stays HIGH while the capacitor is charging and stays LOW while the capacitor is discharging

The Discharge transistor turns ON when the internal flip-flop is SET (Output HIGH)

False

The Control Voltage pin (Pin 5) is often bypassed with a capacitor to filter out noise and stabilize the reference voltages

True

The 555 timer can only operate in astable mode