⏱️ 555 Timer: The Workhorse IC

The 555 timer is one of the most popular ICs ever made.

Introduced in 1972, it's still manufactured by the billions every year.

Why? Because it's:

• Versatile (timing, oscillation, pulse generation)
• Simple (minimal external components)
• Reliable (works from 5V to 15V)
• Cheap (cents per chip)
• Robust (hard to kill)

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🎯 What Can the 555 Do?

The 555 timer can operate in three modes:

Mode	Function	Output
Monostable	One-shot pulse generator	Single pulse when triggered
Astable	Free-running oscillator	Continuous square wave
Bistable	Flip-flop	Toggle between states

We'll focus on monostable and astable (most common).

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🏗️ Inside the 555 Timer

The Block Diagram

The Pins (8-pin DIP)

Pin	Name	Function
1	GND	Ground
2	TRIG	Trigger input (starts timing)
3	OUT	Output (high or low)
4	RESET	Reset (active low)
5	CTRL	Control voltage (usually bypassed)
6	THRES	Threshold (stops timing)
7	DISCH	Discharge (internal transistor to ground)
8	Vcc	Power supply (+5V to +15V)

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🔍 How It Works Internally

The Voltage Divider

Three 5kΩ resistors divide Vcc:

• Upper comparator reference: $\frac{2}{3}V_{cc}$
• Lower comparator reference: $\frac{1}{3}V_{cc}$

Comparator Action

Upper comparator:

• Monitors THRESHOLD (pin 6)
• When THRESHOLD > $\frac{2}{3}V_{cc}$ → resets flip-flop → output goes LOW

Lower comparator:

• Monitors TRIGGER (pin 2)
• When TRIGGER < $\frac{1}{3}V_{cc}$ → sets flip-flop → output goes HIGH

Discharge Transistor

• Connected to pin 7
• ON when output is LOW → grounds external timing capacitor
• OFF when output is HIGH → allows capacitor to charge

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The Magic Numbers

Remember these voltages for a 555 on 5V supply:

• Trigger threshold: $\frac{1}{3} \times 5V = 1.67V$
• Upper threshold: $\frac{2}{3} \times 5V = 3.33V$

These determine when the timer starts and stops!

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🎯 Monostable Mode: The One-Shot

What It Does

When triggered, output goes HIGH for a precise time period, then returns to LOW.

Perfect for:

• Debouncing buttons
• Generating precise delays
• Pulse stretching
• Timeout circuits

The Circuit

• Vcc to pin 8 and pin 4 (RESET tied high)
• Pin 2 (TRIG) through pull-up resistor to Vcc
• Trigger button from pin 2 to ground
• Pin 5 (CTRL) bypassed with 0.01µF to ground
• Timing components: R between Vcc and pin 7/6
• Timing capacitor C between pin 6/7 and ground
• Pin 3 (OUT) to load (LED + resistor)
• Pin 1 to ground

The Timing Formula

$$T = 1.1 \times R \times C$$

Where:

• $T$ = output pulse width (seconds)
• $R$ = timing resistor (ohms)
• $C$ = timing capacitor (farads)

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📐 Design Example: 10-Second Timer

Goal: LED turns on for 10 seconds when button pressed

Design:

Choose $C = 100\mu F$ (common value)

Calculate $R$:

$$R = \frac{T}{1.1 \times C} = \frac{10s}{1.1 \times 100 \times 10^{-6}} = 90.9k\Omega$$

Use standard value: $R = 91k\Omega$

Result:

• Press button → LED on for ~10 seconds
• Automatically turns off
• Can be re-triggered immediately

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⏱️ Timing Component Selection

Timing Range	Resistor	Capacitor
Microseconds (1-100µs)	1kΩ - 100kΩ	10pF - 1nF
Milliseconds (1-100ms)	10kΩ - 100kΩ	10nF - 1µF
Seconds (1-100s)	100kΩ - 10MΩ	1µF - 1000µF
Minutes (1-10min)	1MΩ - 10MΩ	10µF - 100µF

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Practical Limits

• Minimum R: 1kΩ (chip can source ~200mA max)
• Maximum R: 10MΩ (leakage currents cause errors)
• Minimum C: 100pF (stray capacitance matters)
• Maximum C: 1000µF+ (watch leakage)

For very long times, consider using a microcontroller instead!

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🔄 How Monostable Works (Step by Step)

Initial State: Output LOW, capacitor discharged

1. Trigger Applied (pin 2 pulled below 1/3 Vcc):

• Lower comparator triggers
• Flip-flop sets
• Output goes HIGH
• Discharge transistor turns OFF

2. Capacitor Charges (through R):

• Voltage rises exponentially toward Vcc
• Output stays HIGH

3. Threshold Reached (pin 6 exceeds 2/3 Vcc):

• Upper comparator triggers
• Flip-flop resets
• Output goes LOW
• Discharge transistor turns ON
• Capacitor rapidly discharges

4. Ready for Next Trigger

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🚀 Monostable Applications

1. Button Debouncer

Mechanical switches bounce: one press = multiple pulses.

Solution: 555 monostable with T = 10-50ms

• First bounce triggers timer
• Output stays HIGH (ignoring subsequent bounces)
• Clean single pulse to microcontroller

2. Missing Pulse Detector

Monitor periodic signals (heartbeat, sensor pulses).

Setup: Trigger from signal, T = 1.5× expected period

• Normal pulses keep retriggering → output stays HIGH
• Missing pulse → timeout → output goes LOW (alarm!)

3. Precise Delay

Need a delay that's independent of software?

Use 555: Button press → delay → activate relay

• Purely hardware
• No microcontroller needed
• Very reliable

4. Touch Switch

Capacitive touch sensor triggers 555 monostable.

• Touch detected → LED/device on for set time
• Auto-off after timeout

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💡 Design Tips for Monostable

1. Prevent False Triggering:

   • Pull pin 2 HIGH with 10kΩ resistor
   • Add 0.01µF capacitor from pin 5 to ground

2. Reset Pin:

   • Tie pin 4 to Vcc (through 10kΩ) for normal operation
   • Ground pin 4 to force output LOW (external reset)

3. Bypass Control Voltage:

   • Always add 0.01µF cap from pin 5 to ground
   • Prevents noise-induced false triggers

4. Timing Accuracy:

   • Use 1% tolerance resistors
   • Use quality capacitors (film or ceramic C0G)
   • Actual time = 1.1RC ± 1%

5. Retriggering:

   • 555 can be retriggered during timing
   • New trigger extends the timing period
   • Useful for watchdog timers

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🔧 Troubleshooting Monostable Circuits

Problem	Likely Cause	Solution
Output always HIGH	Trigger pin floating low	Add pull-up resistor to pin 2
Output always LOW	Reset pin floating low	Tie pin 4 to Vcc
Random triggering	Noise on trigger/control	Add bypass caps, check wiring
Wrong timing	Wrong R or C values	Recalculate, measure components
Won't trigger	Trigger not going low enough	Check trigger level (<1/3 Vcc)
Won't reset	Capacitor not charging/discharging	Check connections to pins 6/7

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🧪 Lab Exercise 1: Build a Reaction Timer

Objective: Measure reaction time

Circuit:

• Button triggers monostable
• LED lights for exact 1 second
• User must press stop button before LED goes off
• If successful, second LED lights

Components:

• NE555 timer
• Resistors: 10kΩ (pull-up), 91kΩ (timing), 330Ω (LED)
• Capacitors: 10µF (timing), 0.01µF (bypass)
• LEDs, buttons
• Optional: Add second 555 for scoring

Learning Goals:

• Monostable operation
• Timing component calculation
• Button interfacing
• Cascading logic

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🎲 Fun Project: Electronic Dice

Circuit:

1. 555 astable oscillator (very fast, we'll learn this next!)
2. Feeds binary counter (we'll learn digital circuits later)
3. Press button → counter stops at random number
4. Display on 7-segment display

Your contribution: The 555 timing circuit!

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✅ Monostable Summary

Key Points:

• One-shot pulse when triggered
• Output HIGH time = $1.1RC$
• Trigger: Pull pin 2 below 1/3 Vcc
• Threshold: When pin 6 exceeds 2/3 Vcc, output goes LOW
• Can be retriggered during timing
• Pin 4 (RESET) forces output LOW when grounded

Common Uses:

• Delays and timeouts
• Debouncing
• Pulse stretching
• Missing pulse detection
• Touch switches

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🎓 Looking Ahead

In the next lesson, we'll explore Astable Mode where the 555 generates continuous oscillations!

We'll build:

• Flashing LEDs
• Square wave generators
• PWM controllers
• Alarm circuits
• And more!

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📚 Further Reading

• Build a monostable circuit and measure timing accuracy
• Experiment with different R and C values
• Try retriggering during the timing period
• Measure voltages at pins 2, 6, and 7 with oscilloscope
• Design a practical debouncer for your next microcontroller project