NE555

The NE555 integrated circuit, more commonly called the 555 timer, is an extremely versatile and popular IC used for a wide variety of timing, delay, pulse generation, and oscillator applications. It was first introduced in 1971 by Signetics (which was later acquired by Philips, and now NXP Semiconductors) and has remained an electronics staple due to its ease of use, low cost, and stability.

1. Internal Architecture

The 555 IC is a monolithic circuit containing a combination of analog and digital components. Internally, it consists of:

Two voltage comparators: They compare the voltage at their input pins with reference voltages.
An SR flip-flop: This is a digital circuit with two stable states, used to store one bit of information and change state based on comparator outputs.
A discharge transistor (NPN): This transistor is used to discharge an external timing capacitor.
A voltage divider network: Composed of three identical resistors, this network creates two reference voltages: 1/3 of the supply voltage (VCC) and 2/3 of VCC. These reference voltages are used by the comparators.
An output driver stage: This stage provides a high-current output capable of driving various loads.

2. Pin Configuration (DIP-8 Package)

The standard NE555 is available in an 8-pin Dual In-line Package (DIP). The pin configuration is as follows:

Pin #	Name	Function
1	GND	Ground (0V)
2	TRIG	Trigger (active-low input)
3	OUT	Output signal
4	RESET	Reset (active-low)
5	CTRL (CV)	Control voltage
6	THR	Threshold (comparator input)
7	DIS	Discharge (NPN transistor)
8	VCC	Power supply (+4.5V to +15V)

GND (Ground): Connected to the negative power rail (0V).
Trigger (TRIG): An input that initiates the timing cycle. Triggering is active-low; timing begins when the voltage on this pin falls below 1/3 VCC.
Output (OUT): The output pin where the timed pulse or oscillation is available. The output can sink or source up to 200mA, making it capable of directly driving LEDs, relays, or small speakers.
Reset (RST): An active-low input that can interrupt the timing cycle and reset the output to LOW, regardless of the trigger and threshold inputs' states.
Control Voltage (CV): An input that allows the user to modify the internal 2/3 VCC reference voltage used by the threshold comparator. By applying an external voltage to this pin, the timing characteristics can be altered. When unused, it's often connected to ground via a small capacitor (typically 0.01 µF) for noise filtering.
Threshold (THR): An input used to determine the end of the timing cycle in monostable mode or to help set the oscillation frequency in astable mode. When the voltage on this pin exceeds 2/3 VCC (or the voltage set by the control voltage pin), the flip-flop is reset.
Discharge (DIS): An open-collector output connected to the drain of the internal NPN discharge transistor. It's used to discharge the external timing capacitor at specific times during the timing cycle.
VCC (Supply Voltage): Connected to the positive power rail. The 555 timer can operate over a wide supply voltage range, typically from +4.5V to +16V (some versions can go up to +18V).

3. Operating Modes

The NE555 can be configured to operate in three main modes:

Monostable Mode (One-Shot): In this mode, the 555 acts as a single-pulse generator ("one-shot"). When a trigger signal (a brief LOW pulse on the TRIGGER input) is applied, the output goes HIGH and remains so for a predetermined duration. This duration is determined by the values of an external resistor (R1) connected between VCC and the DISCHARGE pin, and an external capacitor (C1) connected between the DISCHARGE pin and GND. The output remains HIGH for approximately T = 1.1 * R1 * C1. After this time, the output goes LOW and stays LOW until another trigger signal is received. Applications include timers, delay circuits, pulse stretchers, and missing pulse detectors.
Astable Mode (Free-Running Oscillator): In astable mode, the 555 oscillates continuously, producing a square wave output. The frequency and duty cycle of the output waveform are determined by two external resistors (RA and RB) and an external capacitor (C). RA is connected between VCC and the DISCHARGE pin, RB between the DISCHARGE pin and the THRESHOLD pin, and C between the THRESHOLD pin and GND.
- The HIGH time (output HIGH) is approximately T_high = 0.693 * (RA + RB) * C.
- The LOW time (output LOW) is approximately T_low = 0.693 * RB * C.
- The oscillation frequency is approximately f = 1 / (T_high + T_low) = 1.44 / ((RA + 2RB) * C).
- The duty cycle (percentage of time the output is HIGH) is approximately D = (RA + RB) / (RA + 2RB).
Bistable Mode (SR Flip-Flop): Although less commonly used directly as a standalone SR flip-flop (as dedicated ICs exist for this purpose), the 555 can be wired to operate as a Set-Reset flip-flop. The TRIGGER and RESET inputs can be used as SET and RESET inputs respectively, and the OUT pin provides the flip-flop's state. The THRESHOLD pin is generally unused in this configuration.

4. NE555 Applications

Due to its versatility, the NE555 is used in numerous applications, including:

Timers and delays: Controlling circuit activation/deactivation durations.
Oscillators: Generating clock signals for various systems.
Pulse generators: Creating pulses of specific durations and frequencies.
Alarms and sounders: Producing audio signals.
Motor control: Pulse-width modulation (PWM) for speed control.
Strobes and flashers: Generating pulses to turn lights on/off at specific frequencies.
Relay control circuits: Providing pulses to activate/deactivate relays.
Electronic toys and games.
Automotive electronics.

5. NE555 Advantages

Ease of use: Requires few external components for many configurations.
Versatility: Can be configured in multiple modes for various timing and oscillation applications.
Stability: Provides relatively stable timing with proper external components.
Wide supply voltage range: Operates with different voltages.
High output current capability: Can directly drive modest loads.
Low cost: A very affordable integrated circuit.
Wide availability: Easily obtainable from many suppliers.

Summary

The NE555 is an extremely popular and versatile timer IC that can be used in a wide range of timing, pulse generation, and oscillation applications. Its simplicity, robustness, and low cost have made it a staple for electronics hobbyists, students, and professionals for decades.