The 555 Timer Integrated Circuit (IC) is a highly stable and versatile timing IC designed by Hans Camenzind in 1971 and commercially released by Signetics Corporation as the SE555/NE555. Due to being the first and only commercial timing IC available at the time, it was also referred to as the "IC Time Machine." Today, it is used in a wide range of electronic devices, from toys and kitchen appliances to spacecraft. The 555 timer can operate in three distinct modes: monostable, astable, and bistable.
Internal Structure and Pin Functions
The 555 timer has a complex internal structure consisting of 25 transistors, 2 diodes, and 15 resistors. As a block diagram, it comprises two comparators, a flip-flop, a voltage divider, a discharge transistor, and an output stage. The voltage divider creates two reference voltages at 1/3 and 2/3 of the supply voltage (VCC) using three identical 5 kΩ resistors. The supply voltage can range from 4.5 to 18 volts.
The 555 timer has eight pins with the following primary functions:
- Pin 1 (Ground/GND): Connected to the negative supply potential, i.e., circuit ground.
- Pin 2 (Trigger): Input of the lower comparator, used to set the flip-flop and raise the output (Pin 3) to a HIGH state. Triggering occurs when the voltage at this pin drops below 1/3 of VCC. In monostable mode, it marks the start of the timing sequence.
- Pin 3 (Output): Output of the 555 timer. When the chip is in HIGH state, it provides a voltage approximately 1.5–1.7 V below VCC; in LOW state, it provides approximately 0 V. It can source or sink up to 200 mA of current.
- Pin 4 (Reset): Used to reset the flip-flop and force the output to a LOW state. Pulling this pin to a voltage below approximately 0.7 V resets the circuit. It is typically held at VCC to prevent accidental resets.
- Pin 5 (Control Voltage): Provides direct access to the 2/3 VCC reference point of the voltage divider, which sets the upper comparator’s threshold. This pin allows adjustment of the timing period and is usually connected to ground via a 0.01 µF capacitor to suppress noise.
- Pin 6 (Threshold): Input of the upper comparator, used to reset the flip-flop and lower the output to a LOW state. This occurs when the voltage at this pin exceeds 2/3 of VCC.
- Pin 7 (Discharge): Connected to the open collector of an NPN transistor. When this transistor turns on, it shorts Pin 7 to ground. The timing capacitor is typically connected between this pin and ground and discharges through it when the transistor conducts.
- Pin 8 (Supply/VCC): Positive supply voltage terminal of the 555 timer IC.
555 Timer Integrated Circuit
Operating Modes
1. Monostable Mode (Monostable Multivibrator)
The monostable multivibrator (MMV) is often called a one-shot multivibrator. In this mode, the output has one stable state and one quasi-stable state. The output is normally LOW. When an external triggering pulse (negative-going) is applied to Pin 2, the output at Pin 3 goes HIGH and remains so for a predetermined time before returning to LOW. This duration is determined by an external RC network consisting of resistor RA and capacitor C.
- Operation: Initially, when Pin 3 is LOW, the internal discharge transistor is ON and the capacitor C is shorted to ground. When a negative pulse is applied to Pin 2, the voltage drops below 1/3 of VCC, causing the lower comparator to trigger and reset the flip-flop. As a result, the discharge transistor turns OFF and the output at Pin 3 goes HIGH. This transition moves the output from its stable state to the quasi-stable state. With the discharge transistor OFF, capacitor C begins charging through resistor RA toward +VCC. When the capacitor voltage exceeds 2/3 of VCC, the upper comparator triggers, setting the flip-flop. The transistor saturates, discharging capacitor C, and the timer output returns to LOW. Thus, the output returns from the quasi-stable state to the stable state.
- Pulse Width Calculation: The duration for which the output remains HIGH (tp) is calculated by the formula: tp = 1.0986 * RA * C (approximately 1.1 * RA * C), where RA is in ohms and C is in farads. Pulse widths can range from microseconds to several seconds.
- Applications: Monostable mode is used in circuits requiring timed operations or to turn external components (LEDs, motors, etc.) on for a fixed duration. For example, it can be used to create a circuit where an LED lights up for a set time after pressing a button or to trigger an alarm.
2. Astable Mode (Astable Multivibrator)
In astable mode, the 555 timer functions as an oscillator or free-running multivibrator. In this mode, the IC has no stable state and continuously switches between HIGH and LOW states without any external trigger, producing a continuous stream of pulses or oscillations.
- Operation: The Trigger (Pin 2) and Threshold (Pin 6) pins are connected together and to an external capacitor. When power is first applied, the capacitor is uncharged, so the voltage at Pins 2 and 6 is near zero. The lower comparator triggers the flip-flop, causing the output at Pin 3 to go HIGH and transistor T1 to turn OFF. The capacitor begins charging through resistors R1 and R2. When the capacitor voltage reaches 2/3 of VCC, the upper comparator resets the flip-flop, causing the output to go LOW and T1 to turn ON. The capacitor now discharges through resistor R2 and the discharge transistor on Pin 7. When the capacitor voltage drops below 1/3 of VCC, the lower comparator triggers again, setting the flip-flop, raising the output, turning T1 OFF, and restarting the charging cycle. This continuous charge-discharge cycle generates a square wave output at Pin 3.
- Frequency and Period Calculation: The high and low times (t1 and t2) and frequency (f) are calculated as follows:
- t1 (High Time) = 0.693 * (R1 + R2) * C
- t2 (Low Time) = 0.693 * R2 * C
- Period (T) = t1 + t2
- Frequency (f) = 1 / T or f = 1 / (0.693 * C * (R1 + 2 * R2))
- Duty Cycle = t1 / T = (R1 + R2) / (R1 + 2R2)
- Applications: Astable mode is used in applications requiring continuous signals such as LED flashers, audio generators, motor control, and sirens. Frequency and duty cycle can be adjusted using a variable resistor (potentiometer).
3. Bistable Mode (Bistable Multivibrator)
In bistable mode, the output of the 555 timer is stable in both HIGH and LOW states. The output changes state only when triggered by an external trigger or reset pulse and remains in the new state until the next trigger. This mode provides no timing function and causes the 555 timer to operate like a flip-flop.
- Operation: Typically, the Trigger (Pin 2) and Reset (Pin 4) pins are connected to external push buttons. Pin 6 (Threshold) is connected to ground. When a LOW signal is applied to Pin 2 (Trigger), the output goes HIGH. When a LOW signal is applied to Pin 4 (Reset), the output goes LOW.
- Applications: This mode can be used in applications requiring separate on/off switches for a machine or to change direction in a robot upon collision detection. However, simpler and more appropriate circuits are available for these functions.
Output Current and Driving High Loads
The output at Pin 3 of the 555 timer can source or sink up to 200 mA of current. This is sufficient for small LEDs or low-current devices. However, to drive higher loads such as motors, LED strips, or other devices requiring more than 200 mA, it is recommended to connect a transistor (NPN transistor or MOSFET) to the output. This increases the current-handling capacity of the 555 timer and enables control of larger loads.
