555 Timer and Its Applications

The 555 timer is a linear IC, which works as a monostable multivibrator, an astable multivibrator, a Schmitt trigger, a function generator with output waveforms (such as square waves), time delay pulses, and pulse-width modulation (PWM) and pulse position modulation (PPM) circuits have various electronic control applications. Every year, millions of 555 IC units are being produced by various manufacturers to meet the industrial and commercial applications.

The 555 IC was designed by Hans R. Camenzind in the year 1971 while working for Signetics Corporation. In the early 1970s, Signetics Corporation produced 555 timer in the trade names SE555 timer and NE555 timer for military and commercial applications, respectively.

The 555 Timer is a precision timing circuit that can produce pulses of accurate and highly stable time delays from microseconds to hours. It is mostly used in practical circuits as flip-flop in monostable, bistable, and astable forms. From its applications, it is known as the IC time machine.

The 555 IC is used mostly for timer functions in commercial electronic circuits. In the timer applications, the duration or length of the output pulses is determined by charging and discharging a capacitor through resistors connected externally to a 555 timer. The duty cycle of the output pulse is adjustable by timing circuit components R and C. The 555 timers to operate on supply voltages ranging from +5 V to +18 V. They are compatible with TTL (Transistor-Transistor Logic) and CMOS (Complementary Metal Oxide Semiconductor) logic circuits.

Packages of 555 Timer

The 555 ICs are available in a standard dual in-line package (DIP), 8-pin mini DIP or 14-pin DIP. Today, the 555 and 556 DIP packages are the most popular packages. The SE555 and NE555 timer ICs are available in both metal can package known as T package and epoxy plastic package known as V package. The type of packaging and application is indicated in military or commercial use by their trade names such as NE555V, NE555T, SE555V, and SE555T.

Meanings of Pin Connections of IC 555

• Ground: The pin (1) is connected to the common or negative terminal of a power supply. At the ground terminal, the voltage level will be at zero potential. Voltages will be measured with reference to this terminal.
• V_CC: The power supply voltage (V_CC) to 555 IC is connected externally to IC pin (8). It ranges from 5 V to 15 V (4.5 V to 16 V), and for some military-designed packages, it extends up to 18 V.
• Output: The primary output of the 555 IC timer can be high level or low level. The output from the IC is available at pin (3). 

1. High level (state): When the supply voltage V_CC = 5 volts and 15 volts, respectively, the high state will be 3.3 V and 13.3 V. It means that the output voltage will be less than 1.7 volts below the supply voltage V_CC. The output saturation levels depend on the magnitudes of V_CC.
2. Low level (state): When the supply voltage VCC = 5 V, the low state will be 0.25 V at 5 mA and would sink up to 200 mA. When V_CC = 15 V, the output low voltage will be of the order of 2 V.
3. Rise and fall times are as fast as 100 ns.

• Trigger: Trigger input voltage is connected to lower (or trigger) comparator. It is connected to 555 IC pin (2). It controls the output states of R–S flip-flop. When the trigger input falls below \left ( \frac{1}{3}V_{cc} \right ), the output voltage rises and interval in output pulse starts. The trigger could be accomplished from a slow ROC (rate of changing) waveform or even from pulses. The triggering voltage lies between +V_CC and the ground terminal. The current requirement is typically 500 nA.
• Control voltage: A control voltage pin (555 IC Number 5) is connected to the reference point on the input side of the upper (or trigger) comparator. It is connected to a point where 2/3 of VCC is available on the voltage divider circuit consisting of three resistors of 5 kΩ each. It has indirect access to the lower (or trigger) comparator reference voltage also. The voltage control ranges from 1 V below VCC down to 2 V above the ground voltage. External signal to control voltage pin, IC output timings can be altered in monostable operation. The control voltage may vary from 45% to 90% of VCC. In an astable mode, applying an external control voltage will make it work as a frequency modulator (FM). If this pin is not used, then a capacitor around 10 nF is connected from this pin to the ground to reduce any parasitic noise and false triggers.
• Reset: This pin (4) is used to set the output to the low state by resetting (disabling) the flip-flop circuit, regardless of the states of any other input. Reset pulse voltage greater than 0.5 V with more than 0.1 mA current capability is needed to obtain the latch reset (flip-flop output). The pulse width should be typically more than 0.5 μs.
• Threshold: The threshold voltage is one of the inputs to the upper (threshold) comparator. It is applied at pin (6). When this voltage is below to above of V_CC, it will reset the flip-flop and will set the output to a low state.

Block Diagram of 555 IC Timer

The internal blocks of 555 IC are as follows: 

1. Two comparators (trigger comparator and threshold comparator)
2. R–S flip-flop
3. Three 5 kΩ resistors
4. Reset transistor
5. Discharge transistor
6. Power amplifier
7. OR Gate

Applications of 555 IC Timer

1. Monostable multivibrator: It works as a one-shot pulse generator.
2. Astable multivibrator: It works as a free-running pulse generator (oscillator).
3. Bistable multivibrator: It works as a flip-flop (Schmitt trigger).

Other applications of 555 IC timer are found in:

1. DC-DC converters and digital logic probes
2. Waveform generators (ramp and square wave generator)
3. Converts an analog voltage to a pulse length in analog to digital conversion
4. Analog frequency meters and tachometers
5. Accurate clock signals
6. De-bounce switches
7. PWM (Pulse Width Modulation) and PPM (Pulse Position Modulation) circuits
8. Traffic signal light control circuits
9. Temperature measurement and control devices