User:Pramodkumar1804

555 INTEGRATED CIRCUIT (TIMER OPERATION)

The 555 integrated circuit is an extremely versatile timer that can be used in many different applications. This IC is a monolithic timing circuit that is a highly stable controller capable of producing accurate time delays or oscillations. Additional terminals are producing are provided for triggering or resetting if desires. In the time delay mode of resistance and a capacitor. For a stable operation as an oscillator, the free running frequency and the duty cycle are both accurately controlled with two external resistors and one capacitor. The circuit may be triggered and reset on falling waveforms, and the output structure can source or sink up to 200ma or drive TTL Circuits.

This integrated circuit contains nearly 25 transistor, a diode or two, and more than 10 resistors. Obviously, if you built this IC from separate components, it would be many, many times larger than on a monolithic chip.

The 555 timer offers timing from microseconds through hours and operates in both astable and monostable modes. It has an adjustable duty cycle, and the output can drive TTL devices. Its output can operate in normally on and normally off modes and the IC offers a frequency stability of 0.005% per degrees centigrade.

Applications for the 555 chip include precision timing, pulse generation, pulse width modulation, pulse position modulation, sequential timing, and missing pulse detection.

IC 555-ASTABLE OPERATIONS: -

If the circuit is connected as shown in figure (pins 2 and 6 connected). It will trigger itself and free run as a multivibrator. The external capacitor charges through Ra and Rb and discharges through Rb only. Thus, the duty cycle may be precisely set by the ratio of these two resistors. In this mode of operation the capacitor charges and discharges between 1/3 Vcc and 2/3 Vcc. As in the triggered mode, the charge and discharges times, and therefore, the frequency are independent of the supply voltage. Figure shows the actual waveforms generated in this mode of operation.

The charge time (output high) is given by: t1	= 0.685 (Ra + Rb) C

And the discharge time (output low) by: t2	= 0.685 (Rb) C

Thus, the total period is given by: T	= t1 + t2	 = 0.685 (Ra + 2Rb) C

The frequency of oscillation is then:

f =       1.46 (Ra + 2Rb) C

IC 555-MONOSTABLE OPERATIONS: - In the monostable mode of operation, the timer functions as a one shot. Referring to figure the external capacitor is initially held discharged by a transistor inside the timer. Upon applications of a negative trigger pulse to pin 2, the flip-flop is set, which releases the short circuit across the external capacitor and drives the output high. The voltage across the capacitor increases exponentially with the time constant.

t = Ra C

When the voltage across the capacitor equals 2/3 Vcc. The comparator resets the flip-flop, which, in turn, discharges the capacitor rapidly and drives the output to its low state. Figure shows the actual waveforms generated in this mode of operation.

The circuit triggers on a negative going input signal when the level reaches 1/3 Vcc. Once triggered, the circuit will remain in this state until the set time is elapsed, even if it is triggered again during this interval. The time that the output is in the high state is given by: t= 1.1 Ra C

Applying a negative pulse to the reset terminal (pin 4) during the timing cycle discharges the external capacitor and causes the cycle to start over again. The timing cycle will now commence on the positive edge of the reset pulse. During the time the reset pulse is applied, the output is driven to its low state.