555 Timer IC

555 timer Chip is low cost chip used in thousands of applications and is still widely available.it can be configured to make monostable, astable and bistable to perform various operations. this IC is used in various application such as Battery chargers, Flashing circuits, Motor speed control circuits with PWM and delay circuits. These devices are precision timing circuits cabple of producing accurate timing delays or oscillations. In Astable mode the frequency and duty cycle is controlled by two external resistors and one capacitor, and in monostabble mode only one resistor and a capacitor can control the Time interval.

Monostable Operation [1]

The external capacitor is initially held discharged by a transistor inside the timer. Upon application of a negative trigger pulse of less than 1/3 V to pin 2, the flip-flop is set which both releases the short circuit across the capacitor and drives the output high.the voltage across the capacitor then increases exponentially for a period of t = 1.1 R C, at the end of which time the voltage equals 2/3 VCCA. The comparator then resets the flip-flop which in turn discharges the capacitor and drives the output to its low state. Figure below shows the waveforms generated in this mode of operation. Since the charge and the threshold level of the comparator are both directly proportional to supply voltage, the timing interval is independent of supply.

monostable circuits

During the timing cycle when the output is high, the further application of a trigger pulse will not effect the circuit so long as the trigger input is returned high at least 10 µs before the end of the timing interval. However the circuit can be reset during this time by the application of a negative pulse to the reset terminal (pin 4). The output will then remain in the low state until a trigger pulse is again applied. When the reset function is not in use, TI recommends connecting the Reset pin to V to avoid any possibility of false triggering.

Astable Operation

If the circuit is connected as shown in Figure below (pins 2 and 6 connected) it will trigger itself and free run as a multivibrator. The external capacitor charges through RA+ RB and discharges through R . Thus the duty cycle may be precisely set by the ratio of these two resistors.

Astable Multivibrator

Astable multivibrator schematic

reference: NE555p Timer IC Datasheet