Application Note : 555 Timer

Authors :  Milton Mah
                Michael Holden
                Korrey Scott

The purpose of this document is to illustrate the implementation of a commercially available integrated circuit package that contains the circuitry necessary to realize monostable and astable multivibrators having precise characteristics. The 555 timer is the most popular of such IC's and will be discussed in this application note.

In the design of any kind of system the need frequently arises for signals having prescribed standard waveforms, which in our case was a system clock pulse. The 555 timer can be configured to operate in a monostable mode in which it functions as a 'one-shot' pulse of predefined length or it can operate in the astable mode where it basically operates as a free running oscillator.

The functional diagram of the 555 timer is as shown below:


The circuit consists of two comparators, an SR flip-flop, and a transistor that operates as a switch. One power supply is used for operation, which is typically 5 V. A resistive voltage divider is also employed, and consisting three equal valued resistors is connected across Vcc and established the reference voltages for the two comparators. These are Vth = 2/3 Vcc for comparator 1 and Vtl = 1/3 Vcc for comparator 2.

Implementing a Monostable Multivibrator using the 555 Timer

In the monostable mode, the timing is controlled by one external capacitor and one external resistor. In the stable state the flip-flop will be in the reset state, and therefore Q' output will be high, turning on transistor Q1. Transistor Q1 will be saturated, and thus Vc will be close to 0 V, resulting in a low level at the output of comparator 1. The voltage at the trigger input terminal is kept high (greater than Vtl), and thus the output of comparator 2 will also be low. Note also that in this reset state, Q will be low and thus Vo will be close to 0 V. To trigger the timer a negative input pulse must be applied to the trigger input terminal. As Vtrigger dips below Vtl, the ouput of comparator 2 goes high, thus setting the flip-flop. The output Q goes high and Q' goes low, turning off transistor Q1. The capacitor C charges up through the resistor R and its voltage Vc rises exponentially toward Vcc. This state prevails until Vc reaches and exceeds Vth of comparator 1 thus resetting the flip-flop. The transistor then turns on and provides a path that discharges the capacitor. The length of the pulse T can be expressed as:
                                                                                        T = RC ln 3 = 1.1RC
The configuration is as follows:

Implementing an Astable Multivibrator using the 555 Timer

In the astable mode, two external resistors and one capacitor is used. The diagram is as shown below:



Assuming initially that C is discharged and the flip-flop is set, we have Vo high and transistor Q1 off. The capacitor will charge up through the series combination of Ra and Rb, and the voltage across it, Vc, will rise exponentially toward Vcc. As Vc crosses the level equal to Vtl, the output of comparator 2 goes low. At this point nothing occurs until Vc reaches and begins to exceed the threshold of comparator 1, Vth. At this instant, the output of comparator 1 goes high and resets the flip-flop, and Vo goes low, Q' goes high, and transistor Q1 is turned on. The capacitor C begins to discharge through Rb and the collector of Q1. The voltage Vc decreases exponentially with a time constant of RbC toward 0 V. When Vc reaches the threshold of comparator 2 Vtl, the output of comparator 2, goes high and sets the flip-flop. The output Vo then goes high, and Q' low, turning off Q1. Capacitor C begins to charge through the series equivalent of Ra and Rb, and its voltage rises exponentially toward Vcc with time constant (Ra + Rb)C. This rise continues until Vc reaches Vth, at which time the output of comparator 1 goes high, resetting the flip-flop, and the cycle continues. The charge time T1 is given by:

T1 = 0.69(Ra + Rb)C and the discharge time is given by: T2 = 0.69RbC The total period can therefore be expressed as: T = 0.69 (Ra +2Rb)C The duty cycle can be derived from T1 and T2 as:

                                                                                Duty Cycle = (Ra + Rb)/(Ra + 2Rb)

Note that the duty cycle will always be greater than 0.5 or 50%. It approaches 0.5 if Ra is selected much smaller than Rb.

In conclusion, the 555 timer can be implemented as a monostable or astable multivibrator. It contains precise characteristics and it is very reliable. With the use of a few components these are easy to construct with minimal calculations.