TITLE: 555 Timer OM INSTITUTE OF TECHNOLOGY Subject: Analog Electronics ( ) Semester: 03 Prepared By:

Oscillators ◊We have looked at simple oscillator designs using an inverter, and had a brief look at crystal oscillators. ◊In this presentation, we introduce the 555 timer; a versatile device that is easier to calculate, design and configure in a variety of ways.

A Versatile Device ◊The 555 Timer is one of the best known IC’s. ◊The 555 is part of every experimenter's tool kit ◊Capable of creating a wide variety of circuits, including: ◊Oscillators with adjustable frequency and Duty Cycle ◊Monostable Multivibrators ◊Analog to digital Converters ◊Frequency Meters ◊Many other applications…. ◊The clock on the Vulcan Board is generated by a 555 timer.

555 Timer Configurations ◊In this course we will use the 555 timer in 2 modes: ◊Astable ◊With some calculations, we can determine the values of the capacitor and the 2 resistors (Ra and Rb) for astable operations. ◊Monostable ◊We can determine the value of the resistor and the capacitor with a simple formula for one-shot operations. ◊There are many other configurations and applications for this device.

555 Layout Also available: 556 (two-555’s in one DIP package) 555 in a “metal can” configuration Ground Trigger Output Reset Vcc Discharge Threshold Control Ground Trigger Output Reset Vcc Discharge Threshold Control

Operation of the 555 Timer Astable Multivibrator

Oscillator Configuration ◊Externally, the 555 requires an RC circuit to create the time delays required for the time high and the time low. ◊Standard configuration requires ◊common capacitor ◊a resistor for the charge cycle ◊a resistor for the discharge cycle

General Configuration ◊Basic connections: ◊Ground ◊Vcc ◊Note: some 555 timers may function at voltages other than 5 volts. ◊Reset (active low) ◊Output Ground Trigger Output Reset Vcc Discharge Threshold Control

General Configuration ◊Specialized connections: ◊Trigger ◊monitors low voltage ◊Threshold ◊monitors high voltage ◊Discharge ◊path to ground, to discharge the capacitor ◊Control ◊specialized input ◊filtering ◊special applications Ground Trigger Output Reset Vcc Discharge Threshold Control

Astable Configuration #1 (“Standard” Configuration) Ground Trigger Output Reset Vcc Discharge Threshold Control Vcc Ra Rb C Vcc

Astable Filter Cap 0.01μF Calculated Values Note: Duty Cycle must be > 50%

Astable Configuration #1 (“Standard” Configuration) Minimum duty cycle > 50%

Calculations: Astable THTH TLTL Notes: The value is a factor associated with the charge/discharge cycle of the 555 timer. Duty Cycle must be > 50% Time High, Time Low Set

Sample Calculation ◊Design an oscillator with a frequency of 200Hz with a duty cycle of 78%. 1.Determine Period (T): 2.Determine T H and T L : Time High, Time Low Set

Since there are 2 variables in the T L equation, select C: 4.Determine R B by using the T L equation: C=10μF Sample Calculation Time High, Time Low Set

Determine the value for R A : Sample Calculation Time High, Time Low Set

Calculations: Astable Notes: The value is a factor associated with the charge/discharge cycle of the 555 timer. Duty Cycle must be > 50% Frequency, Duty Cycle Set

Internal Operation of the 555 Timer ◊Understanding the internal operation of the 555 timer is important ◊The device combines various circuit theories. ◊Various aspects provide a preview of other circuits, such as the Analog to Digital decoders. ◊Understanding its internal function makes it easier to create new designs with the device

Comparator ◊The comparator is an operational amplifier (op- amp) configuration. ◊The comparator compares 2 analog voltages and provides a digital output. + - If V+ > V-, the output is a digital 1 If V- > V+, the output is a digital

Reference and Comparators The Comparators provide digital logic to an SR Latch Comparators compare voltage levels. If “+” is higher, output = 1 If “-” is higher, output = 0 A 3-resistor voltage divider provides reference voltages

Reference and Comparators Pin 8: Vcc connection Pin 5: “Control” connection, used with filter cap. Pin 1: Gnd connection Pin 6: “Threshold” connection (high Comparator) Pin 2: “Trigger” connection (low Comparator)

Latch and Output The Q’ output controls the transistor (“on” or “off”). The SR Latch holds its output states The transistor acts like a switch. The reset input can be used to enable or disable the timer. The buffer has important electrical functions

Latch and Output Pin 4: Reset connection (active low reset) Pin 7: “Discharge” connection Pin 3: Digital Output

Charge Animation Discharge Animation