1. ELEC207 Linear Integrated Circuits
University of Nizwa
Faculty of Engineering and Architecture
Electrical and Computer Engineering
ELEC207 Linear Integrated Circuits
Week 12 & 13
Atef Abu Salim
Fall 2014/2015
12/10/2018

2. 555 Timer Features of IC 555 Timer Reliable, Cheap, User-friendly IC
With just a few external components it can be used to build many useful circuits such as monostable and astable multivibrator, waveform generators etc.
It is basically a timing circuit that produces accurate time delays.
Features of IC 555 Timer
Adjustable duty cycle i.e. time delays ranging from few microseconds to several hours
Two operating modes: monostable (one-shoot) multivibrator and astable (free-running) multivibrator
Operates with a supply voltage in the range of +5V to +18V
Its output is compatible with TTL, CMOS and op-amp circuits.

3. 555 Timer – Functional Circuit Diagram
555 Timer – Pin out Diagram
555 Timer – Internal Circuit Diagram

4. 555 Timer SR-Flip-Flop Transistor Switch
The 555 timer consists of two comparators, an SR flip-flop, a transistor Q that operates as a switch and a voltage divider with three equal resistors.
SR-Flip-Flop
The flip-flop is a two-state device whose output can be either a high voltage level (set) or a low voltage level (reset). The state of the output depends on the input signals as shown in the Table.
Inputs
Outputs S R
Remark NC 1
RESET
SET ? NA
Transistor Switch
VQ’ S
VQ’=5V, Switch S is closed
VQ’=0V, Switch S is open

5. 555 Timer – An Over view Pin 1- Ground.
Pin 2- Trigger signal input. When it is less than 1/3Vcc, Flip-flop is set and output is high.
Pin 3- Output.
Pin 4- Reset input. i.e. For resetting the output to low, it can be grounded.
Pin 5- Control voltage input for changing the reference voltages.
Pin 6- Threshold input. When it is more than 2/3Vcc, Flip-flop is reset and output is low.
Pin 7- Discharge terminal. Connected internally to the collector of NPN transistor. When the output is high, transistor is off and acts as an open circuit. When output is low, the transistor is saturated and acts as a short circuit.

6. MONOSTABLE ( ONE SHOT) MULTIVIBRATOR Using 555 Timer
The 555 timer can be operated as a monostable multivibrator by connecting an external resistor and capacitor.
It has one stable state (o/p ‘low’) and one temporary state (o/p high) which is produced when a trigger pulse is applied.

7. MONOSTABLE ( ONE SHOT) MULTIVIBRATOR Using 555 Timer

8. MONOSTABLE ( ONE SHOT) MULTIVIBRATOR Using 555 Timer

9. EXAMPLE:
1. If for a monostable multivibrator, the time period for which output is high is 500ms, find out the value of resistor. Assume C=10µf.
Ans.

10. ASTABLE ( Free Running) MULTIVIBRATOR Using 555 Timer
The 555 timer can be operated as a astable multivibrator by connecting an external resistor and capacitor. It does not have a stable state and oscillates between high and low states.

11. ASTABLE ( Free Running) MULTIVIBRATOR Using 555 Timer

12. ASTABLE ( Free Running) MULTIVIBRATOR Using 555 Timer

13. EXAMPLE:
1. Design an astable multivibrator with frequency 1KHz, for a time period ratio Ton:Toff=2:1. Assume C=10nf.
Ans.

14. Voltage Controlled Oscillator (VCO) using 555 Timer
A VCO can be made from an Astable Multivibrator with variable control voltage.
The control voltage is externally set by the potentiometer.
With change in the control voltage, the upper threshold voltage changes and thus the time required to charge and discharge the capacitor changes. As a result the frequency of the output voltage changes.
Less R included in circuit => Control voltage increases => Capacitor take more time to charge and discharge => Frequency decreases.
Vice versa for More R included in the circuit.

15. Voltage Controlled Oscillator (VCO) - IC 566
Provides simultaneous square wave and triangular wave outputs as a function of input voltage.
The triangular wave is generated by alternately charging the external capacitor C1 by one current source and then linearly discharging it by another.
The charge-discharge levels are determined by the Schmitt trigger which provides the square wave output.
Both the output waveforms are buffered and send to the output terminals 3 and 4.

16. Voltage Controlled Oscillator (VCO) - IC 566

17. IC 566 VCO – Typical connection
The frequency of oscillation is determined by an external resistor R1, capacitor C1 and the voltage VC applied to the control terminal 5.
The frequency of the output waveforms is approximately
The maximum output frequency is 1MHz.
Power Supply 10V - 24V
2KΩ<R1<20KΩ

18. Example:
In the circuit of the VCO shown in Fig 1,if +V=12V, R2=1.5kΩ, R1=R3=10kΩ and C1=0.001μF.
a. Determine the frequency of the output waveforms.
b. Compute the modulation in the output frequencies if VC is varied between 9.5V and 11.5V.
c. Draw the square wave output waveform if the modulating input is a sine wave.
Solution: a. b.

19. c. During the positive half-cycle of the sine wave input, the control voltage VC will increase. Therefore the frequency of the output waveform will decrease and the time period will increase, Exactly the opposite action will take place during the negative half-cycle of the input.