CH-2 ELECTRONIC TIMERS Marks: 16 (24)
Introduction to IC Timer- IC555:
The IC555 is more versatile IC. It can produce precise time delays. IC555 timer is highly stable integrated circuit capable of functioning as an accurate time delay generator. It is very flexible to design large number of applications by simply connecting few external components. IC555 can be used to design blinker circuit/ dancing light, long delay timer, water level indicator and so many mini projects.
The lC555 timer is available in 8 pin and 14 pin dual in line package or in circular TO-99 metal can with 8 leads.
Functional Block Diagram of IC555:
IC consists of three equal resistors of 5kΩ each. It forms voltage divider and gives the reference voltages for the two comparators. Hence the name of IC is IC555.
The threshold comparator (upper comparator) activates when voltages at pin No. 6 goes higher than 2/3Vcc.
Trigger comparator activates (lower comparator) when voltage at pin No. 2 becomes less than 1/3Vcc.
When threshold input is greater than or equal to 2/3Vcc, the output of threshold comparator becomes high and it resets the R-S flip-flop (R=1, =1).
The high out put of flip-flop turns ON the discharge transistor Q1. Output is taken from, so it is low.
When the trigger input is less than or equal to 1/3Vcc, the output of trigger comparator becomes high and it sets the R-S flip-flop (S=1, =0).
The low out put of flip-flop turns OFF the discharge transistor Q1. Output is taken from, so it is high.
Capacitor connected externally to pin No. 7 discharges through discharge transistor Q1 and timing cycles completes.
Pin Configuration of IC555:
Ground: All voltages are measured with respect to this pin.
Trigger: The output of timer depends on the amplitude of external trigger pulse applied to this pin.
Output: Load can be connected to this pin.
Reset: Once timing cycle has been initiated by trigger pulse, circuit becomes insensitive to further triggering. In order to interrupt the timing cycle the reset pin is used. When reset terminal is not being used, it should be connected to Vcc.
Control voltage: The timing cycle can be modified by applying DC voltage to control voltage terminal. If it is not used it should be connected to ground through capacitor of 0.0lμF.
Threshold: This is non inverting terminal of upper comparator. When voltage at this pin is greater than or equal to 2/3Vcc, o/p of comparator goes high.
Discharge: This pin is connected internally to collector of transistor Ql. When o/p is high Ql is off and acts as an open circuit.
Vcc: The supply of +5V to +18Vin applied to this pin with respect to ground (pin l).
Monostable multivibrator using IC555:
Monostable state circuit has one state stable (ON or OFF). It remains in stable state for period determined by circuit components.
In this mode of operation, timer acts as a one shot. External connections are as shown in circuit diagram.
Working:
When trigger terminal is connected to ground momentarily, the output of trigger comparator becomes high and it sets the flip-flop (S=1,=O).
Here timing cycle starts and output goes high. Transistor Ql becomes cut off and external capacitor C starts charging through R. This capacitor is connected to threshold terminal. When voltage across capacitor C becomes equal to 2/3Vcc, upper comparator resets flip-flop (R=l, =l) and o/p goes low.
Now transistor Q1 conducts and capacitor discharges through it. This completes the timing cycle.
The circuit remains stable in this condition till the next trigger pulse is applied.
This is charging timer. Its period is given as T= l.l RC.
Astable multivibrator using IC555:
In astable state, circuit has two stable states (ON & OFF). It alternates automatically between the two states and remains in each state for a time determined by circuit constants.
It will trigger itself and free run as a multivibrator. Hence it is also called free running multivibrator. It generates square wave.
Working:
When supply is made ON, transistor, Q1 (inside IC) is OFF and capacitor C charges through RA and RB.
When voltage across it reaches to 2/3Vcc, upper comparator resets flip-flop (R=1, =l).
So transistor Ql becomes ON and capacitor discharges through RB and Q1.
During discharging when voltage across it reaches to l/3Vcc, lower comparator sets the flip-flop and S=1, =O.
Therefore Q1 becomes OFF and capacitor is prevented from further discharging.
Again capacitor starts charging through RA and RB and cycle repeats.
The capacitor charges to 2/3Vcc and discharges to 1/3Vcc i.e. charging and discharging of capacitor is between 2/3Vcc and l/3Vcc.
The charging period is -
Ton = 0.693 (RA+RB) x C
Discharging period is-
Toff = 0.693RB x C
Total period, T=Ton+Toff.=0.693(RA+2RB) x C
Frequency, F =
Duty cycle=
Bistable multivibrator using IC555:
A bistable multivibrator is one of the easiest circuits that can be built using a 555 timer. It doesn’t require a capacitor for charging and discharging.
Working:
The trigger and reset pins (pins 2 and 4 respectively) are connected to the supply through two resistors R1 and R2 so that they are always high.
Two switches are connected between these pins and ground in order to make them go low momentarily. The switch at the trigger input will act as S (SET) input for the internal flip-flop. The switch at the reset input will act as reset for the internal flip-flop.
When the switch S1 is pressed, the trigger terminal is shorted to ground through the resistor R1. Hence, the trigger pulse will momentarily go low and the output of the timer at pin 3 will become HIGH. The output remains HIGH.
When the switch S2 is pressed, the reset terminal is shorted to ground through the resistor R2. This pin is internally connected to the RESET terminal of the flip-flop. When this signal goes low for a moment, the flip-flop receives the reset signal and RESETs the flip-flop.
Hence, the output will become LOW and stays there until the trigger is applied
FSK (frequency shift keying) using IC555:
In wireless communication and computer peripherals, the binary data or code is transmitted by means of carrier frequency that is shifted between two preset frequencies. Since carrier frequency is shifted between two preset frequencies, the data transmission is said to use a ‘frequency shift keying’ (fsk) technique.
Working:
The output frequency of the signal was based on the input digital signal given to the base of the transistor.
When the given input was high that is of logic 1 the PNP transistor is off and IC 555 timer works in the normal Astable mode of operation giving out the series of square wave pulses.
Here the resistors RA, RB and Capacitor C1 are selected in such a way to obtain desired output frequency 1070 Hz.
When the input binary data is logic 0, the PNP transistor is on and its connects the resistance RC across resistance RA. The resistor RC is selected in such a way that the value of output frequency is1270Hz.
Thus the resultant output FSK will give frequency of 1070Hz when input is high and frequency of 1270 when input is low. Thus by this way the FSK signal was obtained using NE555.
Formula for Frequencies-
PWM (Pulse Width Modulation) using IC555:
PWM is a digital technique of pulse modulation where the pulse width of carrier is varied according to instantaneous amplitude of modulating (message) signal
Pulse width modulator circuit needs an external clock signal to work. The clock signal must be fed to the trigger input (pin 2) of this pulse width modulator circuit. The modulation input should be fed to pin 5, and you’ll get the pulse-width-modulated output at pin 3.
Formula-
PPM (Pulse Position Modulation) using IC555:
It is another technique of pulse modulation, where the position of pulse is varied according to the instantaneous amplitude of input modulating signal.
Basically it is astable multivibrator. The control voltage pin 5 is connected to modulating signal. The output position changes according to modulating signal.
PAM (Pulse Amplitude Modulation) using IC555:
It is another technique of pulse modulation, where the amplitude of pulse is varied according to the instantaneous amplitude of input modulating signal.
IC555 can be used to generate PAM with one NPN transistor connected at output. The IC555 is used in astable mode to generate pulse train to get samples of information (audio) signal.
Its frequency should be at least twice that of audio signal. This pulse train output is fed to base of NPN transistor. The collector of transistor is coupled with low frequency audio (information) signal.
The output at the collector of transistor is PAM wave. The amplitude of pulses generated by IC555 varies in accordance with the instantaneous amplitude of information signal.
Applications of timers:
(i) Spot welding (vii) In X-ray machine.
(ii) Ultrasonic flaw detector (viii) In traffic light control
(iii) Dye casting (ix) Photosensitive alarm
(iv) Photographic process (x) Washing machine.
(v) Electric oven
(vi) Consumer electronic items like TV, VCR, CD/DVD player etc.
Questions
Marks wise Questions: 1M(2), 2M(2), 3M(2), 4M(2)
1. Fill in the blanks-
Internal discharge transistor of IC555 becomes ON when -------- terminal voltage crosses 2/3 supply voltage.
Trigger (b) Discharge (c) Threshold (d) Reset
IC555 is used as astable multivibrator, its frequency determining capacitor maximum discharges upto ----------.
(a)OV (b) 2/3Vcc (c) 1/3Vcc (d) 3/2Vcc
In IC555, the discharge terminal is ———— of the internal transistor.
(a) Emitter (b) Base (c) Collector (d) O/p of flip-flop
When pin 2 of IC555 is permanently connected to ground, the o/p is —————.
(a)High (b) Low (c) OV (d) Changes alternately
Output frequency of astable multivibrator using IC555 is independent of —————
(a) Resistor RA (b) Resistor RB (c) Capacitor C (d) Supply voltage.
In timer, if resistor R is kept fixed the time (T) can be changed by changing value of ----------. [M-09]
Basic timer consists of ------------.
(a) Only resistor (b) Only capacitor (c) Resistor and capacitor [M-12, M-17]
IC ------------- is a digital timer. [O-11]
If value of condenser is 10 microfarad and resistance is 10 megaohm, then the time constant will be ---------
(i) 100sec (ii) 10sec (iii) 1 sec [M-14]
2. Match the following-
A B
(1) IC555 (1) Digital clock
(2)Free running oscillator[M-17] (2) IC555/ street light control
(3) Timer [M-10,M-15] (3) Astable multivibrator/free running oscillator
3. True or false-
IC555 can be used as monostable multivibrator when pins 6 and 7 are shorted.
IC555 can be used as astable multivibrator when pins 6 and 2 are shorted.
The reference voltage of threshold /upper comparator in IC555 is 1/3Vcc. [M-11]
The reference voltage of trigger comparator in IC555 is 2/3Vcc.
Control voltage is used to interrupt the time cycle in IC555.
In monostable multivibrator using IC555, time period is T = 1.1RC.
Astable multivibrator circuit is square wave generator. [M-09, M-15]
The +Vcc pin number of IC555 is 1. [M-09]
Timer is used in X-ray machine. [O-11]
Monostable multivibrator has two stable states. [M-16]
4. Answer the following-
Draw the functional block diagram of IC555. [M-09, M-10,M-16, 17]
Draw pin out diagram of IC555 and label it. [M-10, M-11, O-11, M-13,M-15M-16]
Explain Monostable multivibrator using IC555 with circuit diagram.[O-09, M-11, 12, 13]
Explain the working of astable multivibrator using IC555. [O-11, M-13, M-15,17]
Enlist applications of timer. [O-11, M-15,M-16]
Draw the circuit diagram of timer lC555 in astable/free running multivibrator/square wave generator mode of operation. Explain its working. Give its application. [M-09, M-10, M-11]
State function of (i) Threshold comparator (ii) Trigger comparator (iii) Discharge transistor in IC555. [M-09]
Define astable and monostable states in multivibrator. [M-09, M-17]
Why IC555 is numbered as 555? [M-13]
Explain use of IC555 as pulse amplitude modulator (PAM).
Explain how IC555 can be used as pulse position modulator (PPM).
Explain the use of IC555 as FSK generator.
Explain use of IC555 as pulse width modulator (PWM).
Explain Bistable multivibrator using IC555 with circuit diagram.
