555 Timer

555 Timer 555 is an IC used to generate a clock. The two attributes of a clock are Frequency Duty cycle. Both of these can be changed using this IC, h...

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555 Timer 555 is an IC used to generate a clock. The two attributes of a clock are  Frequency  Duty cycle.

Both of these can be changed using this IC, however the duty cycle is always <50%.

There are two modes in which 555 can run.

MONOSTABLE MODE As the name suggests; in this mode the output is stable in only one (mono) state i.e. ‘off’ state. Thus it can stay only for a finite time, if triggered, to the other state i.e. ‘on’ state. This time can be set choosing appropriate values of resistances in the formula:

ASTABLE MODE In this mode; the output is stable neither in ‘high’ state nor in ‘low ’ state. Hence it oscillates from one state to another giving us a square wave or clock. We can set the clock frequency and Duty cycle D by the formulae:

NOTE: Capacitor C2 is just to filter the noise and its value can be suitably chosen to be 0.01μF. It can also be neglected.

4029 Counter With the clock made, we are ready to count the number of pulses passed into the circuit. Note that any kind of counting requires a memory (you got to know that you have just counted ‘3’ to go to ‘4’!). Hence 4029 can also be used as a memory element that remembers its immediate previous state.

Pin no.

Name

Pin Function

1

Parallel Load

2 3 4 5 6 7

Output Bit 3 Parallel Input Bit 3 Parallel Input Bit 0 Clock Enable Bar Output Bit 0 TC Bar

8 9

Vss (Gnd) Binary/Dec

10

Up/Down Count

11 12 13 14 15 16

Output Bit 1 Parallel Input Bit 1 Parallel Input Bit 2 Output Bit 2 Clock Pulse Vdd (Live)

If given high; loads the value of Parallel Input bit into the output bits. Low for normal operation. Most significant bit of output Most significant bit of parallel input Least significant bit of parallel input Low on this pin enables counting as per the clock received Least significant bit of output Output bit that gives a low when the count is complete. Can be used to signal the end of counting. Needed for powering To choose b/w binary and hexadecimal modes (low for decimal mode and high for Binary mode) To choose b/w up counting and down counting modes (low for down counting mode and high for up counting mode) 2nd bit of output 2nd bit of parallel input 3rd bit of parallel input 3rd bit of output Clock pulse is given here Needed for powering

7447: BCD to 7 segment display decoder For displaying the number in the counter output on a seven segment display (i.e. 7 LEDs making up a figure of ‘8’ as in a general calculator. See fig. ) we need to decode the 4 bits and match them to the 7 pins for lighting the LEDs corresponding to the number. This work is done by 7447.

Pin No. Name

Pin Function

1 2 3

Input Pin B Input Pin C Lamp Test Bar

4 5 6 7 8 9-15 16

BI / RBI RBI Input Pin D Input Pin A Gnd Output Pins a-g of 7447 Vcc

2nd bit of Input 3rd bit of Input To check whether all LEDs are glowing (High for normal function, Low to glow all LEDs) Keep high for normal function Keep high for normal function Most significant bit of Input Least significant bit of Input For powering the IC To be connected to 7 segment display For powering the IC

NOTE:  The COM pins are to be connected to Vcc via 220 ohm resistor. 

The dot pin is just for display of decimal point and essentially only makes the upper and lower sides distinguishable from each other for a single display. Without the asymmetry produced by dot how will we be able to see which side is upper and which is lower?

LOGIC GATES 4069 : NOT GATE

4081: 2 INPUT AND GATE

4073: 3 INPUT AND GATE

4071: 2 INPUT OR GATE

4075: 3 INPUT OR GATE

4070 : 2 INPUT XOR GATE

Note : Other gates such as NOR, NAND are also available.

4051 : 8X1 multiplexer/demultiplexer Pin Symbol

Pin Function

E

Kept low to enable normal functioning of MUX/DEMUX Supply Voltage Ground Supply Voltage Selection Pin Inputs Independent Input or Output Pins Common Input or Output Pin Supply Voltage

VEE VSS S1 , S2 , S3 Y0 – Y7 Z VDD

4052 : 4X1 multiplexer/demultiplexer Pin Symbol

Pin Function

E

Kept low to enable normal functioning of MUX/DEMUX Supply Voltage Ground Supply Voltage Selection Pin Inputs Independent Input or Output Pins Common Input or Output Pin Supply Voltage

VEE VSS S1 , S2 1Y0 – 1Y3, 2Y0 – 2Y3 1Z, 2Z VDD

Note : 2X1 and 16X1 MUX/DEMUX are also available and has similar pin configuration.