Automatic Street Light 555 timer based

 

Construction of Automatic street light circuit

Figure   Automatic street light circuit

Construction

>> Pin 4 and Pin 8 are short circuited ; Because Pin 4 is Active low pin and if it is leaved free and it will reset the flip flop , so to avoid such condition the pin 4 is connected to a ‘high’ state

>> Pin 2 and 6 are short circuited; By doing this we tied together upper comparator and lower comparator pins (upper comparator Non inverting and lower comparator Inverting.

>>  The LDR connected between Pin 2 and Pin 4; where it works as switch

>> A 0.01 uF capacitor is connected between Pin 5 and Pin 1 , to suppress any electrical noise generated within circuit

>> The power supply is connected between Pin 1 and Pin 8 ( 9V )

>> The output is generated at pin 3 , it  depends on the ‘trigger pin ‘

>> The first led represents street light , it is connected between Pin 3 and Pin 8  with a 220 ohm resistor to protect the led

Components

555 Timer	LED
Dc Battery	Printed Circuit Board
LDR	Resistor
	______________
Capacitor	Connecting Wires

 

 

Internal circuit Diagram and description of 555 timer

SR Flip Flop

In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. A flip-flop is a bistable multivibrator. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic. Flip-flops and latches are a fundamental building block of digital electronics systems used in computers, communications, and many other types of systems.

The design of such a flip flop includes two inputs, called the SET [S] and RESET [R]. There are also two outputs, Q and Q’. The diagram and truth table is shown below.

S=1, R=0—Q=1, Q’=0

This state is also called the SET state.

S=0, R=1—Q=0, Q’=1

This state is known as the RESET state.

S=0, R=0—Q & Q’ = Remember

If both the values of S and R are switched to 0, then the circuit remembers the value of S and R in their previous state.

S=1, R=1—Q=0, Q’=0 [Invalid]

This is an invalid state because the values of both Q and Q’ are 0. They are supposed to be compliments of each other. Normally, this state must be avoided.

 

 Comparator

In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals and and one binary digital output . The output is ideally

 

 NOT Gate (inverter)

The output of NOT gate is the complement of the input.

 

 

Working Of Automatic Street Light using 555 Timer

Figure   Automatic Street Light

The basic circuit of Automatic street light is shown above ,some of the configuration is not shown to make it simpler and neat .We are providing Vcc at Pin 8  with 9V DC battery ,As you can see in above figure , there are three 5K resistor  in series , these works as a voltage divider So at point Q  , the voltage  value will be 2/3 of vcc (6V) ,also at point R value is 1/3 of vcc(3V)

Pin 2 and Pin 6 are tied together by doing this, the upper comparator and lower comparator ,Non inverting and inverting are shorted  and the common point name give in fig is ‘P’. The Two ends of Light Dependent resistor are connected between Pin 8 and node P.

The  R1 resistor connected at P node and Pin 1 , it controls the current flowing the LDR hence it controls the sensitivity of the circuit .The outpout of Two comparator is connected at S and R ends of flipflop.

Condition 1 : During Day time

Figure   : During Day time circuit

During day time LDR conducts ,and voltage at point P will almost equal to Vcc ,R1 value is selected such that it passes maximum current when LDR conducts . so at Pin 2 the volage is equal to 9 V  and at node Q it is 6v , the comparator compares the two value and generates a digital output which is equal to 1 in this case;  hence value of R is 1 . Now at Lower comparator end,The value of  node 6 value is 9v and Node r value is 3v  so ,comparator generates  S = 0

RESET CONDITION
S=0 & R=1	Q= 0 & Q’=1

There is a inverter after Q’ in diagram; which inverts Q’=1  hence pin 3 output is '0'

One may think ,what will happen during Sunset ; in situation where LDR Partially conducts and voltage at Node P  is between 6v(PIN Q) and 3v(PIN R) .This situation generates SR no change condition ,Q Node Potential is greater than Pin 2 , so comparator produces Digital 0 ; hence R=0, Also At lower comparator end Pin 6 potential is greater than Node R , it generates 0 ;S=0

RESET CONDITION
S=0 & R=1	Q= 0 & Q’=1
S=0 & R=0	Q= 0 & Q’=1
There will be no change at output

 

Condition 2 : During Night

Figure- During Night circuit

During night,LDR acts as open circuit ,and voltage at point P will almost equal to O V  , so at Pin 2 the voltage is equal to 0 V  and at node Q it is 6v , the comparator compares the two value and generates a digital output which is equal to 0 in this case;  hence value of R is 0 . Now at Lower comparator end,The value of  node 6 value is 0v and Node r value is 3v  so ,comparator generates 1;hence S is 1

SET CONDITION
S=1 & R=0	Q= 1 & Q’=0

There is a inverter after Q’ in diagram; which inverters Q’=0  to Pin 3 output 1.

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