by user

we have already work on the DC circuits, now we will learn to switch the AC appliances with our previous circuit, we will go through the project-2 “develop smartness to your LED”  in this content we will replace the LED to AC BULB by adding extra ac and dc isolated trigger circuit [relay]. this circuit is applied to only switching below 230 volts and 7 amp resistive load devices. it does not properly work with inductive load. 

 it is a common circuit which is used to trigger the AC appliances it will be used in further projects ahead.

STEP-1 Things required :- 

  • 230-volt ac source.
  • 5 volt, 1amp dc power adopter.
  • light-dependent resister [LDR].
  •  comparator ic [LM358].
  • potentiometer [10k ohm].
  • 3mm /5mm led.
  • resister [50 ohm,1k ohm and 10k ohm].
  • 5volt relay.
  • diode [1N4007].
  • transistor [BC547].
  • AC bulb.
  • multimeter.
  • breadboard and jumper wires [red, blue, yellow].
  • screwdriver.

STEP -2 :- circuit and components connection

figure 4.0.jpg
  • 5volt adapter:- it is ac to dc step down converter, which converts the 230-volt ac into 5-volt dc output.
  • comparator:-  LM358 dual comparator ic each comparator compares two variable voltage and gives the output LOW (0) and HIGH (1) voltage.
  • potentiometer:- it is a manual variable resistor, it is used to set the reference voltage at the negative terminal of the comparator ic.  
  • LDR:-  (light dependent resister ) its resistance vary according to the intensity of light fall on it, it is used to detect the intensity of light.
  • transistor:- bc547  is an  NPN transistor using to as a switching device, which triggers the relay. when the output of the comparator is HIGH than turn off the bulb because bulb connectivity is given to NC [normally closed] terminal so the circuit is not complete.  whereas the comparator’s output LOW than bulb will ON and relay at normally
  • Diode:- diode is a unidirectional device that allows moving current only one direction, so it is used to pass the reverse unwanted current from the coil, the result of it relay’s coil demagnetize fast, so it increases the switching speed of relay.
  • Relay:- it is an electromechanical switch, which magnetizes to complete the path by giving the particular voltage to it at normally open [NO] connection, and complete the path when demagnetize at normally close [NC] connection.   

See Also: Learn How to Develop Smartness to the LED in 30 mins

STEP-3 :- implement on bread board

  • put all the components on the bread board, as  shown in the figure below. 



  • connect the negative output terminal of the adapter to the upper BLUE row and the positive output terminal connected to the RED row of the breadboard. [ RED  row show +5 volt. with reference BLUE row that is NEGATIVE row ]. 
  • connect the both upper RED and lower RED rows, and also BLUE to BLUE rows.

jump the 4th terminal of LM358 to lower BLUE  row and 8th  pin to upper RED  row by using jumper.  

 figure 4.2.jpg

  • connect the potentiometer 1st  and 3rd   pin to positive and negative row respectively, 2nd  pin connects with the 6th pin of comparator ic by yellow wire.
  • one terminal of LDR  is connected to the positive row, and second, connect with one terminal of resister to  PIN 7, and the second terminal of 10k ohm resistor is connected to the BLUE  row.
figure 4.3.jpg
  • connect the resister of 1k ohm to the positive row and the positive terminal of the LED.
  • negative terminal of LED put at the output of the comparator ic [PIN  5]
FIGURE 4.5.jpg

  • the base terminal of the transistor connected to the output pin 5  of the comparator ic through 50-ohm resister.
  •  the emitter of transistor connects to the negative row of the breadboard.
  • collector pin will be connected to the one pin of the relay’s coil. and the second terminal of the coil is connected to the positive terminal row of the breadboard.
figure 4.6.jpg
figure    4.6
  • now we are going to connect the high voltage AC connection. don’t plug-in the adapter to the ac supply, until all connections will be done.
  • connect neutral wire of ac supply directly to the one terminal of the bulb, and phase wire to the middle pin of three serial pins of the relay.
  • remaining bulb terminal connects to the NC [normally closed ]  terminal of the relay. 
  • check the circuit continuity and also check short-circuiting at positive and negative rows by using a multimeter.
  • if getting continuity at positive and negative than check the connection properly. and don’t connect the battery at this condition.
  • plug-in the adapter to the ac supply. and don’t touch the relay circuitry or where the ac supply is connected, until the adapter plug-out from ac supply.

figure 4.7.jpg
figure 4.7      


  • after connecting supply,  set the potentiometer through  screwdriver, led may be ON or  OFF. and also relay create the sound when it will trigger.
  • if led is OFF,  at this situation turn the potentiometer at one direction or other until led will turn ON. and then turn the pot on the reverse direction as it was rotating until at the point where the led get turn  OFF.
  • if led is ON, then turn the potentiometer at one direction or other until at that point where led is turn OFF.

STEP-4 :-  test the result  of the circuit  

  • after the proper set up we observed that whenever the LDR is in darkness then the led and bulb turn on automatically and turn OFF  when it goes under the light. 
  • we can set, the intensity at which we want to turn the led off and on by rotate potentiometer.   

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