Opamp Based Mini Project Part-2: Tripping Relay by sensing LDR

Opamp Based Mini Project: Photoresistor (LDR) sensed relay Tripping



In this post, let us see in detail about how the opamp controls the relay coil by sensing the light using LDR.
Before proceeding further it is recommended to read the Post: Opamp Based Mini Project- Part-1The circuit given below is the modified circuit of the part-1 post's circuit.

This circuit is constructed, to energize the relay coil (which will trip the relay circuit) when the LDR is in the presence of darkness.
You can use this core concept to develop your real time applications/projects.

relay_coil_turns_off_when_cell_in_dark

Points to remember about Opamp:

When Opamp is in open loop configuration,

  1. If the inverting input(-ve terminal) is high, then output will be low.
  2. If the non-inverting input(+ve terminal) is high, then the output will be high.

Points to remember about Photoresistor (Cadcell):

  1. In the presence of darkness, the resistance of photoresistor(LDR) is very high. ie, in the range of Mega Ohms.
  2. In the presence of light, the resistance of LDR is very less. ie, around 50 Ohms.

Circuit Construction:

The function of each component in the circuit is discussed in detail in Opamp Based Mini Project- Part-1 Post.

Circuit Operation:

This circuit turns the relay coil ON when the LDR is in darkness and turns OFF when the LDR is in the presence of light.



LDR is in presence of light:

  • Assume that the potentiometer connected to the non-inverting terminal has been adjusted to provide a 6V to it.
  • When  LDR is in presence of light, it has a low resistance.
  • So less than 6V is applied to the non inverting input.
  • Since the inverting input has higher voltage than non inverting input, the output is low.
  • Thus there will be no base current to the BJP and also LED  will be in OFF condition.

LDR is in presence of darkness:

  • When the cadcell is in the presence of darkness, its resistance increases (greater than 4.7KOhms).
  • When the resistance is more than 4.7KOhms, a voltage of about 6V is applied to the inverting input.
  • This causes output of Opamp to a high state.
  • Consequently, the LED and transistor turns ON.

You may also like to read:

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Thanks for reading... Please share you face any difficulty to build the circuit...

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