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LDR Darkness Sensor Circuit

This circuit uses an LDR (Light dependent resistor). The resistance in an LDR is variable and as light falls upon it, the resistance decreases. The resistance in a typical LDR can fall from 1M ohm to 5K ohm.

When combining an LDR with a Transistor which is being used as a switch we can use these to turn on a light as darkness falls. This can be used in a Variety of applications, such as a camera shutter controller, security light, Childs night light and automatic lights on a car or boat.

The components that we are going to need for this experiment are listed below and we have put a link and product number next to each one for quick reference:

  • 1 x LDR (Product no: 115-005 or 115-001)
  • 1 x Breadboard & Jumper wires (Product no: 214-002)
  • 1 x 9v Battery
  • 1 x 9v Battery Clip (Product no: 230-003)
  • 1 x BC547B Transistor (Product no: 101-008)
  • 1 x 100k Variable resistor (Product no: 226-004)
  • 1 x 470 ohm resistor (Product no: 212-003)
  • 1 x LED (Product no: 110-001)

The transistor we are using is the BC547B which is an NPN transistor. Transistors have 3 legs which are: Collector, Base and Emitter. These have to be connected into a circuit the correct way. Use the image below the circuit diagram to work out the correct legs for our transistor.

With your breadboard and transistor in front of you we are going to insert the transistor into the following holes on the breadboard.

  • The Collector to pin hole h12
  • The Base to pin hole h13
  • The Emitter to pin hole h14

Next we are going to insert the variable resistor. A standard resistor could be used with a value of around 50k ohm. This would mean that the sensitivity of your detector is fixed. By using a variable resistor of 100k ohm this allows us to alter the sensitivity of the circuit.

  • The two outside pins of your variable resistor should go to a7 and a9, allowing the centre pin to pin hole b8

By keeping the variable resistor to the outside of the board it allows you to adjust this without knocking the other components.

  • Insert the LDR legs with one leg to pin hole e1 and the other leg to f1. (The LDR can go either way round and is not polarity sensitive)

Using the right hand side positive and negative strips insert the resistor R2 470 ohm to the following:

  • One leg into the right hand side positive strip row 22 and the other leg to pin hole i22

The resistor R2 is the basic level of resistance that is required to protect our LED. If the LED was to be substituted for a buzzer the resistor would not be required. If the circuit is to be used with a higher voltage (Up to 30v DC) then the value of R2 would need to be increased to protect the LED.

Now we need to connect the LED as follows:-

  • Positive leg of the LED (Longest leg) to pin hole g22
  • Negative leg of the LED (Shortest leg to pin hole g20

Now that we have all the components inserted into our bread board we just need to connect them together with the breadboard jumper wires.

  • One jumper cable into pin hole j1 and the other end to the negative strip on the right hand side of the breadboard.
  • One jumper cable from the right hand side positive strip and the other end to pin hole c9
  • One jumper cable to pin hole c8 and the other end to pin hole a1
  • One jumper cable to pin hole b1 and the other end to pin hole g13
  • One jumper cable to the negative strip on the right hand side and the other end pin hole j14
  • One jumper cable into pin hole j12 and the other end to pin hole j20

We can now connect the battery clip and battery. Insert the battery clip to the positive and negative columns of the right hand side power strip. You may want to do this on the bottom rows so that the battery is kept out of the way.

Depending on how the variable resistor is set and how dark your room is the LED may or may not now light up.

  • Turn the variable resistor fully anticlockwise which is setting the least resistance

Place your finger over the top of the LDR and you will now see that the LED lights up.

You can adjust the variable resistor to set the sensitivity of the LDR. Try going from a well lit room to a darker room and see how the circuit reacts.