Introduction: Simple Light Detector With Sensitivity Control

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Light detectors are one of the most popular sensor and they are commonly found in many real-world applications. They are widely used by electronic hobbyists and projects because they are practical and intriguing yet surprising easy to construct. This instructable will guide and show you how easy it is to breadboard your own light activated Light Emitting Diode (LED) with minimal tools and materials. The whole project is simple enough for beginners and should take at most 10 minutes to construct. This implementation can be used for an educational demo or applied directly to the practical world.

Step 1: Gather Parts and Tools

The following are the list of materials and tools required for this project. These are widely available and can be easily obtained from sites like All quantities are single unless specified otherwise.

  • Breadboard
  • A few Jumper Wires
  • 9 Volts Battery
  • 9 Volts Battery Clip
  • Light Dependent Resistor (LDR)
  • Light Emitting Diode (LED) with any color of choice
  • TLC3704 Quad Comparator (only one of its four comparators will be used) (Alternatively, you can use the single LM311N Comparator with 8pin)
  • 3362P-103-ND 10K Ohms Variable Resistor
  • 1K Ohms (Brown-Black-Red) Resistor X2
  • 330 Ohms (Orange-Orange-Brown) Resistor
  • Variable Resistor Trimmer Pen (or a small screwdriver)
  • Needle-nose Pliers (not essential but useful for breadboarding)

Step 2: Understand How It Works

The schematic diagram for the circuit is given in the picture above.

Like its name suggests, a comparator compares two given voltages. The pair of 1K ohms resistors create a voltage divider and provide a 4.5 volts reference for the comparator. The variable resistor and LDR both form another pair for a second voltage divider. When light falls on the LDR, its resistance lowers and that voltage divider provides a voltage lower than 4.5 volts. The comparator produces no output (0 volts). When light is absent, the resistance of the LDR and the voltage increases. When the voltage increases over 4.5, the comparator activates its output and supplies 9 volts to power the LED.

An interactive flash animation of how the circuit works can be accessed by clicking here or on the file below.

Step 3: Install Shorter Components

It is always easier to place the shorter components on the breadboard first before placing the taller ones. Hence we shall being with the shorter components.

1. Insert the TLC3704 Integrated Circuit (IC) Chip between the middle divider of the Breadboard.

Placing the chip between the divider ensures that its pins do not get short-circuited. Place its pins starting from the column labelled 6 until the 12th column on the breadboard. Make sure the half circle (pin 1 indicator) is towards the right side (Picture 1).

TIP: If the chip is brand new, gently bend the chip's two rows of pins inwards slightly to ease insertion into the board.

CAUTION: The pins of an IC are delicate and break off easily without gentle care.

2. Insert the Variable Resistor in the middle of the top row from column 2 to 4.

This variable resistor has 3 pins. We will only be using two. Inserte the center pin into row H and the other two pins into row G (Picture 2).

3. Insert two short Jumper Wires connecting J4 to J6 and G5 to G7.

The needle-nose pliers easily facilitates placement of these small Jumper Wires (Picture 3). Grip them gently by the middle and firmly push them through their designated holes. Placing these wires with bare hands is still possible but requires more time. After places these two short Jumper Wires, your board should look like the one in Picture 4.

4. Insert the remaining 4 longer Jumper Wires connecting J3 and J10 to the Positive Rail, F4 to E4, and A10 to the Negative Rail.

This completes the placement of all Jumper Wires (Picture 5).

Step 4: Install Taller Components

Now that we have laid out all the shorter components, we can proceed to finishing up with our taller components.

1. Bend the two legs of each resistor forming a U shape for easy insertion.

After bending the legs of your resistors, they should look like the ones in Picture 1.

TIP: It is better to use your bare hands to make smoother bends rather than using the pliers to make sharp ones. After repeated sharp bending with the pliers, the pins break off easily.

CAUTION: Never make a bend right at the point where the pin comes out of the component as pins will mostly likely break off after a few more of such bends.

2. Insert a 1K Ohm (Brown-Black-Red) Resistor across H5 and the Positive Rail.

Completing this step should give you Picture 2.

TIP: Resistors are not polarized components. This means that you can connect them any order you like and they would still function.

3. Insert the other 1K Ohm (Brown-Black-Red) Resistor across F5 and the Negative Rail.

Completing this step should give you Picture 3.

4. Insert last 330 Ohm (Orange-Orange-Brown) Resistor across G12 and D17.

Completing this step should give you Picture 4.

TIP: The only purpose this resistor severs is to limit the current powering the LED and protect it from burning out. Thus this resistor is paired in series together with the LED. This pair can be replaced with a 9 Volts Relay to activate a switch or trigger any event.

5. Insert the longer leg of the LED into position A17 and the shorter leg into the Negative Rail.

An LED is a polarized component. The longer leg indicates the positive terminal. Completing this steps gives you Picture 5.

Tip: If it is impossible to indicate your LED's polarity by the pin length (because of previous vicious clipping), look into the transparent casing of the LED. You will notice the two pins entering through the base and what appears to be two flags at the top of these two "flagpoles". The shorter flag corresponds to the positive terminal.

CAUTION: Ensure that your LED terminals are inserted correctly or your LED will not function.

6. Insert the LDR across A4 and the Negative Rail.

Like resistors, LDRs are not polarized. You can insert its pins without regard to polarity. You circuit should look like the one in Picture 6 when you have successfully completed this step.

Step 5: Connect the Power Supply

Before we can configure and test the circuit, we need to attach the 9 Volts Battery that will power our light detector.

1. Unwind and attach the Battery Clip to the 9 Volts Battery

If the 9 Volts Battery came with safety cap, remove it first. The Battery Clip should only allow you to snap it to the Battery in one orientation for the correct polarity (Picture 1).

2. Connect the Red Lead to the Positive Rail on the breadboard.

See Picture 2.

3. Connect the Black Lead to the Negative Rail on the breadboard.

This would instantly supply power to your light detector. The LED on your circuit may or may not light up depending on your Variable Resistor but do not worry about that, we will configure the circuit in the next step. Successful completion of this step should produce something close to Picture 3.

Step 6: Configure the Light Detector

This step will allow you to setup and adjust the sensitivity of your light detector for different working environments. Repeat this step as often as you need to change the sensitivity of your light detector.

1. Transfer circuit to a desired working environment.

A place with moderate but sufficient light is a good place to start for testing the circuit (Picture 1).

2. Turn the Trimmer of the Variable Resistor all the way clockwise with the Trimmer Pen (or small screwdriver).

At this point, if your LED has not already been lighted, it should light up (Picture 2).

3. Slowly turn the Trimmer counter-clockwise until the LED just turns off.

This is where you get to control the sensitivity of your light detector. For higher sensitivity, adjust trimmer as exact as possible to the point where the LED turns off. To decrease sensitivity, turn a little ways more after the LED has turned off.

CAUTION: Ensure that the LDR is not obstructed from the light source while you perform this step.

4. Remove the Trimmer Pen.

The light detector is now configured for this environment.

5. Cover the LDR with your hand to test the circuit.

The LED should come on as you cover the LDR. Covering the LDR simulates a darker or nighttime environment. The LED should turn on as it detects the absence of light (Picture 3).

Step 7: Take This Project Further

The fully functional light detector turns an LED on when it detects a lower intensity of light. It can be configured to be highly sensitive and is capable of detecting the slight lowering of the sun's intensity as it is partially covered by passing clouds. A direct application of the circuit would be to turn on a night light as your room light goes out or to keep a porch light on during the night. The 330K Ohm Resistor and LED can be replaced with virtually anything to trigger a signal or an event. For example, a 9 Volts Relay would be used to make an absence-of-light activated switch. To make this circuit operate in reverse (the LED turns on when there is light), simply just exchange the positions of the Variable Resistor and the LDR.