DIY Automatic Light Fence

A light fence circuit is used to detect the presence of any human or object in a particular area. The detecting range of the Light Fence Circuit is about 1.5 to 3 meters. It's pretty simple to design the circuit using LDR and Op-amp. This portable circuit can work smoothly with a commonly available 9V battery, and the alarm sound generated from the buzzer is loud enough to detect the presence of a human, vehicle, or object.

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1. LM741 Op-Amp × 1
2. 555 Timer × 1
3. BC557 - PNP Transistor × 1
4. LDR × 1
5. Potentiometer × 1
6. Buzzer × 1
7. LED × 1
8. Resistor (210, 1K, 5.7K, 100k, 1M) × 1
9. Capacitor (0.1uf, 10uf) × 1

A Light Dependent Resistor (also known as a photoresistor or LDR) is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light-sensitive devices. They are also called photoconductors, photoconductive cells, or simply photocells.

They are made up of semiconductor materials that have high resistance. There are many different symbols used to indicate a photoresistor or LDR, one of the most commonly used symbols is shown in the figure below. The arrow indicates light falling on it.

555 timer is one of the most used IC in electronics, especially for triggering purposes. Whether a simple project involving a single 8-bit microcontroller and some peripherals or a complex one involving a system on chips (SoCs), 555 timer working is concerned. Depending on the manufacturer, the standard 555 timer package includes 25 transistors, two diodes, and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). Variants consist of combining multiple chips on one board. However, 555 is still the most popular.

A 555 timer working as a flip flop or multi-vibrator has a particular set of configurations.

Pin 1. Ground: This Pin should be connected to the ground.

Pin 2. TRIGGER: Trigger pin is dragged from the negative input of comparator two. The Lower comparator output is connected to the SET pin of the flip-flop. A negative pulse (< Vcc/3) on this Pin sets the Flip flop, and the output goes High.

Pin 3. OUTPUT: This Pin also has no particular function. This is the output pin where the Load is connected. It can be used as a source or sink and drive up to 200mA current.

Pin 4. Reset: There is a flip-flop in the timer chip. The reset pin is directly connected to the flip-flops (Master Reset). This active Low Pin is typically connected to VCC to prevent accidental Reset.

Pin 5. Control Pin: The control pin is connected to the negative input pin of comparator one. Output Pulse width can be controlled by applying a voltage at this Pin, irrespective of the RC network. Typically this Pin is pulled down with a capacitor (0.01uF) to avoid unwanted noise interference with the working.

Pin 6. THRESHOLD: Threshold pin voltage determines when to reset the flip-flop in the timer. The threshold pin is drawn from the positive input of the upper comparator. If the control pin is open, a voltage equal to or greater than VCC*(2/3) will reset the flip-flop. So the output goes low.

Pin 7. DISCHARGE: This Pin is drawn from the open collector of the transistor. Since the transistor (on which discharge pin got taken, Q1) got its base connected to Qbar. Whenever the output goes low, or the flip-flop gets reset, the discharge pin is pulled to the ground, and the capacitor discharges.

Pin 8. Power or VCC: It is connected to a positive voltage (+3.6v to +15v).

The complete circuit diagram for Automatic Fence Lighting with Alarm is shown above. LDR is placed facing toward the entrance, and a potentiometer is used to adjust the device's sensitivity. You can also switch between the negative Pin of the battery and LDR's grounded Pin to manually control this security system.

Here, the op-amp IC is used as a voltage comparator, and the 555 timer IC is placed in an astable mode. The LDR and the potentiometer create a voltage divider circuit. The output of this divider circuit will change according to the intensity of light that falls on the LDR. The divider is connected to the inverting Pin of the Op-amp IC. The non-inverting Pin is associated with the supply through a 5.7Kohm resistor, so the voltage value at the non-inverting is fixed. You can replace this resistor with a 10K potentiometer to adjust the voltage per the requirement.

We can adjust the device's sensitivity using the potentiometer VR1 connected in series with the LDR. When the voltage at the non-inverting input is greater than or equal to the reference voltage, the output (at pin 6) of the op-amp IC output (PIN 6) goes HIGH. Learn more about the working of op-amp by following the various op-amp-based circuits.

According to the circuit diagram, when LDR detects any activity, the output of the Op-amp IC goes LOW, and PNP transistor T1 starts conducting. Hence, the LED starts glowing, and the 555 timers IC gets triggered. Here, 555 timer IC is in Astable mode, and R3, R5, and C1 provide a preset time delay.

So whenever some person or object enters the prohibited area, his shadows will be sensed by the LDR, and the circuit triggers the alarm.

Download Gerber files from here.