Building an Auto Street Light Circuit using IC 741 and Transistor

Building an Auto Street Light Circuit using IC 741 and Transistor, which turns street lights on at night and off during the day, is a simple but efficient way to save energy.

This design employs a relay to operate the street light, an LDR (Light Dependent Resistor) for light detection, BC547 transistor used to drive the relay and an operational amplifier IC 741 as a comparator.

Because of its dependability, affordability and simplicity of construction, the circuit is perfect for small-scale applications or educational settings.

Circuit Working:

Parts List:

Building this auto street light circuits main function is to turn the lightbulb on at night and off during the day.

As a result, we employed LDR as a light dark sensor.

R1 and LDR combine to create a potential divider.

Preset VR1gives the sensitivity level to non inverting input of IC 741.

The reduced resistance of the LDR during the day causes a larger voltage at the IC 741s inverting input.

The resistance of the LDR rises at night, which lowers the voltage at the inverting input.

The IC 741 does a comparison between the reference voltage at the non inverting input (+) and the voltage at the inverting input (-).

If the reference voltage is lower than the inverting input voltage, the ICs output is low.

A lower inverting input voltage results in a higher output.

Current can pass via the relay coil when the transistor BC547 is engaged by a HIGH IC output.

When the relay flips on, the linked street light turns on as well.

The resistance of the LDR decreases with increasing ambient light, which raises the inverting input voltage.
The transistor is deactivated, the relay and street light are turned off, and the IC output turns LOW.

Formulas and Calculations:

Below are the formulas for calculating Auto Street Light Circuit:

Voltage across the LDR and R1 (Voltage Divider Rule):

V_LDR = (R_LDR / (R_LDR + R1)) * Vcc

where,

• R_LDR is the resistance of the LDR.
• R1 is the fixed resistor in the divider 100k.
• Vcc is the supply voltage +12V.

Reference Voltage at Non-Inverting Input:

V_ref = (RV1 / (RV1 + R3)) * Vcc

where,

• RV1 is the variable resistor 10k
• R3 is the fixed resistor stabilizing the non inverting input.
• Vcc is the supply voltage +12V.

Base Current for the Transistor (BC547):

I_B = (V_out – V_BE) / R6

where,

• V_out is the output voltage of the IC 741 around 12V when high.
• V_BE is the base emitter voltage of the BC547 around 0.7V
• R6 is the base resistor 1k.

Calculation:

If V_out = 12V, then:
I_B = (12V – 0.7V) / 1kΩ = 11.3 mA

Relay Current (Collector Current):

I_C = β * I_B

where,

• β is the current gain of the BC547 around 100-200.
• I_B is the base current.

Calculation:

If I_B = 11.3 mA and β = 100:

I_C = 100 * 11.3 mA = 1.13 A

Power Dissipation in the Resistors:

P = I² * R

where,

• I is the current through the resistor.
• R is the resistance value.

How to Build:

To build an Auto Street Light Circuit using IC 741 and Transistor follow the below mentioned steps for connections:

• Gather all the components as mentioned in the above circuit diagram.
• Connect pin 2 of IC1 741 to pin 6 through resistor R5.
• Connect a resistor R1 between the positive supply and pin 2 of IC1 and connect a LDR between pin 2 of IC1 and GND.
• Connect pin 3 of IC 741 to center leg of VR1 and second leg to positive supply through resistor R3, and 3rd leg of VR1 to GND through resistor R4.
• Connect pin 4 of IC1 741 to GND.
• Connect pin 6 of IC1 741 to base of transistor Q1 through resistor R6.
• Connect pin 7 of IC1 741 to positive supply of 12V.
• Connect collector of transistor Q1 to one terminal of relay and other terminal to positive supply, connect emitter of transistor Q1 to GND.
• Connect the cathode of diode D1 to positive supply and anode to collector of transistor Q1.
• Connect a 220V bulb and 220V AC supply in series between the NC pin and the Common pin of the 12V relay

Conclusion:

This IC 741 based auto street light circuit shows how to control street lights in an easy to understand manner.

It is perfect for energy conservation and runs effectively with few parts.

This circuit is appropriate for streets, gardens and other outdoor lighting applications as it can be adjusted for different light situations with the right calibration.

References:

 Design And Fabrication Of Automatic Street Light Control System