Automatic Night Lamp Circuit with Photocell

Automatic night lamp use photocell to check light or dark.
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When dark the lamp is ON.

When light the lamp goes OFF.

It is good for street, garden and security lights.

Parts used in circuits are LDR (ORP12), op-amp (uA741), relay which controls the AC lamp.

Low light makes relay ON and lamp ON.

This Automatic Night Lamp Circuit with Photocell uses 9V DC power for best work.

Circuit Working:

Parts List:

The automatic night lamp uses a uA741 operational amplifier as a voltage comparator.

An LDR (ORP12) and a resistor R1 form a voltage divider sends voltage to pin 3 of the op-amp.

During the day the LDR has low resistance by making the voltage at pin 3 higher than the reference at pin 2.

This keeps the op-amp output low, by keeping transistor, relay and lamp OFF.

At night, the LDRs resistance increases by lowering the voltage at pin 3 below the reference at pin 2.

The op-amp output then goes high by turning ON the transistor and activating the relay and lighting the lamp.

Formulas with Calculations:

Below are the formulas with calculations for Automatic Night Lamp Circuit with Photocell:

Automatic night lamp work by voltage divider with LDR (ORP12) and R1.

Formula:

V_LDR = (Vcc × R1) / (R1 + R_LDR)

Vcc = 9V and R1 = 10k.

In bright light the R_LDR = 1k and V_LDR = 8.18V.

In dark the R_LDR = 500k and V_LDR = 1.76V.

Reference voltage at pin 2:

V_REF = (Vcc × R3) / (R2 + R3)

R2 = 10k, R3 = 10k, V_REF = 4.5V.

Relay turns ON when V_LDR < V_REF.

In bright light V_LDR is 8.18V and greater than 4.5V so relay is OFF and lamp is OFF.

In dark V_LDR is 1.76V and less than 4.5V so relay is ON and lamp is ON.

This shows the circuit will switch ON the lamp in dark and switch it OFF in bright light.

How to Build:

To build a Automatic Night Lamp Circuit with Photocell following steps are needed to follow:

• Assemble all the components shown in the circuit diagram.
• Connect pin 2 of IC1 between the LDR (ORP12) and resistor R1.
• Connect pin 2 of IC1 to pin 6 of IC1 through resistor R4.
• Connect pin 3 of IC1 as a voltage divider between resistor R2 and resistor R3.
• Connect pin 4 of IC1 to the ground of the circuit.
• Connect resistors R5 and R6 from pin 6 of IC1 to ground.
• Join pin 7 of IC1 to the positive supply of the circuit.
• Connect the base of transistor Q1 between resistors R5 and R6.
• Wire the collector of transistor Q1 to the relay coil terminals through diode D1.
• Connect the emitter of transistor Q1 to the ground of the circuit.
• Wire the load and the 230V AC supply in series with the common and normally open (NO) contacts of the relay.

Conclusion:

This Automatic Night Lamp Circuit with Photocell turns the lamp ON in darkness and OFF in bright light.

It is simple to build with low cost and is useful for outdoor or security lighting.

The sensitivity can be adjusted by changing resistor values in the circuit.

References:

µA741 General-Purpose Operational Amplifiers datasheet (Rev. G)