Automatic night lamp use photocell to check light or dark; when dark the lamp is ON and when light the lamp goes OFF.
This type of circuit is good for street, garden and security lights and also the parts used in circuits are LDR (ORP12), op-amp (uA741), relay which controls the AC lamp.
Also, low light makes relay ON and lamp ON and hence, this Automatic Night Lamp Circuit with Photocell uses 9V DC power for best work.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors (All resistors are 1/4 watt unless specified) | 10k | 3 |
| 270k | 1 | |
| 4.7k | 1 | |
| 1k | 1 | |
| 1M Preset | 1 | |
| Photocell LDR ORP12 | 1 | |
| Semiconductors | IC uA741 | 1 |
| Transistor BC547 | 1 | |
| Diode 1N4007 | 1 | |
| Relay 9V SPDT | 1 | |
| Lamp 230V | 1 |
The automatic night lamp uses a uA741 operational amplifier as a voltage comparator and an LDR (ORP12) and a resistor R1 form a voltage divider and 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 and 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 and then 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 and in dark V_LDR is 1.76V and less than 4.5V so relay is ON and lamp is ON.
As a result, 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 follow below steps for connection:
- First, assemble all the components shown in the circuit diagram.
- Next, connect pin 2 of IC1 between the LDR (ORP12) and resistor R1, then connect pin 2 of IC1 to pin 6 of IC1 through resistor R4.
- Now connect pin 3 of IC1 as a voltage divider between resistor R2 and resistor R3.
- After that, connect pin 4 of IC1 to the ground of the circuit and also connect resistors R5 and R6 from pin 6 of IC1 to ground.
- Join pin 7 of IC1 to the positive supply of the circuit.
- Then 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 and then connect the emitter of transistor Q1 to the ground of the circuit.
- Finally, wire the load and the 230V AC supply in series with the common and normally open (NO) contacts of the relay.
Conclusion:
To conclude, this Automatic Night Lamp Circuit with Photocell turns the lamp ON in darkness and OFF in bright light, also it is simple to build with low cost and is useful for outdoor or security lighting.
Moreover, we can adjust the sensitivity by changing the resistor values in the circuit.