This circuit is like a smart switch for your night light.
It uses a special light sensor LDR to see how bright it is outside.
At night, when it gets dark enough the circuit turns on your lamp automatically.
Then, in the morning when it gets light again the circuit turns the lamp back off.
It does this using a tiny chip IC 555 that acts like a timer.
Circuit Working:
Parts List:
| Category | Component | Quantity | Details |
|---|---|---|---|
| Resistors | 1k | 1 | 1/4 watt |
| 220Ω | 1 | 1/4 watt | |
| Potentiometer | 1M | 1 | |
| Light Dependent Resistor (LDR) | 1 | ||
| Capacitors | Ceramic C1 | 1 | 0.01µF |
| Electrolytic C2 | 1 | 100µF, 25V | |
| Semiconductors | IC | 1 | 555 |
| Transistor | 1 | BC547 | |
| Diode | 1 | 1N4007 | |
| LEDs | Red LED (5mm) | 1 | 20mA |
| Relays | 12V Relay | 1 |
This circuit automatically illuminates the premises of a house around 6 pm and switches off in the morning.
It operates using the light sensing property of an LDR.
Unlike other LDR based circuits, this design ensures a smooth transition without lamp flickering enabling the use of CFL and fluorescent lamps.
It utilizes the schmitt trigger action of the timer IC 555 for precise switching.
The circuit employs an LDR and timer IC 555 for automatic switching.
The LDR exhibits very high resistance around 10 Meg in darkness, but only 100 ohms or less in light.
This property makes it ideal for triggering lamp activation based on the presence or absence of sunlight.
In this circuit, the LDR triggers the timer IC 555 configured as a schmitt trigger.
The timer IC 555 features two internal comparators: the threshold comparator and trigger comparator.
The set and reset actions of these comparators can be utilized for ON/OFF operations.
In this setup, the IC 555 acts as a bistable with schmitt trigger action.
The upper comparator threshold comparator of the IC trips at 2 to 3 of the supply voltage, while the lower comparator trigger comparator trips at 1 to 3 of the supply voltage.
Both comparators inputs pin 6 and pin 2 are shorted and connected to the junction of the LDR and the Preset VR1 in the circuit.
During daylight, the LDR allows more current causing the current into the upper comparator pin 6 to exceed 2 to 3 Vcc resetting the ICs internal Flip Flop.
Simultaneously, the current into the lower comparator pin 2 surpasses 1 to 3 Vcc resulting in a low output from IC1.
When the light on the LDR diminishes its resistance increases reducing the current flowing to the upper and lower comparators of IC1.
This decreases the input voltage of the upper comparator below 2 to 3 Vcc and that of the lower comparator below 1 to 3 Vcc, leading to a high output from pin 3 of IC1.
This triggers T1, and the relay connected to its collector turns on allowing AC to reach the lamp through the Common and NO normally open contacts of the relay.
Only the phase line is connected through the relay contacts while the neutral line remains unaffected.
VR1 adjusts the sensitivity of the LDR to the specific light level at which the lamp turns on.
Capacitor C2 maintains the base voltage of T1 for clean switching action and also prevents relay clicking.
Diode D1 eliminates back e.m.f when T1 switches off.
It is important to place the LDR away from the lamp light while ensuring it receives daylight.
Use a 12V relay and refer to the wiring connections in the diagrams.
Formulas:
From the above circuit a resistive divider network is made up of a potentiometer and a LDR.
The threshold or control voltage input of the inbuilt comparator op amp of the IC 555 can be linked to this divider network.
Assuming that Rpot is the resistance of the potentiometer (when set to a certain value Rpotadj) and LDR resistance are both given:
Total Resistance of the Divider (Rtotal):
Rtotal = RLDR + Rpotadj
The voltage at the junction (Vjunction):
The voltage divider formula may be used to determine the voltage at the junction, which is linked to pin 5 of IC 555:
Vjunction = VCC × Rpotadj / Rtotal
Notes:
Varying the potentiometer Rpotadj will alter the Vjunction threshold voltage, which in turn influences the internal comparator of the IC 555.
Make sure the resistor values selected fit the IC 555s working range and are suitable for the intended use.
Generally speaking, the IC 555 performs best at voltages between GND and VCC.
The IC 555 may be utilized for a variety of timing and control applications, such as light detection using the LDR, thanks to the variable voltage input that its resistive divider architecture gives it.
Test the circuit to make sure it is operating properly and adjust the resistor values according to the needs of the particular circuit.
How to Build:
To build a simple Automatic Night Lamp Circuit you need to follow the below mentioned steps:
- Connect the LDR in series with preset VR11M with one leg at positive power supply.
- Connect the junction of LDR and VR1 to pin 6 threshold input and pin 2 trigger input of IC 555.
- Connect R2 220Ω with the base of transistor T1.
- Connect the pin 3 to ground through R1 1k and red LED.
- Connect pin 1 to ground directly.
- Connect pin 4 reset to Vcc to disable the reset function.
- Connect pin 5 to the junction of C1 0.01 capacitor and ground.
- Connect the pin 4 and pin 8 directly to positive supply.
- Connect the collector of T1 BC547 to the relay coil and connect the emitter of T1 to ground.
- Connect the common terminal of the relay to the phase line live wire of the lamp.
- Connect the NO normally open terminal of the relay to the lamps neutral line.
- Connect a diode D1 1N4007 across the relay coil with the anode connected to the collector of T1 and the cathode connected to Vcc to suppress back e.m.f.
- Adjust VR1 to set the sensitivity of the LDR for the desired light level at which the lamp should turn on.
- Apply power to the circuit.
- The lamp should turn on automatically when the ambient light falls below the set threshold and turn off when the light level rises above the threshold.
Note:
- Always double check the connections and component values before applying power to avoid damage to the components.
Conclusion:
An automatic night lamp circuit using IC 555 is a simple and effective way to automatically control the illumination of a lamp based on the ambient light level.
By utilizing an LDR and the versatile IC 555 this circuit provides a cost effective solution for applications where automatic lighting control is desired such as night lamps, street lights and security lighting.