Simple Automatic Day Night Switch Circuit

Saving electricity is important especially in cities where there is not enough to go around anymore.

This article shows you how to build a simple automatic light switch that turns things on, and off depending on how light or dark it is outside.

This can help you save money on your electric bill.

What is a Automatic Day Night Switch Circuit:

An automatic day-night switch circuit also known, as a light sensitive switch or twilight switch is designed to automatically turn a load such as lights on during the night or in low light conditions and turn it off during the day or in bright conditions.

This type of circuit is commonly used in outdoor lighting, streetlights and other applications where automatic control based on ambient light levels is desired.

Working Explanation:

Parts List:

The circuit employs two transistors Q1 and Q2 and an LDR.

When light falls on the LDR the increased resistance causes Q1 to turn on, activating the relay.

Conversely, in the absence of light Q1 turns off deactivating relay.

This dynamic switching mechanism allows for the effective control of various appliances during day and night cycles.

By adopting this simple automatic day night circuit one contributes to the collective effort of electricity conservation aligning with the imperative need to address the challenges of contemporary energy demands.

Formulas and Technical Details:

1. Light Dependent Resistor LDR Resistance:

R_LDR ∝ 1 / I_LDR

where:

• R_LDR​ is the resistance of the LDR.
• I_LDR​ is the current flowing through the LDR.

2. Transistor Q1 Activation Threshold:

V_T1 = V_BE1 + I_LDR × R_LDR

where:

• V_T1​ is the threshold voltage for Q1.
• V_BE1 is the base-emitter voltage for Q1.

3. Transistor Q2 Activation Threshold:

V_T2 = V_BE2 + ( V_CC − I_LDR × R_LDR )

where:

• V_T2​ is the threshold voltage for Q2.
• V_BE2​ is the base-emitter voltage for Q2.
• V_CC​ is the supply voltage.

4. Relay Activation Time Constant:

τ = R_Relay × C_Relay

where:

• R_Relay is the resistance of the relay coil.
• C_Relay is the capacitance of the relay.

Note:

These formulas provide insights into the relationships between resistance, current and voltage thresholds within the circuit.

Adjustments to the simple automatic day night circuit components, such as the variable resistor, can be made based on these formulas to fine tune sensitivity and optimize performance according to specific environmental conditions.

Circuit Construction:

• Gather all the required materials and lay them out on a clean workspace.
• Ensure the resistors, capacitors, and diodes are selected based on their compatibility with the circuit specifications.
• Integrate the LDR into the circuit.
• As ambient light varies the LDRs resistance changes influencing the states of Q1 and Q2.
• Connect NPN transistors Q1 and Q2 to the circuit.
• Q1 activates during daylight, while Q2 takes over when darkness descends activating the relay.
• Ensure that the relay is appropriately connected to the right side transistor.
• This arrangement enables the desired appliances, to be turned on and off correctly with the day night cycle.
• Incorporate the 100k variable resistor to fine tune the sensitivity of the circuit.
• Adjustments can be made to accommodate specific environmental conditions.
• Connect the circuit to a 5V to 12V DC battery ensuring that the relay specifications align with the chosen working voltage.

References

DESIGN AND CONSTRUCTION OF AN LDR 

Design and Analysis of Image as a street light switch