Simple Day Night Activated Switch Circuit using a Triac and LDR

This article shows you how to build a circuit that automatically turns things on and off depending on how light it is outside.

You only need a few parts: a special switch called a triac, a light sensor LDR and a resistor and capacitor.

This circuit can be used to turn on lamps at night and off during the day or other things like alarms.

It works with your houses regular electricity and does not need any extra motors or switches.

Building circuits with high voltage can be dangerous.

Be with adult supervision or someone who knows how to work with electricity safely.

Circuit Working:

Parts List:

This day night switch circuit, employing a triac and Light Dependent Resistor LDR, is exceptionally simple making it cost effective and easy to construct.

However, it comes with a particular limitation it lacks flexibility and does not allow electronic adjustments to control or modify the switch.

Unlike a mechanical switch it utilizes either an SCR or triac.

Various solutions have been devised to automatically turn on lights when it becomes dark.

Typically, a DC power supply and an electromechanical relay are required.

Nevertheless, this project minimizes the number of components needed by directly connecting to the AC power supply.

The crucial element in the day night triggered triac switch circuit is the LDR photoresistor which exhibits sensitive resistance.

In quebec, its resistance varies based on the intensity of the light it receives registering 200 ohms at night and a few thousand ohms during daylight.

The capacitor C1 functions as a voltage divider supplying the photosensitive LDR and the triac gate.

In normal light conditions the voltage decreases as the LDR resistance decreases.

Conversely, at night the resistance of the photoresistor increases leading to an elevated triac activation current, thereby operating and illuminating the connected lights.

Formulas:

You must construct a circuit that can switch a 220V lightbulb on or off dependent on the light level measured by the LDR in order to produce a Day Night Activated Switch Circuit.

The formula for configuring the circuit components are listed below.

Calculate the Voltage Divider:

The voltage VLDR across the LDR is determined by:

V_LDR​ = V_supply ​× R_LDR​​ / R_LDR​ + R_fixed​

where,

• The supply voltage to the LDR resistor network is denoted by Vsupply.
• The resistance of the LDR or VLDR is dependent on the intensity of the light.
• R fixed denotes the fixed resistor 1k in the above circuit.

Impact of Capacitor:

The voltage will be stabilized or filtered by the 1µF capacitor C.

The approximate charging time of the capacitor is:

t = R × C

where,

• The RC circuits time constant is denoted by t.
• If a filter is being utilized, the resistance (in series with the capacitor) is denoted as R.
• C represents capacitance.

Using a TRIAC as the switching element, this circuit will turn the light on or off depending on the light intensity sensed by the LDR.

How to Build:

To build the triac based day night activated switch circuit following process is mentioned below.

Triac Connection:

• Connect the MT1 and MT2 terminals of the triac to the AC mains supply.

LDR and Resistor:

• Connect one leg of the LDR to the MT1 terminal of the triac.
• Connect the other leg of the LDR to one end of the resistor.
• Connect the other end of the resistor to the gate terminal of the triac.

Capacitor Connection:

• Connect one leg of the capacitor C1 to the junction of the LDR and resistor.
• Connect the other leg of the capacitor to the MT2 terminal of the triac.

Diac Connection:

• Connect one leg of the diac to the gate terminal of the triac.
• Connect the other leg of the diac to the junction of the LDR, resistor and capacitor.

Indicator LED (Optional):

• Connect an LED across the MT1 and MT2 terminals of the triac for visual indication.

Power Supply:

• Connect the circuit to the AC mains power supply.

Important Points:

• Ensure all connections are secure and insulated to prevent any short circuits.
• Use appropriate safety measures when working with AC mains electricity.
• Choose components based on your specific requirements and refer to datasheets for accurate specifications.

Conclusion:

To conclude, kindly note once the connections are made the circuit should automatically switch off connected electrical appliances during daylight and activate them during the night.

Adjustments can be made by experimenting with resistor values or choosing an appropriate LDR based on the ambient light conditions in your location.

If you are not familiar with electronics, it is recommended to seek guidance from someone with experience or consult relevant documentation.

References:

Can I use a TRIAC as an AC switch and an LDR as a switch controller to turn on and off a LED bulb?

Datasheet BT136