Simple Day Night Activated Switch Circuit using a Triac and LDR

This article teaches how to construct a circuit that in response to the amount of light outside automatically switches objects ON and OFF.

All you need is a light sensor LDR, a resistor, a capacitor and a special switch known as a triac.

This circuit is commonly used to activate alarms which shut ON lighting at night and turn them OFF during the day.

It does not need any additional motors or switches which runs on homes standard electricity.

Creating high voltage circuits could prove risky.

Perform under the supervision of an adult or someone who is familiar with safe electrical work practices.

Circuit Working:

Parts List:

This day night switch circuit which uses a triac and a light dependent resistor LDR is extremely simple and making it cost effective and easy to build.

It has a specific disadvantage also, however because it is solid and does not allow electronic modifications or control of the switch.

When compared to a mechanical switch it makes use of a Triac or SCR.

Many ways to make lights switch ON automatically as it becomes dark have been developed.

Usually an electromechanical relay and a DC power source are needed.

However by connecting directly to the AC power source this solution reduces the amount of components required.

Due to its sensitive resistance the LDR photoresistor is an essential component of the day night triggered triac switch circuit.

It registers 200 ohms at night and a few thousand ohms during the day in Quebec depending on how much light it receives.

The photosensitive LDR and the Triac gate are supplied by the capacitor C1 which serves as a voltage divider.

Under normal lighting circumstances the voltage drops as the LDR resistance drops.

As a result the photoresistors resistance rises at night resulting in a higher Triac activation current that powers and flashes the connected lights.

Formulas:

To create a Day Night Activated Switch Circuit one can build a circuit that can turn ON or OFF a 220V lightbulb based on the light level detected by the LDR.

The circuit component configuration formula is shown below:

Calculate the Voltage Divider:

The voltage VLDR across the LDR is calculated by:

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

where,

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

Capacitor Impact:

The 1µF capacitor C will filter or regulate the voltage.

The capacitor will charge in around the following time:

R × C = t

where

• The symbol for the RC circuits time constant is t.
• The resistance (in series with the capacitor) is indicated by R if a filter is being used.
• Capacitance is represented by C.

Depending on the light intensity detected by the LDR this circuit will either turn the light on or off using a TRIAC as the switching element.

How to Build:

To build a Simple Day Night Activated Switch Circuit using a Triac and LDR following steps are mentioned below.

The triac connection:

• The triacs MT1 and MT2 terminals should be connected to the AC mains supply.

Resistor and LDR:

• Connect the triacs MT1 terminal to one leg of the LDR.
• Join one end of the resistor to the opposite leg of the LDR.
• Connect the other end of the resistor to the triacs gate terminal.

Connection of the Capacitor:

• Wire the capacitor C1s first leg to the resistor LDR junction.
• Connect the capacitors opposite leg to the triacs MT2 terminal.

Diac Connection:

• The diacs leg should be connected to the triacs gate terminal.
• Join the opposite leg of the diac to the LDR, resistor and capacitor junction.

Optional Indicator LED:

• An LED can be connected between the triacs MT1 and MT2 terminals to provide visual indication.

Source of Power:

• Connect the AC mains power supply to the circuit.

Key Points:

• To avoid any short circuits make sure every connection is safe and insulated.
• When handling AC mains power take the proper precautions.
• Select components according to your particular requirements and consult datasheets for exact details.

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.

One can make adjustments by changing the resistor values or selecting an LDR that is suitable with the locations natural light levels.

It is advised to check proper datasheet or ask an experienced person for advice if unfamiliar with electronics.

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