In this post we learn how to make a transformerless automatic darkness activated LED lamp circuit, using a couple of transistors and capacitive based power supply, eliminating the use of any bulky transformer.
Compact Transformerless Design
Although the concept may look pretty familiar and common, the main feature of the circuit is its low current consumption and compactness.
The power supply used here is a capacitive type, thus no transformer is incorporated making the circuit very compact and fixable in any small corner of the particular premise.
Why use LEDs
The use of LEDs in place of a filament bulb makes the application very power economic and efficient. The proposed LED automatic day night lamp switch circuit diagram shows red LED being used, however white LEDs would suit the application better, as that would help illuminate the area better than the red LEDs.
How to Install the LDR
The LDR must be positioned such that the light from the LED does not fall on it, only the ambient light which is to be sensed is required to reach the LDR.
How the Entire Circuit Works
The proposed transformerless automatic day night LED lamp circuit may be understood through the following points: The input 220 V mains suply is applied across the 10 Ohm resistor and the other neutral point.
The 10 Ohms resistor helps to cancel out the initial surge or the voltage rush that might otherwise be potentially harmful to the further stages of the circuit. The MOV or the varistor placed after the 10 Ohm resistor enhances the protection feature of the unit and grounds all surges that might sneak in after the 10 Ohm resistor.
The capacitor drops the mains voltage current to lower levels and the bridge rectifier made up the four diodes rectify the voltage to DC.
The 1000uF capacitor filters the rectified voltage and the smooth DC is applied to the control circuit consisting the two transistors.
The first transistor is wired up as a comparator, which compares the potential difference across the variable resistor and conducts when the voltage across it rises to saturation levels.
The above rise in the voltage level takes place when the relevant magnitude of light falls on the LDR surface.
Once the resistance of the LDR falls below the set threshold due to higher ambient light, the transistor conducts. The collector of the above transistor instantly grounds the base of the next transistor and switches it OFF.
The associated LED lights connected to the collector of the second transistor are also immediately switched OFF. The opposite reaction takes place when the light over the LDR falls below the set threshold, probably during dusk when the sun sets.
The LEDs light up again and remain switched ON until the day beaks and the ambient light over the LDR reaches the set high threshold level. The following figure shows a simple LED automatic day, night lamp circuit.
WARNING: THE CIRCUIT IS NOT ISOLATED FROM MAINS AND THEREFORE IS LETHAL, IF TOUCHED IN POWERED ON CONDITION WITHOUT A PROPER ENCLOSURE.
Modifying the Above Design for Activating a 220V Lamp with a Triac
The above triac based design can be further improved by using an opamp controller for achieving a cleaner automatic switching action of the lamp during darkness, as shown below: