The voltage divider consisting of R1, LDR, R2, and the LDR (Light Dependent Resistor) has the characteristic of presenting very low resistance under strong light conditions and very high resistance in darkness.
The values of R1 and R2 have been chosen so that the voltage across R2 can vary from a minimum of about 1V (black) just below the supply voltage (in bright light); this variation is almost linear. This voltage is applied to the inverting input (-) of operational amplifier U1. The non-inverting input (+) is a voltage controlled by adjusting the trimmer.
The op-amp output becomes logic high when the voltage across R2 is below the level set by the trimmer. Suppose at a given moment, the illumination on the resistor gradually decreases. In this condition, the value of the LDR starts to increase, and consequently, the voltage across R2 decreases. The voltage applied to the op-amp’s + pin is compared by each comparator with the voltage set by the potentiometer. Therefore, the output of U1 goes high, causing the LED to emit light and activating the relay.
Twilight Switch Diagram
The Photoresistor
Photoelectric cells available in the market come with different characteristics, but the typical value is 1Mohm in darkness.
Here is a table illustrating this feature.
Test
Power the twilight switch circuit with a 12 V DC voltage, adjust the potentiometer to the middle, ensure that the indicator light is off when the cell is in full light. Now, try casting a shadow on the cell; the indicator light should turn on, and the relay should close.
Beware of Light!
The photoelectric cell should be placed away from the light emitted by lamps connected to our device, and even car headlights can disrupt the circuit’s operation.
Circuit board photos of the Twilight Switch
Source: ingdemurtas.it – CC