circuit photocell to detect light intensity. At full light the resistance of the photocell will be a little ten ohms, and in darkness it will rise to several hundred ohms.
The op amp IC1 wired as a comparator uA741 here. In the darkness of photocell resistance increases and so the voltage at the inverting input of IC1 would be smaller than the reference voltage at the non-inverting input. The output of IC1 to go to positive saturation and the transistor switch ON to activate the relay. In this way the lights glow is connected through relay contacts. Diode works as a diode D1 encourage free.
Photocell
A light sensor (light sensor), which varies its resistance between two terminals based on the number of photons (light) it receives. Used for photographic light meter, automatic in-the-evening street lights and other applications that are sensitive to light, also called "light dependent resistor" (the LDR) and "photoresistor."
The photocell's semiconductor material typically cadmium sulfide (CDS), but other elements are also used. Photocells and photodiodes are used for similar applications, but the photocell through the bi-directionally flow, while the diodes are unidirectional.
Photocells come in various packages like this from PerkinElmer. When the photocell receives more photons, lowered resistance between two terminals.