It works as a dark key with the LDR that you see in the diagram . The circuit consists of four resistors, one opamp, two potentiometers, one LDR, one switching transistor and one lamp. In the circuit, the value of the resistor R1 is directly proportional to the internal resistance of the LDR and operates as a voltage divider between 6-12V.
Depending on the internal resistance of LDR1, the function of the RV1 potentiometer was used for the degree of sensitivity according to the amount of light that would fall on the LDR1. The RV1 is also a voltage divider. Therefore, the resistance value divided by the LDR1 and R1 materials is continuously compared by the resistance value U1 divided by the RV1. Assuming that the RV1 potentiometer is 6V, and that LDR1 is also in the dark, the voltage at the non-inverting input of U1 will be a value of approximately 6V or slightly more than that of the U1 (741 opamps) because the internal resistance of LDR1 will be high. there will be a positive voltage.
This will trigger Q1 through R3 and give L1 light. A slight decrease in the internal resistance of the LDR1 will be observed with the light emitting of L1, which will pull the opamp output to 0V and will go to the T1 transistor and L1 will go out. The only thing to note is that, assuming that there is no R4 resistance, there will be a constant voltage on the opamp output and this will prevent the transistor Q1 from going to the cutting. The resistance of R4 will draw the voltage of the Q1 transistor on the base to the chassis. Since the RV2 potentiometer will react against opamp 741 , the gain will change. When the resistance of the RV2 potentiometer is large, the opamp will operate as a comparator, and when it is small it will function as an amplifier.
741 Op Amp Pinout