The LDR and trimmer form a variable voltage. In daylight, LDR low → non-inverting input <6V → output low → relay off. As light falls, LDR resistance rises → input voltage rises. At threshold (~6V), output goes high. The 1MΩ feedback resistor slightly raises the threshold voltage, requiring light to increase significantly before turning off.
The answer lies in .
: Hobbyist building a burglar alarm, student preparing a lab project, technician troubleshooting a control system, ham radio operator designing an audio filter. Master Handbook Of 1001 More Practical Electronic Circuits
While page counts vary by edition, 1001 circuits usually break down as: The LDR and trimmer form a variable voltage