LDR light switch

This is a LDR operated switch circuit, the LED’s ON when no light is present in the room.

circuit diagram and my construction pictures:

LDR light switch
LDR Light Switch
LDR Light Switch

LDR (light dependent resistor) is a passive optical transducer, the resistance of LDR decreases with increase in light intensity, LDR or photoresistor is applied in light sensing detecting circuits, light or dark activated switches.  The resistance of LDR is in the order of few MΩ in the dark and few hundreds of Ω in light….

working: RV1 (variable resistor) sets the reference voltage at the non-inverting pin(V+) of opamp(LM358).  LDR and R1(10K) forms voltage divider network ,  the voltage at the inverting pin(V-) of opamp is low when the LDR in dark , the LDR resistance in the dark is around 10MΩ so the less the voltage drop across the R1(10K) therefore V- is LOW, the voltage at the non-inverting pin is very high which is also the reference voltage set by the trimmer potentiometer(RV1) .

so, Vout = (V+) – (V-)  ( since V- <<  V+)   therefore Vout =V+  ,  the Vout is high, the MOSFET comes into conduction,simply LEDs ON in the dark.. the drain current of the MOSFET  set by the R2 and R3 resistors.  since  the MOSFET is voltage controlled the drain current is high the more the voltage drop across the R3 which is Vgs (gate to source voltage). the Vgs of the MOSFET must be greater than the threshold voltage(2.5V for NTD4963).

D1-D6 are the power LED’s , R4-R9 current limiting resistors, c1 and c2 are decoupling cap’s to remove noise.



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