Category Archives: Sensor circuits

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.


IR sensor module

IR emitter:
It is a LED emits infrared light (940nm typically) applications of IR LED: like optical counters, free air data transmission, infrared remote control, card readers , etc….

photodiode (type of optical transducer)

photodiode is a high speed diode used to convert current to voltage, it is reverse biased with constant voltage.

Sensor module circuit diagram:

IR sensor module schematic

IR sensor module schematic

TCRT5000 sensor
opamp LM358
R2 and R3 for biasing the non-inverterting terminal of opamp(LM358), voltage at non-inverting terminal = Vin(1.8K)/(22K+1.8K) = 0.378V (378mV).. that is also the voltage at the pin1 of opamp with refered to ground(GND) when no IR light on photodiode. when full IR light on photodiode the Vout = 3.5V (which is max)

R1 sets the reverse bias voltage across D1(photodiode), in this condition the current flowing through the D1 is in the range of nA typically.

R4 sets the constant current that flows through the IR-LED D2
the forward voltage range is around 1.25 to 1.5V typically, the max current it can handle is 60mA so R4 = (Vin-1.25)/20mA = 187.5ohms if Vin = 5V (220ohms on board)

C2 and C1(decoupling caps) C1 electrolytic reduces the noise voltage created by the low frequency and mid frequency and C2 the low ESR ceramic cap reduces the high frequency noise in power supply lines

here is my construction, it looks ugly, ahh but working fine 😀

IR sensor module circuit

IR sensor module circuit

the output of sensor module connected to the scope, as you can see the voltage varies, the photodiode starts conduction when the IR light from the emitter is reflected on it, Vout is proportional to the current that flows through the photodiode.