e-Gizmo Photosensitive LDR Light Sensor Module with Analog Out

Operating Voltage (VCC): 5.0V DC
Primary Sensing Element: 5mm CdS Photoresistor (LDR connected across J1)
Comparator IC: LM311 Differential Comparator (U1)
Voltage Divider Pull-Up Resistor (R2): 10 k-ohmsThreshold Potentiometer (RV1): 6.8 k-ohm trimmer with 1 k-ohm limit resistors (R1, R4)
Analog Output Voltage Range: VCC (Dark) down to approx. 0V (Bright Light)Digital
Output Logic: Active-LOW (Normally HIGH at VCC, pulls LOW to 0V on trigger)Output Pull-Up Resistor (R5): 4.7 k-ohms
Status LEDs: D2 Green (Power), D1 Red (Digital Trigger)
Main Header Spacing (J2): 2.54 mm (0.1 in) pitch

THEORY OF OPERATION

1. Photoresistor (LDR) & Analog Signal Generation
The sensing element is a Cadmium Sulfide (CdS) Light Dependent Resistor connected across J1 Pin 3 (IN) and GND. It forms a voltage divider circuit with fixed resistor R2 (10 k-ohms), which is pulled up to VCC (+5V).

The resulting analog voltage V_ANA at node IN is given by:
V_ANA = VCC * (R_LDR / (R2 + R_LDR))
- Dark Environment: R_LDR is extremely high (typically greater than or equal to 1 M-ohm). Because R_LDR is much greater than R2, the voltage V_ANA remains high, close to VCC (+5V).
- Bright Environment: R_LDR drops significantly (typically 1 k-ohm - 10 k-ohms). As R_LDR decreases relative to R2 = 10 k-ohms, V_ANA falls towards ground (0V).
Filter capacitor C1 (100 nF) is connected in parallel across the LDR to ground, smoothing rapid light fluctuations and removing high-frequency electrical noise from V_ANA. This signal is exposed on J2 Pin 1 (ANA) for direct measurement by microcontroller ADCs.

2. Threshold Reference Voltage Generation
A stable comparison threshold voltage V_ref is established using a resistive divider network comprising R1 (1 k-ohm), trimmer potentiometer RV1 (6.8 k-ohms), and R4 (1 k-ohm) connected between VCC and GND:
V_ref(min) = VCC * (R4 / (R1 + RV1 + R4)) = 5V * (1 k-ohm / 8.8 k-ohms) = approx. 0.57V
V_ref(max) = VCC * ((RV1 + R4) / (R1 + RV1 + R4)) = 5V * (7.8 k-ohms / 8.8 k-ohms) = approx. 4.43V
Adjusting RV1 allows precise tuning of V_ref at Pin 3 (non-inverting input V+) of the LM311 comparator (U1).

3. Digital Switching via LM311 Comparator
The LM311 comparator (U1) continuously compares the filtered analog sensor voltage V_ANA at Pin 2 (inverting input V-) against reference voltage V_ref at Pin 3 (non-inverting input V+).
a. Dark / Below-Threshold Condition (V_ANA > V_ref):
-V- (Pin 2) is higher than V+ (Pin 3).
- The LM311 internal output transistor remains OFF (open-collector).
- Resistor R5 (4.7 k-ohms) pulls Pin 7 (DIG) HIGH to VCC (+5V).
- Red LED D1 remains OFF because there is no ground path through Pin 7.
b. Bright / Threshold Reached Condition (V_ANA <= V_ref):
- As ambient brightness increases, R_LDR drops and V_ANA falls below V_ref.
- V- (Pin 2) becomes less than or equal to V+ (Pin 3).
- The LM311 output transistor turns ON, pulling Pin 7 (DIG) LOW to 0V.
- Current flows from VCC through R6 (2.4 k-ohms) and Red LED D1 into Pin 7 to ground, illuminating D1.
- The DIGITAL OUT pin (J2 Pin 2) provides a clean Active-LOW logic output (0V).

APPLICATION & CALIBRATION GUIDELINES

Wiring:
- Connect VCC (J2 Pin 4) to +5V DC power supply.
- Connect GND (J2 Pin 3) to system ground.
- Connect ANA (J2 Pin 1) to an analog input (e.g., A0 on Arduino) for intensity reading.
- Connect DIG (J2 Pin 2) to a digital input or interrupt pin.

Calibration:
- Expose the LDR to the desired light level at which the output should trigger.
-Turn potentiometer RV1 until Red LED D1 just turns ON.
-When light level exceeds this calibrated threshold, DIG goes LOW (0V) and D1 illuminates.