IoTClass Inside the measurement
A closer look · 01

Sensor

Look inside
a sensor.

Light-dependent resistor

An LDR, or photoresistor, changes its electrical resistance with light: more light makes its resistance fall. Connected to a fixed resistor as a voltage divider, it turns that resistance change into a changing voltage. The IoT device reads this voltage through an analog-to-digital converter (ADC), which gives a numerical count.

Check which resistor is connected toward the supply and which toward ground. Swapping the LDR and fixed resistor reverses whether darkness makes the count rise or fall, so it reverses how a light-switch threshold should work.

01

Recognize the element

Recognize the element: Light-dependent resistor

ldr: sensor-family detailLight leads to a disc-shaped photoresistor with a winding track and two leads. Cropped from the approved sensor-family comparison; conceptual, not to scale. Conceptual light
Recognize the light-sensitive element. In this readout, a divider turns its changing resistance into a voltage for the controller.
02

Peel the teaching model

From case to sensing element

Choose a layer to see the measurement path. The enclosure and board layout are invented teaching choices, not a particular product.

The full text version below describes every layer. All measurement explanations are available without 3D.
Conceptual, not to scale
Case: disc with a zig-zag trackA photoresistor changes resistance as the light level changes. [Photoresistor Compression Loop]

Drag to rotate; pinch to zoom. With the view focused: arrow keys rotate, + / − zoom, 1–3 choose a layer, Home resets rotation and zoom. Step advances to the next layer; Reset returns to the case and default view. Layer buttons select a view directly. The page does not move on its own.

Layer 1 of 3 / Outside

Case: disc with a zig-zag track

A photoresistor changes resistance as the light level changes. [Photoresistor Compression Loop]

Teaching view: A round disc with a dark zig-zag track sits on two silver leads. The disc represents the resistor whose resistance changes with light.

The measurement to rememberA light-dependent resistor (LDR) changes resistance with light; a voltage divider turns that change into a voltage. [Photoresistor Compression Loop]

Full text version · every layer

1 · Case: disc with a zig-zag track

A photoresistor changes resistance as the light level changes. [Photoresistor Compression Loop]

Teaching view: A round disc with a dark zig-zag track sits on two silver leads. The disc represents the resistor whose resistance changes with light.

2 · Board / module: external voltage divider

The divider voltage depends on both the LDR resistance and the fixed resistor. [Photoresistor Compression Loop]

Teaching view: The two leads connect this loose LDR to an external circuit. Pairing it with a fixed resistor makes a voltage divider that turns resistance changes into voltage changes.

3 · Element: light-sensitive resistor

The course example shows resistance decreasing as illumination increases. [Photoresistor Compression Loop]

Teaching view: The inner patch and zig-zag track mark the light-sensitive resistor. Light frees electric charges in its semiconductor material, allowing current to flow more easily and lowering its resistance. With this resistor connected to the supply and a fixed resistor connected to ground, brighter light raises the voltage at their junction.

Explore the layers in order to follow the teaching model.

03

Listen

Package narration

Read transcript
Asha is building a connected desk lamp that switches on when her workspace gets dim. She chooses a light dependent resistor and tests how its reading changes when she shades it. Her sensor has a layer of cadmium sulfide semiconductor between metal contacts. Light absorbed in that layer frees electrons to move through the material. More mobile charge carriers make it conduct more easily, so its resistance falls as the light gets brighter. Asha connects the sensor between the supply and a fixed resistor to ground. Together they form a voltage divider. The voltage at their meeting point changes with the resistance ratio. An analog to digital converter turns that voltage into a number for her lamp controller. Use the course's bright desk example: a three point three volt supply, a ten kiloohm fixed resistor to ground, and a sensor resistance of three kiloohms. Multiply three point three by ten, then divide by three plus ten. The result is about two point five four volts. Asha expects darkness always to raise the reading. In this circuit, darkness raises the sensor's resistance and lowers the voltage. Swapping the sensor and fixed resistor reverses that direction. The takeaway: light changes the sensor's resistance, and the divider wiring decides which way the reading moves.

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