Building a Light-Activated Switch with an LDR

Overview

A light-activated switch (or dark-activated night light) is a classic analog sensing circuit that automatically switches a load—such as an LED, buzzer, or relay—on or off based on ambient light levels. By combining a Light Dependent Resistor (LDR) with a fixed resistor (or potentiometer) in a voltage divider and feeding the output into the base of an NPN bipolar junction transistor (BJT), you can build a reliable threshold switch without requiring a microcontroller.

In this tutorial, you will learn how the LDR voltage divider works, calculate component values for reliable switching around the transistor's 0.7V base-emitter threshold, and design both schematic and PCB layouts in tscircuit.

export default () => (
  <board width="36mm" height="26mm">
    {/* 9V Power Input Header */}
    <pinheader
      name="J1"
      pinCount={2}
      pitch="2.54mm"
      footprint="pinrow2"
      pinLabels={["VCC", "GND"]}
      pcbX={-13}
      pcbY={0}
      schX={-6}
      schY={0}
    />

    {/* LDR (Photoresistor) modeled as top leg in dark-activated divider */}
    <resistor
      name="R_BIAS"
      resistance="10k"
      footprint="0805"
      pcbX={-5}
      pcbY={6}
      schX={-2}
      schY={3}
    />
    <resistor
      name="R_LDR"
      resistance="100k"
      footprint="axial_p5.08mm"
      pcbX={-5}
      pcbY={-6}
      schX={-2}
      schY={-3}
    />

    {/* Base Protection Resistor */}
    <resistor
      name="R_BASE"
      resistance="1k"
      footprint="0805"
      pcbX={1}
      pcbY={0}
      schX={1}
      schY={0}
    />

    {/* NPN Switching Transistor (e.g., 2N2222 / MMBT2222A / BC547) */}
    <transistor
      name="Q1"
      type="npn"
      footprint="sot23"
      pcbX={7}
      pcbY={-2}
      schX={4}
      schY={-1}
    />

    {/* LED Load and Current-Limiting Resistor */}
    <resistor
      name="R_LED"
      resistance="330"
      footprint="0805"
      pcbX={7}
      pcbY={7}
      schX={4}
      schY={4}
    />
    <led
      name="D1"
      color="yellow"
      footprint="0805"
      pcbX={13}
      pcbY={3}
      schX={4}
      schY={2}
    />

    {/* Power Nets */}
    <trace from=".J1 > .VCC" to="net.VCC" />
    <trace from=".J1 > .GND" to="net.GND" />

    {/* Voltage Divider Wiring */}
    <trace from=".R_BIAS > .pin1" to="net.VCC" />
    <trace from=".R_BIAS > .pin2" to="net.VDIV" />
    <trace from=".R_LDR > .pin1" to="net.VDIV" />
    <trace from=".R_LDR > .pin2" to="net.GND" />

    {/* Transistor Base Drive */}
    <trace from="net.VDIV" to=".R_BASE > .pin1" />
    <trace from=".R_BASE > .pin2" to=".Q1 > .base" />

    {/* Collector LED Load Path */}
    <trace from=".R_LED > .pin1" to="net.VCC" />
    <trace from=".R_LED > .pin2" to=".D1 > .anode" />
    <trace from=".D1 > .cathode" to=".Q1 > .collector" />

    {/* Transistor Emitter to Ground */}
    <trace from=".Q1 > .emitter" to="net.GND" />
  </board>
)

---

Components Needed

Reference	Component	Typical Value / Part	Role in Circuit
J1	2-Pin Header	2.54mm pitch (pinrow2)	Connects 5V–9V DC supply (VCC and GND)
R_LDR	Light Dependent Resistor (LDR)	GL5528 (~1kΩ bright light to >100kΩ dark)	Varies resistance inversely with ambient light intensity
R_BIAS	Fixed Bias Resistor (or Potentiometer)	10kΩ – 50kΩ (0805 or pinrow3 trimpot)	Forms a voltage divider with R_LDR to set the light threshold
R_BASE	Base Current-Limiting Resistor	1kΩ (0805)	Limits maximum base current into Q1 when R_LDR is high
Q1	NPN Bipolar Junction Transistor	2N2222, BC547, or SMD MMBT2222A (sot23)	Acts as a low-side electronic switch for the load
R_LED	LED Series Resistor	330Ω – 470Ω (0805)	Limits current through the indicator LED to safe levels (~15mA)
D1	Indicator LED	Red, Yellow, or White LED (0805)	Illuminates when the transistor switch turns on

---

How the Circuit Works

1. The LDR and Voltage Divider

A Light Dependent Resistor (LDR), or photoresistor, is made of a high-resistance semiconductor such as cadmium sulfide (CdS). When photons strike the photosensitive surface, bound electrons jump into the conduction band, dramatically lowering the component's resistance:

• In bright light: R_LDR drops to a few hundred ohms or 1kΩ–2kΩ.
• In darkness: R_LDR rises to 100kΩ–1MΩ.

To convert this resistance change into a control voltage, we pair R_LDR with a fixed resistor R_BIAS across the supply rails (VCC and GND). Using the standard voltage divider equation, the voltage at the midpoint (V_DIV) when R_BIAS is connected to VCC and R_LDR is connected to GND is:

V_DIV = V_CC * (R_LDR / (R_BIAS + R_LDR))

2. NPN Transistor Threshold (V_BE ≈ 0.7V)

An NPN transistor (Q1) remains OFF (cut-off region) as long as its base-to-emitter voltage (V_BE) stays below approximately 0.6V–0.7V. Once V_DIV exceeds 0.7V, current flows into the base terminal, driving Q1 into saturation so current can flow from its collector to its emitter and illuminate the LED (D1).

Let's verify both operating states with V_CC = 9V and R_BIAS = 10kΩ (Dark-Activated Night Light configuration):

1. Bright Environment (R_LDR ≈ 500Ω):

   V_DIV = 9V * (500 / (10000 + 500)) ≈ 0.43V

   Because 0.43V < 0.7V, transistor Q1 stays OFF and the LED remains OFF.

2. Dark Environment (R_LDR ≈ 100kΩ):

   V_DIV = 9V * (100000 / (10000 + 100000)) ≈ 8.18V (clamped to ~0.7V at Q1 base)

   Because the open-circuit divider voltage is well above 0.7V, base current flows through R_BIAS and R_BASE into Q1, saturating the transistor and turning the LED ON.

---

Step-by-Step Circuit Design in tscircuit

Step 1: Build the LDR Voltage Divider

We start by placing a 2-pin power header (J1), the upper pull-up bias resistor (R_BIAS), and the lower photoresistor (R_LDR) connected between net.VCC, net.VDIV, and net.GND. Because an LDR is a two-terminal resistive sensor, we use a <resistor /> with a through-hole axial footprint (axial_p5.08mm) so a standard 5mm photoresistor can be soldered directly onto the board.

export default () => (
  <board width="30mm" height="24mm">
    <pinheader
      name="J1"
      pinCount={2}
      pitch="2.54mm"
      footprint="pinrow2"
      pinLabels={["VCC", "GND"]}
      schX={-5}
      schY={0}
    />
    <resistor
      name="R_BIAS"
      resistance="10k"
      footprint="0805"
      schX={0}
      schY={2.5}
    />
    <resistor
      name="R_LDR"
      resistance="100k"
      footprint="axial_p5.08mm"
      schX={0}
      schY={-2.5}
    />

    <trace from=".J1 > .VCC" to="net.VCC" />
    <trace from=".J1 > .GND" to="net.GND" />
    <trace from=".R_BIAS > .pin1" to="net.VCC" />
    <trace from=".R_BIAS > .pin2" to="net.VDIV" />
    <trace from=".R_LDR > .pin1" to="net.VDIV" />
    <trace from=".R_LDR > .pin2" to="net.GND" />
  </board>
)

Step 2: Add the NPN Transistor and LED Load

Next, we add the <transistor /> element (Q1) configured as type="npn" with a surface-mount sot23 footprint, a 1kΩ base protection resistor (R_BASE), and the collector load consisting of a 330Ω current-limiting resistor (R_LED) in series with an LED (D1).

export default () => (
  <board width="36mm" height="26mm">
    <pinheader
      name="J1"
      pinCount={2}
      pitch="2.54mm"
      footprint="pinrow2"
      pinLabels={["VCC", "GND"]}
      schX={-6}
      schY={0}
    />
    <resistor name="R_BIAS" resistance="10k" footprint="0805" schX={-2} schY={3} />
    <resistor name="R_LDR" resistance="100k" footprint="axial_p5.08mm" schX={-2} schY={-3} />
    <resistor name="R_BASE" resistance="1k" footprint="0805" schX={1} schY={0} />
    <transistor name="Q1" type="npn" footprint="sot23" schX={4} schY={-1} />
    <resistor name="R_LED" resistance="330" footprint="0805" schX={4} schY={4} />
    <led name="D1" color="yellow" footprint="0805" schX={4} schY={2} />

    <trace from=".J1 > .VCC" to="net.VCC" />
    <trace from=".J1 > .GND" to="net.GND" />
    <trace from=".R_BIAS > .pin1" to="net.VCC" />
    <trace from=".R_BIAS > .pin2" to="net.VDIV" />
    <trace from=".R_LDR > .pin1" to="net.VDIV" />
    <trace from=".R_LDR > .pin2" to="net.GND" />

    <trace from="net.VDIV" to=".R_BASE > .pin1" />
    <trace from=".R_BASE > .pin2" to=".Q1 > .base" />

    <trace from=".R_LED > .pin1" to="net.VCC" />
    <trace from=".R_LED > .pin2" to=".D1 > .anode" />
    <trace from=".D1 > .cathode" to=".Q1 > .collector" />
    <trace from=".Q1 > .emitter" to="net.GND" />
  </board>
)

Step 3: Add Adjustable Sensitivity with a Potentiometer

Ambient lighting conditions vary widely between indoor rooms and outdoor twilight. By replacing or pairing the fixed R_BIAS resistor with a <potentiometer /> (R_POT, 50kΩ), you can fine-tune the exact light level that triggers the transistor switch.

export default () => (
  <board width="40mm" height="28mm">
    <pinheader
      name="J1"
      pinCount={2}
      pitch="2.54mm"
      footprint="pinrow2"
      pinLabels={["VCC", "GND"]}
      pcbX={-15}
      pcbY={0}
      schX={-6}
      schY={0}
    />

    {/* 50k Sensitivity Trimpot in series with a 4.7k minimum stop resistor */}
    <resistor
      name="R_STOP"
      resistance="4.7k"
      footprint="0805"
      pcbX={-7}
      pcbY={8}
      schX={-2}
      schY={4.5}
    />
    <potentiometer
      name="R_POT"
      maxResistance="50k"
      pinVariant="two_pin"
      footprint="pinrow2"
      pcbX={-7}
      pcbY={2}
      schX={-2}
      schY={2}
    />
    <resistor
      name="R_LDR"
      resistance="100k"
      footprint="axial_p5.08mm"
      pcbX={-7}
      pcbY={-6}
      schX={-2}
      schY={-3}
    />

    <resistor
      name="R_BASE"
      resistance="1k"
      footprint="0805"
      pcbX={1}
      pcbY={0}
      schX={1}
      schY={0}
    />
    <transistor
      name="Q1"
      type="npn"
      footprint="sot23"
      pcbX={8}
      pcbY={-3}
      schX={4}
      schY={-1}
    />
    <resistor
      name="R_LED"
      resistance="330"
      footprint="0805"
      pcbX={8}
      pcbY={7}
      schX={4}
      schY={4}
    />
    <led
      name="D1"
      color="yellow"
      footprint="0805"
      pcbX={14}
      pcbY={2}
      schX={4}
      schY={2}
    />

    <trace from=".J1 > .VCC" to="net.VCC" />
    <trace from=".J1 > .GND" to="net.GND" />

    <trace from=".R_STOP > .pin1" to="net.VCC" />
    <trace from=".R_STOP > .pin2" to=".R_POT > .pin1" />
    <trace from=".R_POT > .pin2" to="net.VDIV" />
    <trace from=".R_LDR > .pin1" to="net.VDIV" />
    <trace from=".R_LDR > .pin2" to="net.GND" />

    <trace from="net.VDIV" to=".R_BASE > .pin1" />
    <trace from=".R_BASE > .pin2" to=".Q1 > .base" />

    <trace from=".R_LED > .pin1" to="net.VCC" />
    <trace from=".R_LED > .pin2" to=".D1 > .anode" />
    <trace from=".D1 > .cathode" to=".Q1 > .collector" />
    <trace from=".Q1 > .emitter" to="net.GND" />
  </board>
)

Step 4: PCB Layout and Routing

Below is the complete PCB view with explicit component coordinates (pcbX, pcbY) arranged cleanly from left (power input and sensor divider) to right (transistor switch and LED indicator):

export default () => (
  <board width="36mm" height="26mm">
    <pinheader
      name="J1"
      pinCount={2}
      pitch="2.54mm"
      footprint="pinrow2"
      pinLabels={["VCC", "GND"]}
      pcbX={-13}
      pcbY={0}
      schX={-6}
      schY={0}
    />
    <resistor
      name="R_BIAS"
      resistance="10k"
      footprint="0805"
      pcbX={-5}
      pcbY={6}
      schX={-2}
      schY={3}
    />
    <resistor
      name="R_LDR"
      resistance="100k"
      footprint="axial_p5.08mm"
      pcbX={-5}
      pcbY={-6}
      schX={-2}
      schY={-3}
    />
    <resistor
      name="R_BASE"
      resistance="1k"
      footprint="0805"
      pcbX={1}
      pcbY={0}
      schX={1}
      schY={0}
    />
    <transistor
      name="Q1"
      type="npn"
      footprint="sot23"
      pcbX={7}
      pcbY={-2}
      schX={4}
      schY={-1}
    />
    <resistor
      name="R_LED"
      resistance="330"
      footprint="0805"
      pcbX={7}
      pcbY={7}
      schX={4}
      schY={4}
    />
    <led
      name="D1"
      color="yellow"
      footprint="0805"
      pcbX={13}
      pcbY={3}
      schX={4}
      schY={2}
    />

    <trace from=".J1 > .VCC" to="net.VCC" />
    <trace from=".J1 > .GND" to="net.GND" />
    <trace from=".R_BIAS > .pin1" to="net.VCC" />
    <trace from=".R_BIAS > .pin2" to="net.VDIV" />
    <trace from=".R_LDR > .pin1" to="net.VDIV" />
    <trace from=".R_LDR > .pin2" to="net.GND" />
    <trace from="net.VDIV" to=".R_BASE > .pin1" />
    <trace from=".R_BASE > .pin2" to=".Q1 > .base" />
    <trace from=".R_LED > .pin1" to="net.VCC" />
    <trace from=".R_LED > .pin2" to=".D1 > .anode" />
    <trace from=".D1 > .cathode" to=".Q1 > .collector" />
    <trace from=".Q1 > .emitter" to="net.GND" />
  </board>
)

---

Circuit Modifications & Enhancements

1. Reversing Operation (Bright-Activated Switch)

To make the LED turn ON in bright light and OFF in darkness (for example, a solar tracker wakeup trigger or laser tripwire receiver), simply swap the positions of R_LDR and R_BIAS in the voltage divider:

• Connect R_LDR between VCC and net.VDIV.
• Connect R_BIAS (10kΩ) between net.VDIV and GND.

Now, when bright light lowers R_LDR to ~1kΩ, V_DIV rises to 9V * (10k / 11k) ≈ 8.18V, saturating Q1 and turning on the load.

2. Adding Hysteresis (Preventing Twilight Flicker)

When ambient light changes slowly at dusk or dawn, a single transistor stage enters its linear active region and may cause the LED to dim gradually or flicker due to shadow noise. You can eliminate this by feeding net.VDIV into an op-amp comparator (such as an LM393) or Schmitt trigger IC (74HC14) with positive feedback before driving Q1.

3. Driving Heavy Loads with a Relay

To switch high-power AC lamps or 12V DC strips, replace D1 and R_LED with a 5V/9V relay coil between VCC and .Q1 > .collector. Always place a flyback diode (such as a 1N4148 or 1N4007) in reverse parallel across the relay coil (cathode to VCC, anode to .Q1 > .collector) to clamp inductive voltage spikes when the transistor turns off.

Practical Applications

• Automatic Night Lights & Garden Lamps: Automatically illuminate pathways and solar garden LEDs after sunset.
• Street Light Controllers: Switch outdoor lighting grids on at dusk and off at daybreak.
• Optical Beam-Break Alarms: Pair a laser diode pointed at R_LDR to trigger an alarm buzzer whenever an object interrupts the beam.
• Camera Exposure & Day/Night Mode Triggers: Switch security cameras between IR night-vision LEDs and daytime color filters.