Here’s the core idea behind a sound–light controlled lighting fixture (often called “sound-and-light switch”): the lamp turns on only when it’s dark enough (light sensor says “night”) and a sound event (clap/voice/door noise) is detected. It then stays on for a short time and turns off automatically.

How the circuit works (block view)

1) Light sensing (ambient “it’s dark”)

• Sensor: LDR (photoresistor) in a voltage divider with a potentiometer.

• Comparator: An op-amp (LM358) or a tiny comparator (LM393) compares the LDR voltage to a user-set threshold.

• Hysteresis: A little positive feedback (tens to hundreds of kΩ) prevents flicker at the threshold.

• Result: A clean digital “DARK” signal (high or low depending on design).

Typical values

• LDR + 100 kΩ pot as divider to 3.3–12 V.

• Reference: 10–50 kΩ divider.

• Hysteresis: 330 kΩ from output to non-inverting input.

Threshold math (example)
If LDR is on top to VCC and pot to GND:

• V_LDR=V_CC⋅R_pot/(R_LDR+R_pot)​​
  Choose V_ref so that in daylight V_LDR<V_ref (no turn-on), and at night V_LDR>V_ref​.

2) Sound sensing (detect a “clap” or speech burst)

• Microphone: Electret condenser mic, biased through ~2.2 kΩ to 2–5 V, AC-coupled.

• Preamp: One LM358 stage with gain 20–100 and gentle band-pass around speech (≈ 300 Hz–3 kHz) to reject hum & very high frequencies.

• Envelope detector: Diode + RC (e.g., 1N4148, 1 µF & 100 kΩ → ~0.1 s) to convert audio bursts to a “loudness” voltage.

• Comparator with Schmitt trigger: Compares the envelope to a sensitivity pot; hysteresis avoids chatter.

Typical preamp

• High-pass: 100 nF into 10 kΩ → fc≈160f_c \approx 160fc​≈160 Hz

• Gain: A≈1+R_f/R_in​​; e.g., R_f=100 kΩ, R_in=2.2 kΩ → A≈46

• Low-pass: 10 kΩ + 100 nF across feedback or output → f_c​≈160 Hz to a few kHz (set to taste).

3) Logic gating (AND)

• Combine the DARK output and the SOUND trigger with:

  • A discrete AND (two NPN stages or a diode-logic + transistor), or

  • Feed both into a small MCU GPIO, or

  • Gate them inside a comparator stage (e.g., only allow the sound comparator to arm when “dark”).

Result: a single pulse when a sound occurs while it’s dark.

4) Timer / monostable (keeps the light on for N seconds)

• 555 monostable (classic): t≈1.1 R C.
  Example: R=470 kΩ,C=47 μF → $t\approx 24$ s.
  Use the AND pulse to trigger the 555 (active-low TRIG).

• Or use a retriggerable monostable (CD4538) so continued noise extends the on-time.

• Or do it in a microcontroller (ATtiny/STM32/ESP32), which also lets you add debouncing & smarter detection.

5) Power driver (what actually switches the lamp)

• For low-voltage LED strips/bulbs:
  Logic → N-MOSFET (e.g., AO3400/IRLZ44 for higher current) with gate resistor (100 Ω), pull-down (100 kΩ), flyback diode if inductive.

• For AC mains lamps (on/off):
  Use isolation. Recommended:

  • Opto-triac (MOC3063 = zero-cross) → Triac (BT136/BT139) → Lamp.

  • Snubber RC across triac (e.g., 100 Ω + 100 nF, X2-rated cap) for EMI/inductive loads.

  • Zero-cross types reduce EMI; they wait for the next AC zero crossing before turn-on (tiny delay, good for simple on/off).

  • If you need immediate triggering at arbitrary phase (e.g., some special loads), use a non-zero-cross opto (MOC3021) + appropriate filtering.

Safety: If you interface with mains, keep the sensing & logic on an isolated low-voltage supply (e.g., small SMPS module). Avoid capacitive droppers unless you fully understand creepage/clearance and enclosure safety. Fuse and protect.

A practical reference design (low-voltage version, 5–12 V)

Parts

• LM358 (dual op-amp), NE555, Electret mic, LDR

• Pots: 100 kΩ (light threshold), 100 kΩ (sound sensitivity), 470 kΩ (on-time)

• R’s/C’s as below, 1N4148, N-MOSFET for LEDs

Subcircuits

1. Light comparator (LM358A):

   • LDR to VCC, 100 kΩ pot to GND → junction to +IN.

   • 10 kΩ/10 kΩ divider to create mid-rail 2.5–6 V → −IN (with 330 kΩ from OUT to +IN for hysteresis).

   • Output = HIGH when dark.

2. Sound chain (LM358B + LM393):

   • Mic bias 2.2 kΩ to 5 V; AC-couple 100 nF into LM358B with Rin 2.2 kΩ; Rf 100 kΩ; small Cp across Rf (220 pF–1 nF).

   • Envelope: 1N4148 to 1 µF; discharge 100 kΩ to ground.

   • Comparator (LM393): envelope vs 100 kΩ pot; 330 kΩ hysteresis; output = short “HIGH” pulse per sound.

3. AND + monostable:

   • Wire the DARK output and SOUND pulse into a simple AND (two diodes + pull-down, then NPN to pull 555’s TRIG low), or gate in logic.

   • 555 monostable: R=470 kΩ pot + C=47 μF for ~5–30 s adjustable.

4. MOSFET driver:

   • 10–100 Ω gate resistor, 100 kΩ gate-to-GND, MOSFET source to GND, drain to LED- load (low side). Size MOSFET for current & P=I²R_DS(on)

A practical reference design (mains on/off with isolation)

• Supply: Small 5–12 V isolated SMPS to power the low-voltage electronics.

• Sense & logic: Same as above (LM358/LM393/555).

• Output: 555 output → 330 Ω → MOC3063 LED → opto-triac → triac gate (100–180 Ω series). RC snubber across triac. Lamp in series with triac on mains.

• Fuse + MOV for surge protection; respect creepage/clearance.

Design tips & common pitfalls

• Don’t pollute thresholds: Give both comparators proper Schmitt hysteresis (10–20% of full-scale is typical).

• Avoid false triggers: Band-limit the audio (HPF ~200–300 Hz, LPF ~3–5 kHz). Choose envelope τ ≈ 50–300 ms so brief clicks register but continuous hum doesn’t dominate.

• Retiggering: Use retriggerable monostable if you want the light to stay on while noise continues.

• Placement: Keep the mic away from the lamp’s airflow or transformer buzz; keep the LDR out of the lamp’s direct beam (or shield it) to prevent self-oscillation.

• EMI: For triac outputs use a zero-cross opto and an RC snubber; twist/load wires; add an LC mains filter for tough environments.

• Power options: For tiny LED loads, a DC design with a MOSFET is simpler, safer, and quieter.

Minimal parts list (analog version)

• 1× LM358, 1× LM393 (or just LM358 if you reuse as comparator), 1× NE555

• 1× Electret mic + 2.2 kΩ bias, 1× LDR + 100 kΩ pot

• Resistors: 100 kΩ (×4), 330 kΩ (×2), 10 kΩ (×4), 2.2 kΩ (×2), 100 Ω gate

• Capacitors: 100 nF (×3), 220 pF–1 nF (stability), 1 µF (envelope), 47 µF (timing)

• 1× 1N4148, 1× logic-level N-MOSFET (or MOC3063+triac for mains)

• Optional: MOV, fuse, RC snubber, isolated SMPS if using mains