Automatic night light
An LDR turns an LED on the moment it gets dark.
need an LDR · a fixed resistor · an LED + transistor
do read the divider; switch the light below a threshold.
The other half of the workbench: not making things happen, but noticing them. Light, heat, motion, distance, sound, touch — cheap sensors turn a circuit into something that perceives. Here's the field, and what to make it watch for.
An LDR, photodiode, or phototransistor reads brightness — from night lights, mice, cameras.
brightnessA thermistor or digital chip reads temperature; an IR thermopile reads it from a distance.
temperatureA PIR spots a warm body; an accelerometer feels taps, tilt, and orientation.
movementUltrasonic, IR, or time-of-flight measure the gap to whatever's in front.
rangeA mic hears; a piezo feels knocks and vibration.
acousticCapacitive pads sense a finger; load cells and FSRs read weight and pressure.
contactAn LDR turns an LED on the moment it gets dark.
need an LDR · a fixed resistor · an LED + transistor
do read the divider; switch the light below a threshold.
Two light sensors steer it toward the brightest spot (or flee it).
need 2 light sensors · 2 motors · an H-bridge
do drive the wheel on the darker side faster — it turns to the light.
A beam held on a sensor; break it and the alarm fires.
need a laser module · a photodiode · a buzzer
do aim the beam at the sensor; trigger when it drops.
⚠ never aim at eyesLight through a fingertip flickers with each heartbeat.
need an LED · a photodiode · an amp
do shine through a finger, read the pulsing reflection (PPG).
A thermistor and a setpoint switch a fan or heater.
need a thermistor · a fixed resistor · a relay (+ flyback diode)
do compare to a threshold; switch the load with hysteresis.
An IR thermopile reads surface temperature without touching.
need an IR temp sensor · a microcontroller · a display
do point and read — the infrared it emits gives its temperature.
A flame sensor or fast-rising thermistor trips on fire.
need a flame/IR sensor · a buzzer
do watch for the IR signature of flame or a sudden heat jump.
Ultrasonic measures the gap and beeps faster as you close in.
need an ultrasonic module · a buzzer · a microcontroller
do time the echo; speed the beep as distance shrinks.
A PIR catches a warm body moving and switches a load.
need a PIR module · a relay or transistor · a light/buzzer
do wire its output to the switch; add a re-arm delay.
An accelerometer counts steps and reads taps, tilts, orientation.
need an accelerometer/IMU · a microcontroller
do watch the motion pattern — count peaks, detect gestures.
Count turns and which way (knobs — and motor feedback).
need a rotary encoder · two input pins
do read the two channels' order to get direction + count.
An IR sensor counts a spinning mark to read RPM.
need an IR reflectance sensor · a reflective mark
do count pulses per second on the spinning part — multiply out.
A mic triggers on a clap, or pulses lights to the music.
need a microphone module · a relay or LEDs
do detect a level spike (a clap), or follow the envelope.
A piezo reads a tap pattern to release a latch.
need a piezo disc · a microcontroller · a solenoid lock
do match the rhythm of taps; unlock on the right pattern.
Turn a plant, a fruit, or foil into a touch button.
need a touch-sense pin or chip · any conductor
do sense the capacitance a finger adds — no moving parts.
A load cell or FSR reads weight and pressure.
need a load cell + amp (or an FSR + resistor)
do calibrate against known weights; read force as a number.
An MQ sensor warns on smoke, gas, or vapor in the air.
need an MQ gas sensor · a microcontroller · an alert
do watch the level rise; sound off above a threshold.
⚠ not life-safety ratedTemperature, humidity, and pressure — logged and trended.
need a temp/humidity + pressure sensor · a logger
do sample on a timer; a falling pressure hints at weather turning.
A moisture probe triggers a pump or valve when the soil's dry.
need a moisture probe · a pump/solenoid valve · a driver
do read the soil; water a measured dose below a threshold.
Record any sensor over time — the root of all measurement.
need any sensor · a microcontroller · SD or serial
do timestamp each reading; the trend tells you more than the value.
workbench series · light, heat, motion, distance, sound, touch · a reading isn't truth until you check it