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Built-in project · no microcontroller

Motion light

A PIR module, a transistor and a lamp, no board. Click the dome and the light holds for two seconds after the movement stops. There is no microcontroller in it: the simulator solves the 5 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.

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Step by step, with the wiring and the common mistakes: PIR Motion Sensor: How It Works, With an Arduino

Motion lightlive0.000 s 0.00x
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The circuit itself, running here on the simulator. Press what can be pressed; click anything else to open it in the editor.

How it works

There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.

sketch.ino
// A motion light with no board: a PIR, a transistor and a lamp.
//
// Nothing here is programmed. The HC-SR501 module has its own chip on it, and
// what comes out of OUT is already a clean digital signal: high while it can
// see movement, and for the hold time afterwards.
//
//   PIR VCC -> 5 V,  GND -> ground
//   PIR OUT -> 1 k -> the base of a 2N2222
//   emitter -> ground,  collector -> the lamp -> 220 ohm -> 5 V
//
// The output is 3.3 V and not 5, whatever the module is powered from, because
// the regulator on the board feeds its logic. Through 1 k into a base that is
// a good two and a half milliamps, and a 2N2222 with a gain of a hundred will
// pass far more than the lamp's twelve.
//
// Things to try. Click the dome, then click it again and count: the lamp holds
// for two seconds. Now click again right away: nothing, for another two
// and a half, because the module ignores everything after a pulse. Then open
// the PIR's properties and set retrigger to single: now holding the movement
// on gives one pulse and no more.

Parts list

7 parts, plus the jumper wires. Every one is in the editor's parts bin.

How it is wired

6 connections, pin by pin, read from the circuit itself. Each line is one set of pins joined together, by a jumper wire or a breadboard strip.

  • 5 V: Motion sensor pin VCC; Resistor, 220 Ω pin 2
  • Ground: Motion sensor pin GND; Transistor (NPN or PNP) pin E
  • Motion sensor pin OUT; Resistor, 1k Ω pin 1
  • Transistor (NPN or PNP) pin C; LED, yellow pin C
  • Transistor (NPN or PNP) pin B; Resistor, 1k Ω pin 2
  • LED, yellow pin A; Resistor, 220 Ω pin 1

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