Motion Activated Light Sensor

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The Story

My friend Blauvster pointed me to some cheap LED strip lights for sale on Amazon. Later he asked how hard it would be to hook up a PIC with a motion and light sensor to turn the lights on when it's dark and motion is sensed. This should be a pretty simple handy device to use several locations around the house, so here goes.



In order to check for ambient light in this circuit we'll be using a photoresistor. If you're unfamiliar with photoresistors, they are devices in which the resistance across the device changes with the amount of light shining on the device. So the brighter the light on the photoresistor the less resistance it provides. has a great tutorial with example circuits and some very useful equations to help you figure out resistor values and how to connect a photoresistor in your circuit.

Photoresistor Circuit

The equation they use to find the static resistor, R=sqrt(PR_dark*PR_light), will get you close to the value you'll need. My plan is to get a potentiometer close to this value in order to fine tune the circuit. I got a pack of GL5528 photoresistors on Ebay. They range from 1M Ohms in darkness to 10K Ohms in bright light. using these values in the provided equation works out to about 100K Ohms. Plugging 100K into voltage divider equations gives 0.45 Vout at 10K Ohms and 4.55 Vout at 1M Ohm. This is well beyond the limit thresholds for our PIC16F84 controller calls out 0.8V and bellow as a Low signal and 2V and above as a High signal. You'll want to double check the specs and do some experiments with the photoresistors you pick out before settling on a static resistor or potentiometer.

PIR Motion Sensor

You can pick up a Parallax PIR infrared motion sensor from Radioshack for about $10. This is pretty cheap and ready to use out of the box. It has a jumper for selecting high and low sensitivity which, according to the data sheet, switches between 30 foot and 15 foot motion detection. It also has a 3 pin connector for power, ground, and motion detection output. The output goes logic high and stays there as long as motion is detected.

PIR Motion Sensor Module

Note: I have seen similar sensors on ebay for about $2.50. The specs on these sensors will almost undoubtedly be different from the Radioshack Parallax model, but should work in a similar fashion. Here's a nicely packaged, easy to use solution from Amazon.


  • 5V Logic Power
  • 12V LED Power: Solid State Relay & Transistors


PIC Code and Programming

  1. Check for motion(PORTB<7:4> interrupt-onchange)
    1. If NO motion, turn OFF lights
    2. If motion, Check if lights are ON
      1. If lights are ON, keep ON
      2. If lights are OFF, check for ambient light
        1. If ambient light, keep lights OFF
        2. If NO ambient light, turn lights ON
  2. SLEEP

A change-in-state interrupt will wake the chip from sleep mode. We then run through the listed tests here, adjust the light control output accordingly, and return the controller to sleep.

Assembly code for the PIC16F84

;		Motion Light Sensor
;		--------
;	Author: Elliott Long
;	Date:	Aug. 2012
;	4MHz internal resonator used
;	PORTB Change Interrupt used
;	PORTB, 4 = Motion Detecter Input Change on interrupt
;	PORTB, 5 = Photoresistor Input
;	PORTA, 1 = LED Light Strip Output
;	PORTA, 2 = Power Latch Output
 STATUS	equ 03h
 RP0	equ 05h
 PORTA	equ 05h
 PORTB	equ 06h
 INTCON	equ 0Bh
 OPT	equ 81h
 OPT	equ 81h
 TRISA	equ 85h
 TRISB	equ 86h
 C		equ 00h
 Z		equ 02h
 Delay1	equ 0Ch
 Delay2	equ 0Dh
 Delay3	equ 0Eh
 Time	equ 13h
 Brightness	equ 14h
 ORG 0000h
 ORG 0004h	;Interrupt routine
	MOVF PORTB,1 ; you must read PORTB upon Int-on-chg
	;if NO motion turn OFF lights
	GOTO LIGHTOFF	;If PORTB,4 is high(motion) this is skipped
		;if lights are ON, keep ON
		GOTO RTN		;If PORTA,1 is high(lights ON) we return leaving them ON
			GOTO RTN		;if PORTB,5 is high (ambient light) we return leaving lights OFF
			;if NO ambient light, turn lights ON
		BSF PORTA,2		;Set the Power Latch Output PORTA, 2
		BSF PORTA,1		;Set Light Output PORTA,1
		BCF PORTA,2		;Set the Power Latch Output PORTA, 2
		BCF PORTA,1		;Clear Light Output PORTA,1
	RTN	BCF INTCON,0	;Clear Interrupt Flag
		BCF STATUS,RP0	;Bank 0
		BSF INTCON,7	;Global Interrupt Enable
		BSF INTCON,3	;PortB Change Interrupt Enable
		BCF INTCON,0	;Clear Interrupt Flag
 		BSF STATUS,RP0	;Bank 1
 		MOVLW 30h	;Set PORTB 4&5 as Input 110000
		MOVLW 00h	;Set PORTA outputs
		MOVWF TRISA	;Setup RA as outputs
		BCF STATUS,RP0	;Bank 0
		BCF INTCON,0	;Clear Interrupt Flag
		MOVF PORTB,1	;Read PortB to itself
 ;Loop through testing
 LOOP:	;Main Program which just loops to sleep
	;BCF INTCON,0	;Clear Interrupt Flag


This programming works with the circuit always powered. When the PIR sensor attached to pin 4 triggers on motion it causes an interrupt on the PIC which wakes the PIC up from sleep mode. The program then checks for if the LEDs are already on and if so goes back to sleep. If the LEDs are off the program checks the photoresistor input on pin 3 to see if it's dark in the area. If it's dark the program turns the LEDs on and goes back to sleep. If it is not dark the LEDs are left off and the PIC goes back to sleep.

Blauvster threw out the idea of having the entire circuit only draw power when it's dark. This way when the area becomes dark the circuit turns on. Then when motion is sensed the power to the circuit is latched on by a secondary source controlled by the PIC. This way when the LEDs are turned on power to the circuit is not killed. As far as programming is concerned this is a simple addition of turning on the output to RA2 before turning on RA1 to power the LEDs and then turning off that output just after turning off the LED outputs. In progress...