Laser Pointer Switch using arduino

Laser Pointer Switch

This instructable will detail how to make a switch that uses an arduino to sample light. When the light sample reaches a threshold it will trigger a relay that can be used to turn on/off a small appliance (light, radio, fan, etc…) The parts for this instructable can be ordered as a kit:

Jameco Kit

Laser Pointer Switch

Step 1: Review relay and prep wire

unpackage the relay and stretch out the cable. this cable is used to activate the relay but will also be used to power the arduino. cut the wire in about two equal parts. the part with the white clip end will plug into the relay and be soldered into the arduino. the wire with the black clip end will be spliced into the usb cable to supply power to the arduino. this is also a good time to review the relay tutorial for safety information:

Step 2: Modify USB cable

cut the USB cable about 6 inches from the end with the rectangle end.  strip the wires and you will find a red, black, green and white wire.  splice the red and black wires from the relay cable (with the black end) to the red and black wires of the USB cable.  solder the connections and tape the wires.  this new USB cable will be used to power the arduino. 

Step 3: Identify Arduino

find the arduino in the kit of parts and familiarize yourself with the pin holes and orientation of the package. 

Step 4: Component Layout

the components that are soldered onto the arduino need to be placed in specific places.  the right angle header goes into the right side of the arduino.  the photo cell needs to have one leg in the A5 hole and one leg int the Vcc hole.  the resister needs to have one leg in the GND hole and one leg in the A5 hole (with the photo cell) .  the wire from the relay (with the white end) needs to be soldered into the arduino also.  the green wire in the 5 hole, the black wire in the GND hole (with one leg of the resistor) and the red wire in the Vcc hole with one leg of the photo cell).  on some arduinos, the resistor leg and the photo cell leg will not fit in the same hole.  if this happens to you, you can put the resistor in from the back of the arduino and connect the photo cell leg to the resistor leg behind the arduino. 

Step 5: Solder Arduino

for the holes that have a wire and a component leg in the same hole, make sure to put the wire in the hole first then push the component leg in.  after all the components are in the proper place, solder them.

Step 6: Program Arduino

connect your arduino to the FTDI device that you use to program it then upload this code to your arduino:
 Laser Pointer Switch circuit

//adjust the lTrigger number for your light values
int lTrigger = 610;

int photoPin = A5;  //define a pin for Photo cell
int ledPin=5;     //define a pin for relay activator
boolean bLatch = false;
int lLaser = 0;

void setup() {
Serial.begin(9600);  //Begin serial communcation
pinMode( photoPin, INPUT );
pinMode( ledPin, OUTPUT );

void loop()  {
lLaser = analogRead(photoPin);
if (lLaser > lTrigger)   {
bLatch = !bLatch;
Serial.println(lLaser); //display photocell value to serial monitor.
delay(10); //short delay for switch bounce

test your arduino with the serial monitor open to see the light reading values.  adjust the trigger value to a range that works best with your laser pointer.

Step 7: Prepare Power Cord

the power cord is the short extention cord that we will use to power the arduino (with USB block) and the appliance that is switched (with the relay).  this cord will be plugged into a power outlet and the electrical properties can be hazardous.  please be careful when working with this cord and the relay when it is powered.  you will want to cut the power cord a little off of center.  to complete the wiring of the relay we will need a short piece (1.5 in) of the black wire.  cutting the whole wire a little off center will allow us to cut a piece from the black wire to complete the connection.

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