## Step 11: .:Light:. (Photo Resistors) - CIRC09

What We're Doing:
Whilst getting input from a potentiometer can be useful for human controlled experiments, what do we use when we want an environmentally controlled experiment? We use exactly the same principles but instead of a potentiometer (twist based resistance) we use a photo resistor (light based resistance). The Arduino cannot directly sense resistance (it senses voltage) so we set up a voltage divider ( http://tinyurl.com/2sunta ). The exact voltage at the sensing pin is calculable, but for our purposes (just sensing relative light) we can experiment with the values and see what works for us. A low value will occur when the sensor is well lit while a high value will occur when it is in darkness.

The Parts:
• Photo-Resistor (x1)
• 5mm Green LED (x1)
• 10k ohm Resistor (brown-black-orange) (x1)
• 560 ohm Resistor (green-blue-brown) (x1)
• Wire

The Circuit and Plugging Everything In:
A Small Video of Everything Being Plugged in

The Code: - http://tinyurl.com/crdum6''
`/* * A simple programme that will change the  * intensity of an LED based  on the amount of * light incident on the photo resistor. *  *///PhotoResistor Pinint lightPin = 0; //the analog pin the                   //photoresistor is                   //connected to                  //the photoresistor is not                   //calibrated to any units so                  //this is simply a raw sensor value (relative light)//LED Pinint ledPin = 9;//the pin the LED is connected to               //we are controlling brightness so                //we use one of the PWM (pulse                //width modulation pins)void setup(){  pinMode(ledPin, OUTPUT); //sets the led pin to                            //output} /* * loop() - this function will start after setup  * finishes and then repeat */void loop(){ int lightLevel = analogRead(lightPin); //Read the                                        // lightlevel lightLevel = map(lightLevel, 0, 900, 0, 255);          //adjust the value 0 to 900 to  lightLevel = constrain(lightLevel, 0, 255);      //make sure the value is betwween 0 and 255                                                     analogWrite(ledPin, lightLevel);  //write the value}`

Not Working?
• LED is Remaining Dark - This is a mistake we continue to make time and time again, if only they could make an LED that worked both ways. Pull it up and give it a twist.
• It Isn't Responding to Changes in Light. - Given that the spacing of the wires on the photo-resistor is not standad, it is easy to misplace it. Double check its in the right place
• Still not quite working? - You may be in a room which is either too bright or dark. Try turning the lights on or off to see if this helps. Or if you have a flashlight near by give that a try.

Making it Better:
Reverse the response:
Perhaps you would like the opposite response. Don't worry we can easily reverse this response just change.
` analogWrite(ledPin, lightLevel); ----> analogWrite(ledPin, 255 - lightLevel);`
Upload and watch the response change.

Night light:
Rather than controlling the brightness of the LED in response to light, lets instead turn it on or off based on a threshold value. Change the loop() code with.
`  void loop(){     int threshold = 300;     if(analogRead(lightPin) > threshold){       digitalWrite(ledPin, HIGH);     }else{       digitalWrite(ledPin, LOW);         }}`

Light controlled servo:
Lets use our newly found light sensing skills to control a servo (and at the same time engage in a little bit of Arduino code hacking). Wire up a servo connected to pin 9 (like in CIRC-04). Then open the Knob example program (the same one we used in CIRC-08) File > Sketchbook > Examples > Library-Servo > Knob. Upload the code to your board and watch as it works unmodified.
Using the full range of your servo:
You'll notice that the servo will only operate over a limited portion of its range. This is because with the voltage dividing circuit we use the voltage on analog pin 0 will not range from 0 to 5 volts but instead between two lesser values (these values will change based on your setup). To fix this play with the val = map(val, 0, 1023, 0, 179); line. For hints on what to do visit http://arduino.cc/en/Reference/Map .

<p>Very good article, thank you for valuable information!</p>
<p>Hi there! I bought your ARDX kit and it's been fun following the projects. The only problem I've been having is that the breadboard is very stiff and some of the components won't insert. Do you have any tips for warming up the breadboard so wires will go in smoothly?</p>
<p>I'm having a problem with circuit 11. I can't change the delay (it keeps flicking back and forth between green and red at one second intervals no matter how much delay I put in the code), and I can't change the pin to any other pin but 2. I'm trying to integrate this circuit with another one so I need be able to make changes!</p>
<p>can i use this relay to control and lamp?</p>
Hi, when I plug the arduino in the power light comes on and the led flashes but the new device box does not pop up, I'm not able to select a port it. How do I install the drivers, I can't seem to find the arduino on my computer. I'm running Windows 7. Your help is much appreciated. Thank you
<p>If you bought an official Arduino you can find the driver at <a href="http://www.arduino.cc/" rel="nofollow">http://www.arduino.cc/</a><br><br>If you bought a chinese one from eBay, read the description again. Most of them that I've seen use a different USB controller and also have a link to the required driver in their description.</p>
<p>Thanx !! All project suggestions are much appreciated !!!</p>
<p>Can some explain to me the in and out configuration when I have 10 leds. This is to test my learning capability. </p>
N00b here. I've built the Circ01 and Circ02 projects and now as i hit Circ03 the question still remains: <br> <br>Exactly what are the 2-pin Headers for? <br> <br>thanks <br>
<p>I guess the 2 pin headers hold the template in place. Nothing more than that. If you are not using the template then you don't need the headers.</p>
<p>I'm just wondering about the role of the resistors in this example. Couldn't these be replaced with Jumper wires without having any detrimental effect on the solution?</p>
<p>I don't understand why for(int i = 0; i &lt; 8 ; i++) even works. if i is set to 0, then i is less than 8, right? for(int i = 0; i =7 ; i++) does nothing, and for(int i = 0; i == 7 ; i++) works correctly except the LEDs are lit at ~10%. What's happening here?</p>
<p>The first one (for(int i=0;i&lt;8;i++)) works because it tests if i is less then 8 and if it is, it runs the code. for(int i=0,i=7,i++) does nothing because when it starts i equals 0 and fails the test &quot;does i equal 7?&quot; and breaks out of the for loop. </p>
<p>Hey - fun project! I had no problems getting it to work. then I hacked it a bit to count to 1024 (2 extra LEDs). I'm interested in using this to count passers-by. I have a second IR beam-breaking detector that provides a ground when the beam is interrupted. I would like to feed this ground signal into D13 so that when grounded, the binary count would increment by one. Can anyone recommend how to tweak the code to look for a ground on D13 thereby incrementing the count by one? </p><p>Thanks!! M&oslash;</p>
I don't understand the purpose of the wire running to 5v. It connects the positive rail on the breadboard to 5v but I don't see anything on the breadboard connected to the positive rail. Here's my new-to-Arduino question: obviously all of the LEDs ground through the resistors=&gt;negative rail=&gt; GRND on Arduino. Do they get all the power they need (+) from the pins 2~9 being controlled by the code? Right now I'm running it completely without the wire to 5V and it runs but LEDs are dim.
<p>You are correct, you technically do not need the 5v wire from the ardunio to the positive side of the breadboard, the positive voltage comes from the pins (2-9) all the code actually does is turns the voltage on or off through the pins, thus the circuit completes through the negative rail on the breadboard, and lights the LED.</p>
<p>THANK YOU TO INSTRUCTIONALS AND <a href="https://www.instructables.com/member/oomlout/" rel="nofollow">oomlout</a>!!!!!!!!!!!!!!!!!!!!!!!!!!!!</p><p>To Instructionals for having this site and to <a href="https://www.instructables.com/member/oomlout/" rel="nofollow">oomlout</a> for posting the instruction as to how this circuit works. I got to this circuit, hooked it up, nothing---changed out all the parts from Arduino kit, still nothing, took out all the parts, rewired it, still nothing, found a dead laptop, took out the cooling fan, reading voltage(5v) and working amps, (.35a), put in a LED where the motor should be, the LED blinked on/off per 1 sec, the circuit works with a LED but not motor,,,soldered pins TO the laptop fan, installed fan.. nothing,, then i checked the amps across the base to collector, it read like .00004ma, i think, it was pretty low amp reading, it works with a LED but not motor,mmmmm, went to an electronics store about asked a transistor with a higher gain, nice lady there found a TO-18 case that had a gain of 200, came back plugged that part in,,noting CBE and the tab.</p><p>moment of Poooof, making sure I had the CBE correct, plugged in the laptop fan, plugged in power, ANNNNNNNNNNNNNNNNNNNNND,,, the freaking thing works, cycles on 1 sec, off 1 sec. </p><p>wowoow on to the next project</p><p>so to all of you instructable people out there,</p><p>********* DON'T GIVE UP!! KEEP TRYING, YOU'LL GET IT.. ********</p>
<p>can i copy and paste the code or no </p>
I am new to the electronics things,so please forgive me,if my question seems to be a foolish one.My doubt is.. <br>Why the transistor is connected after the motor and why not before??? If i am right current flows from +ve to -ve and in that case the transistor is placed after the current passes through the motor.Why so??In that case how is the transistor helpful in this circuit for amplifying current?? Can you please help me out of this confusion. <br>Thanks
Friend <br>I am about to begin experimenting with Arduino and this kit of experimentation which we teach and explain how to use seems to me to be excellent. I bought a platform type arduino with all the accessories of the Upgrade Industries brand called XBoard which is ideal for experimenting, ideal to attach it to the kit samples. Be the Xboard is 100% compatible with the kit that you recommend? Thank you
I am stuck on the 'controlling a motor' part. I am not sure how I by-pass the 560 Ohm resister. I have removed the red LED but how do I connect the motor?
I am just getting started with Arduino and am having a problem with Circ03. The motor will run, but does not go on and off, and the variable speed just runs at the same speed. <br>
Why do we connect a wire directly to 5v when pin 13 is already getting 5v. I inadvertently pulled off the 5v wire and the circuit still worked. <br> <br>I assume this is just some kind of best practice that is useful 90% of the time but just not in this case? <br> <br>thanks <br> <br>ps. i asked this in another comment, but I'll re-ask here: what purpose do the 2Pin Headers serve? <br>
I just got the kit, and I couldn't get this to work until I changed out the 10K resistor with the other 330 resistor. <br>Troubleshooting was educational, though!
Same here.. I spent over an hour troubleshooting this same project and while I swapped out the resistor for another 10k, I would've never changed to the 330. I was about to pull my hair out and found your comment.. much appreciated.. that leaves some question in my mind as to why this is the case and how someone learning these circuits is supposed to figure something like that out. I love the Arduino but this sparkfun kit has a number of flaws..
The second resistors (560ohm) are listed as also being Red-Red-Red just like 2.2KOhm ones, Probably should read Green-Blue-Brown
This seems like a great kit, but I still cant understand how to make the motor run both ways.. Did anybody manage and is it at all possible?<br>I am also planning to combine several to contol 5 motors. Did anybody try that yet?<br><br>I will be very greatful for your help!<br><br>Pink
how can you make the motor run in forward and reverse? is there a series of code that I can type in, or is it all in how I plug it up? thanks, Andrew
Interesting. I just bought an Osepp board whih is allegedly compatible with the sparkfun arduino.
NEVERMIND- i was using my datasheet for a 2n2222, not the one provided with the link (p2n2222). now i guess i know what that tiny &quot;p&quot; means.
what i thought i knew is now suspect. on the datasheet for the p2n2222ag (and what i thought i already knew) when the flat side of the npn transistor facing me, with a to-92 package,with the pins down, the order from left to right is, emitter, base, and collector on the right. why is this not true on the layout sheet. ?
I tried this out and got it to work. Thanks for the tutorial I learned a thing or two. <br><br>I am however, wondering why the same circuit doesn't work when I replace the 5v and ground from the arduino with a power and ground from a 6v battery pack. Shouldn't it? The motor and transistor are rated for such loads. The only thing I've changed is the power source. I'm trying to make the jump from learning about motor controllers to actually constructing a useful one with it's power independent of the arduinos power.<br><br>I'd appreciate any help any one could point out.
i had a similar problem, where i was powering the arduino and motors from separate power sources.<br><br>It turned out they both had to share the same ground, i just plugged the negative terminal from my battery pack in to the ground rail on my breadboard and then connected a ground pin from my arduino to the same breadboard rail, then everything worked
Thanks for the reply. I've moved on since I posted that but I'll no doubt be reviewing it some day. You might want to post that message in the comments so others can see it and learn.
do i have to press the reset button before uploading new files to arduino uno<br>or i have to erase the older programmes or i just have to upload the new programme
Hello,<br><br>I was reading the datasheet of the 74HC595 and I came across with some values...<br><br>It is written that the max Icc/Ignd is 70mA, and each output pin supports 35mA.<br>If I want, for example, lit 8 leds at the same time, each at 20mA, the total would be 160mA.<br><br>So, wouldnt it damage the 74HC595?<br><br><br>Thanks a lot
ON the in and out code I found an easier way. I reloaded the array so the order is the same order the LEDs are to be lit, then I used loops to kight them from the middle out and unlight them from the outside in. I then changed it around a little to sequence out and then in. <br> <br>Here's the array: <br>int ledPins[] = {5, 6, 4, 7, 3, 8, 2, 9}; <br> <br>Here's the loop: <br>//LEDs light inside to outside <br> for(int i = 0; i &lt;= 7; i++){ <br> int j = i + 1; <br> digitalWrite(ledPins[i], HIGH); //Turns on LED #i each time this runs <br> digitalWrite(ledPins[j], HIGH); //Turns on LED #j each time this runs <br> delay(delayTime); //gets one added to it so this will repeat <br> digitalWrite(ledPins[i], LOW); //Turns off LED #i each time this runs <br> digitalWrite(ledPins[j], LOW); //Turns off LED #j each time this runs <br> } //8 times the first time i will = 0 the final <br> //time i will equal 7; <br> <br>//LEDs light outside to inside <br> for(int i = 4; i &gt;= 0; i--){ //same as above but rather than starting at 0 and counting up <br> int j = i + 1; <br> //we start at seven and count down <br> digitalWrite(ledPins[i], HIGH); //Turns on LED #i each time this runs <br> digitalWrite(ledPins[j], HIGH); //Turns on LED #i each time this runs <br> delay(delayTime); //gets one added to it so this will repeat <br> digitalWrite(ledPins[i], LOW); <br> digitalWrite(ledPins[j], LOW);
void loop() { int value = analogRead(potPin) / 4; analogWrite(ledPin, value); } <br><br>potPin was not decleared in the scope........ everytime i rewrite or copy and paste <br><br>
Hello,<br>This step mentions linking multiple shift registers together to control a larger number of LEDs with just 4 Arduino pins. How is this accomplished? Thanks for your help.
http://www.arduino.cc/en/Tutorial/ShiftOut
Hi there, I am wondering if I can run my dc motor reverse. is there any special code to make it reverse? if there is, please tell me. thank you,
the red wire is normally going in the + rail, pull it out and connect it to the Collector of the transistor and the black wire coming from the motor goes into the + rail
&nbsp;Hey, just thought I'd let you know that the example from arduino.cc uses a pull-down configuration while your circuit uses a pull-up configuration, so the LED on/off is reversed (:
Where is the code for this project? Can't find it anywhere.<br />
The code is just below the video and goes all the way to the right and i mean ALL the way
&nbsp;Hello there, nice job,thanks! Maybe you could help me here.<br /> <br /> I am trying to figure out why is the 2.2k resistor required? I have seen in the transistor datasheet something like &quot;Emitter-Base voltage = 6.0Vdc&quot;, doesn't it mean that we could plug the device directly to Arduino?<br /> <br /> I am trying to figure out if I may use a 2N60B instead, since I have one of these here. I just don't know why would I need a resistor.<br /> <br /> Thanks in advance.<br /> <br /> P.S.: The code appears without line breaking (using google-chrome for Linux) for me.<br />
The &quot;Emitter-Base voltage = 6.0Vdc&quot; statement may be the confusing thing here; The maximum REVERSE voltage that may be applied to&nbsp;the Base-Emitter junction&nbsp;is ~6V But the normal&nbsp;FORWARD drob across the&nbsp;base-emitter junction is approximately 0.7V...&nbsp;&nbsp;The current into the base&nbsp;MUST be limited to prevent the transistor form being distroyed be excessive base current.<br /> <br /> Since the 0.7V forward base voltage is significantly less than the 5V output of the Arduino port pin, then the current flowing into the base of the transistor would&nbsp;likely exceed the rated maximum of both the Arduino&nbsp;pin AND the rated maximum input current od the base of the transistor.<br /> <br /> The function of the resistor is to prevent damage (caused by excessive current) to either the transistor or the Adrion.&nbsp;
&nbsp;Thank you for your attention ;)<br /> <br /> I found the mentioned info in the dataheet. Still I did not understand some details, however. How do I get to the 2.2k value? Am I supposed to assume a 2mA current at the base so that r=v/i=4.3/0.002=2150? How do I get that info in the sheet?<br /> <br /> I am right now using a photocoupler to do the job of swtching the motor, I will make an instructable soon, but I still am not sure about the resistor values.<br /> <br />
If you are using a bipolar transistor (2n2222, 2n3904 etc), read the data sheet...<br /> <br /> 1. determine the current that the transistor must switch and divide it by the Hfe (current gain of the transistor at the desired load current per the data sheet)<br /> <br /> 2. multiply the result by 2 (safety margin)....&nbsp; this is the ideal base current for your application<br /> <br /> 3. subtract 0.7V (the base-emitter drop)&nbsp;from the drive voltage from the Arduino pin&nbsp;(use the data sheet&nbsp;and&nbsp;the Minimum guaranteed Vh out.<br /> <br /> 4. divide this number by the base current you determined in 2 above.<br /> <br /> 5. this is the base resister value you need to use.<br /> <br /> works every time ^_*<br /> <br />
&nbsp;Thanks, that's what I wanted to know: how to determine the base current. Now I know it is obtained by dividing the switched current by the transistor's Hfe.<br /> <br /> Again thanks a lot!<br />