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--- oak:tutorials:photocell [2016/03/20 19:04]
jwhendy created
+++ oak:tutorials:photocell [2016/03/22 21:53]
jwhendy
@@ Line 1: / Line 1: @@
-In this lesson, we will connect a photoresistor to our Oak. There some great background information and an Arduino tutorial on [[https://learn.adafruit.com/photocells|Adafruit]], so please do take a look there for further learning. In essence a photoresistor is simply a potentiometer where light is acting to turn the knob. When there is a lot of light, the resistance drops; with low light, the resistance is higher.
+===== Oak: using a photocell (photoresistor) =====
-==== Requirements ====
+[[http://digistump.com/wiki/_media/oak/tutorials/photocell.png|{{http://digistump.com/wiki/_media/oak/tutorials/photocell.png?400}}]]
-This tutorial will use the following (in addition to an Oak):
+In this lesson, we will connect a photoresistor to the Oak. A photoresistor is like a potentiometer (a variable resistor) except light is acting instead of a knob. When there is a lot of light, the resistance drops; with low light, the resistance is higher. Using this property, we can send a voltage through a photoresistor, measure the resistance via a change in voltage, and obtain a value for how much light is shining on the photoresistor.
-  * a breadboard
+===== Components =====
-  * a photoresistor
-  * jumper wires
-  * a 10k resistor
-  * a USB UART converter, [[http://digistump.com/wiki/oak/tutorials/serial_through_arduino|serial-capable Arduino]], or familiarity with [[http://digistump.com/wiki/oak/tutorials/particle-variable|Particle.variable()]]
-==== Connecting the circuit ====
+^ Part       ^ Quantity          ^Identification^
+| Oak with soldered headers |1| |
+| Breadboard|1| |
+|Photocell|1| |
+|Jumper wires|7| |
+|Resistor, 10k ohm|1|Black-Brown-Orange|
+|3.3V USB UART or an Arduino*|1 | |
-Here is the wiring connection we'll use:
+**Note:** if using an Arduino to read serial, you will need to follow the instructions at the tutorial on [[http://digistump.com/wiki/oak/tutorials/serial_through_arduino|serial with an Arduino]]. In contrast, you don't need //either// of these components if you learn how to use ''Particle.variable()'' as described in [[http://digistump.com/wiki/oak/tutorials/particle-variable|this tutorial]].
+===== Concepts =====
+=== Voltage divider ===
+In the circuit we create below, electricity will flow from the Oak's ''Vcc'' pin into one leg of the photocell. When electricity leaves the photocell, it will see two paths: one to pin '''A0'', and the other to ground through the 10k resistor. This creates what is known as a voltage divider, which is very helpful for measuring small changes in voltage. There's a great writeup of //why// this is required on the Electronics StackExchange Q&A site [[http://electronics.stackexchange.com/questions/70009/why-use-a-pull-down-resistor-with-a-ldr-and-microcontroller|here]]. An Adafruit [[https://learn.adafruit.com/photocells|tutorial]] also features a discussion on adjusting the resistor value to target different ranges of measurement in the section on [[https://learn.adafruit.com/photocells/using-a-photocell|using a photoresistor]].
+The short version is that a voltage divider creates a ratio in the voltages in each path based on their individual resistances. The voltage going to ''A0'' can be found as follows:
+<code>
+A0 voltage = 3.3V (Vcc) * 10k / (10k + photocell_resistance)
+</code>
+The ratio of the resistor going to ground divided by //total// resistance tells us how much will be measured at A0. If the photocell resistance is small, there will be very little difference between ''Vcc'' and ''A0'' with a direct connection. By using the ratio produced by a voltage divider, the ''A0'' voltage will vary a lot more as the resistance of the photocell changes.
+=== ADC ===
+An analog sensor is one that produces a change in electrical properties in response to changes in the thing being measured. This is in contrast to digital sensors which use a communication protocol to report out an actual number. A photocell is an analog sensor in that it's resistance changes in response to light. We do not obtain a direct number from the photocell; instead, we need to measure the electricity going through it and convert //that// to a number of some form.
+An Analog to Digital Converter (ADC) does this for us. It will read in a voltage and compare it to a reference voltage (''Aref''), which serves as the maximum value it //could// read. The measured voltage is then converted into a number ranging from 0 (measured voltage = 0) to maximum (measured voltage = reference voltage), which depends on the microcontroller. The Arduino, for example, converts 0 - ''Aref'' into 0-255. The Oak, converts to a range of 0-1023.
+=== analogRead() ===
+The ''analogRead()'' function is how we access the ADC. When we call ''analogRead(A0)'', we are telling the Oak to convert the voltage it measures at ''A0'' into a number between 0-1023 depending on how big it is. By using the ''delay()'' function, we can adjust how often these readings are taken. We can also use an ''if()'' statement to do something else based on the 0-1023 value.
+=== the if() statement ===
+For the second portion of this tutorial, the code will use an ''if()'' statement. This is one of the fundamental ways to control the Oak's behavior. The format is:
+<code>
+if( some condition ) { do this; }
+</code>
+In our case, the condition we're checking for is whether or not the ''A0'' light reading is above or below 250. If it's above 250, we do nothing. If it's below 250, we turn on the on-board LED. To compare the actual statements above to the actual code below, here are the lines. Can you see how they work?
+<code>
+if(light < 250) { digitalWrite(1, HIGH); }
+if(light >= 250) {digitalWrite(1, LOW); }
+</code>
+===== Circuit =====
+To connect the photocell, we will use the following circuit:
 [[http://digistump.com/wiki/_media/oak/tutorials/photocell.png|{{http://digistump.com/wiki/_media/oak/tutorials/photocell.png?500}}]]
-If you prefer a real life example, here is the setup:
+To complete the circuit, follow these steps:
+  * Insert the photocell legs into two different rows
+  * Connect the 10k resistor from one photocell leg (either is fine) to a new row with a jumper wire
+  * Connect pin A0 of the Oak to the same row containing //both// leads from the resistor and photocell
+  * Connect a ground pin of the Oak to the free lead on the resistor
+  * Connect a Vcc pin on the Oak to the free lead of the photocell
+For reference, here is the setup above in real life:
 [[http://digistump.com/wiki/_media/oak/tutorials/photocell-wiring.jpg|{{http://digistump.com/wiki/_media/oak/tutorials/photocell-wiring.jpg?400}}]]
-We connect one leg of the photocell directly to ''Vcc'', and the other leg to two different "outlets:" one to analog pin, ''A0'', and the other to ground through the 10k resistor. This creates what is known as a voltage divider, which is required for measuring small changes in voltage. There's a helpful writeup of //why// this is required on the Electronics StackExchange Q&A site [[http://electronics.stackexchange.com/questions/70009/why-use-a-pull-down-resistor-with-a-ldr-and-microcontroller|here]]. The Adafruit tutorial also features a discussion on adjusting the resistor value used to target different ranges of measurement in the section on [[https://learn.adafruit.com/photocells/using-a-photocell|using a photoresistor]].
+If you wish to read the photocell output via serial, please take this time to follow the instructions as described [[https://github.com/digistump/OakRestore|here for a UART]] and [[http://digistump.com/wiki/oak/tutorials/serial_through_arduino|here when using an Arduino]].
-==== The code ====
-For an introduction to these sensors, we'll read the light value to get a handle on the expected ranges that will be encountered, and then pick a cutoff value at which point to turn on the built-in LED.
+===== Code: reading the photocell =====
-Reading the photocell is straightforward:
+We will begin by simply reading the value produced by the photocell signal to ''A0'':
 <code>
@@ Line 37: / Line 90: @@
 void setup()
 {
-  // use the on-board LED to illuminate when the button is pressed
-  pinMode(1, OUTPUT);
-  pinMode(1, LOW);
   // initialize the analog pin as an input
   pinMode(A0, INPUT);
+  // start serial communication and create a variable for particle.io access
   Serial.begin(9600);
+  Particle.variable("light", light);
 }
@@ Line 60: / Line 111: @@
   Serial.println(light);
-  // a short delay as we don't need readings every millisecond
+  // a short delay as we don't need extremely fast readings
   delay(250);
@@ Line 66: / Line 117: @@
 </code>
-The code above assumes you will use a USB UART or Arduino to read the data via serial, however you are free to access it through the [[http://digistump.com/wiki/oak/tutorials/particle-variable|Particle.variable()]] if you prefer. Using serial, the output will look like this as you cover and uncover the sensor:
+This code reads ''A0'', stores the number it obtain in the variable called ''light'', sends it ot the computer via serial, and then pauses for 0.25 seconds.
-<code>
+Using serial, the output will look like this as you cover and uncover the sensor:
+<code>
@@ Line 90: / Line 142: @@
 </code>
-Based on these readings, pick a value you want to consider "dark enough;" I'll use 250. If we add some code to our sketch, we can automatically turn on the on-board LED when the sensor is in low light. This is just the modified ''loop()'' section; include everything else from above in the full sketch.
+If you are using ''Particle.variable()'', you would issue a call like this, substituting your device ID and access token in their appropriate locations:
 <code>
+curl https://api.particle.io/v1/devices/id_here/light?access_token=token_here
+</code>
+The result will look like this, where the value in "result" is the output of ''A0''
+<code>
+{
+  "cmd": "VarReturn",
+  "name": "button",
+  "result": 529,
+  "coreInfo": {
+    "last_app": "",
+    "last_heard": "2016-03-19T20:51:30.817Z",
+    "connected": true,
+    "last_handshake_at": "2016-03-19T20:32:30.268Z",
+    "deviceID": "12345678",
+    "product_id": 82
+}
+</code>
+=== Code: turning on an LED when it's dark ===
+Now, we'll use these values to turn on and off the on-board LED at pin 1 based on how light or dark it is. Based on your readings, pick value 50-100 higher than your lowest reading. For the sensor above, 250 was chosen. If we add some code to our sketch, we can automatically turn on the on-board LED when the sensor is in low light. To do this, we add the LED functionality from the [[http://digistump.com/wiki/oak/tutorials/blink|blink tutorial]] and use an ''if()'' statement:
+<code>
+// variable to store our reading
+int light;
+void setup()
+{
+  // enable the on-board LED as an output
+  pinMode(1, OUTPUT);
+  // initialize the analog pin as an input
+  pinMode(A0, INPUT);
+  // start serial communication and create a variable for particle.io access
+  Serial.begin(9600);
+  Particle.variable("light", light);
+}
 // the loop() routine runs over and over again
 void loop()
@@ Line 107: / Line 204: @@
   if(light >= 250) {digitalWrite(1, LOW); }
-  // a short delay as we don't need readings every millisecond
+  // a short delay
   delay(250);
@@ Line 113: / Line 210: @@
 </code>
-Adjust the cutoff values as desired; with this code added, we get a nice effect like this (you may need to click and open in a separate tab to see the animation):
+Adjust the cutoff values as desired; with this code added, we get a nice effect like this:
 [[http://digistump.com/wiki/_media/oak/tutorials/photocell-turn-on.gif|{{http://digistump.com/wiki/_media/oak/tutorials/photocell-turn-on.gif?500}}]]
+===== Conclusion =====
+Congratulations! You just connected to an analog sensor and read the values! Sensors are a crucial component to many projects, so this is an invaluable skill that will serve you well. For a next step, perhaps you could try connecting a different analog sensor? There are many Arduino tutorials for other types of sensors, such as Adafruit's on using the [[https://learn.adafruit.com/tmp36-temperature-sensor/overview|TMP36 temperature sensor]].
-So, what could you use this for? That's up to you, but you now have a new sensor in your toolbox for future projects. Here's some [perhaps silly] ideas to get you going:
+Or perhaps you can think up some projects that might include a photocell? Now that you have this sensor in your toolbox for future project, here's some [perhaps silly] ideas to get you going:
   * an automated night light

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