| 1 | // |
| 2 | // This example shows one way of using an LED as a light sensor. |
| 3 | // You will need to wire up your components as such: |
| 4 | // |
| 5 | // + digital2 |
| 6 | // | |
| 7 | // < |
| 8 | // > 100 ohm resistor |
| 9 | // < |
| 10 | // | |
| 11 | // | |
| 12 | // ----- |
| 13 | // / \ LED, maybe a 5mm, clear plastic is good |
| 14 | // ----- |
| 15 | // | |
| 16 | // | |
| 17 | // + digital3 |
| 18 | // |
| 19 | // What we are going to do is apply a positive voltage at digital2 and |
| 20 | // a low voltage at digital3. This is backwards for the LED, current will |
| 21 | // not flow and light will not come out, but we will charge up the |
| 22 | // capacitance of the LED junction and the Arduino pin. |
| 23 | // |
| 24 | // Then we are going to disconnect the output drivers from digital2 and |
| 25 | // count how long it takes the stored charge to bleed off through the |
| 26 | // the LED. The brighter the light, the faster it will bleed away to |
| 27 | // digital3. |
| 28 | // |
| 29 | // Then just to be perverse we will display the brightness back on the |
| 30 | // same LED by turning it on for a millisecond. This happens more often |
| 31 | // with brighter lighting, so the LED is dim in a dim room and brighter |
| 32 | // in a bright room. Quite nice. |
| 33 | // |
| 34 | // (Though a nice idea, this implementation is flawed because the refresh |
| 35 | // rate gets too long in the dark and it flickers disturbingly.) |
| 36 | // |
| 37 | #define LED_N_SIDE 2 |
| 38 | #define LED_P_SIDE 3 |
| 39 | |
| 40 | void setup() |
| 41 | {} |
| 42 | |
| 43 | void loop() |
| 44 | { |
| 45 | unsigned int j; |
| 46 | |
| 47 | // Apply reverse voltage, charge up the pin and led capacitance |
| 48 | pinMode(LED_N_SIDE,OUTPUT); |
| 49 | pinMode(LED_P_SIDE,OUTPUT); |
| 50 | digitalWrite(LED_N_SIDE,HIGH); |
| 51 | digitalWrite(LED_P_SIDE,LOW); |
| 52 | |
| 53 | // Isolate the pin 2 end of the diode |
| 54 | pinMode(LED_N_SIDE,INPUT); |
| 55 | digitalWrite(LED_N_SIDE,LOW); // turn off internal pull-up resistor |
| 56 | |
| 57 | // Count how long it takes the diode to bleed back down to a logic zero |
| 58 | for ( j = 0; j < 30000; j++) { |
| 59 | if ( digitalRead(LED_N_SIDE)==0) break; |
| 60 | } |
| 61 | // You could use 'j' for something useful, but here we are just using the |
| 62 | // delay of the counting. In the dark it counts higher and takes longer, |
| 63 | // increasing the portion of the loop where the LED is off compared to |
| 64 | // the 1000 microseconds where we turn it on. |
| 65 | |
| 66 | // Turn the light on for 1000 microseconds |
| 67 | digitalWrite(LED_P_SIDE,HIGH); |
| 68 | digitalWrite(LED_N_SIDE,LOW); |
| 69 | pinMode(LED_P_SIDE,OUTPUT); |
| 70 | pinMode(LED_N_SIDE,OUTPUT); |
| 71 | delayMicroseconds(1000); |
| 72 | // we could turn it off, but we know that is about to happen at the loop() start |
| 73 | } |