Diff: main.cpp

Revision:

0:bee32ed0a6b4

Child:

1:f4abb1d24a76

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp	Sat Nov 01 20:29:33 2014 +0000
@@ -0,0 +1,81 @@
+// Program.: Photo_LED
+// Version.: 1.0
+// Chip....: STM32F4xx
+// Date....: November 2014
+// Author..: UJ
+//-----------------------------------------------------------------------------
+
+// This program is a simple demonstration how a normal LED can be used as a
+// photo diode. The hardware platform I have used is the NUCLEO 411RE, but
+// the principle can be used with every modern CMOS-controller.
+//
+// Harware sketch:
+//     LED (20mA, green or red)
+//           |\    |
+//           |  \  |
+//      -----|    \|----
+//     |     |    /|   _|_
+//     |     |  /  |  |   |
+//     |      /       |   | 100 Ohm
+//     |              |___|
+//     |                |
+//     |                |
+//     -                -
+//    |O|              |O|
+//     -                -
+//    Anode Pin        Cathode Pin
+//
+// How it works:
+// In the first phase the LED is turned on in it´s normal mode for 50ms.  
+// In the second phase the LED is driven with a reversed voltage for 10ms
+// (anode negative, cathode positive). This will charge the internal capacity
+// with reverse potential. Then the anode pin is configured as a high-impedance
+// input pin. It takes some time until the voltage at the anode pin will raise
+// above the Schmitt-trigger level. This time is depending on the over all 
+// circuit resistance, but also very depending on the ambient light.
+//
+// Warning:
+// The capacity of the LED is very low (5..35pF). Do not place the board
+// to a conductive ground. Even humidity will change the blink frequency.
+// Do not touch the components.
+//-----------------------------------------------------------------------------
+
+#include "mbed.h"
+//-----------------------------------------------------------------------------
+
+DigitalInOut anode_pin(PC_10);
+DigitalOut cathode_pin(PC_11);
+//-----------------------------------------------------------------------------
+
+int main(void)
+{
+  anode_pin.output();
+
+  while (1)
+  {
+    // Set normal LED operation, turn on LED for 50ms
+    // Setup PC10 as output
+    anode_pin.output();
+
+    // Turn on LED for 50ms
+    // Set anode (PC10) high and cathode (PC11) low
+    anode_pin = 1;
+    cathode_pin = 0;
+    wait(0.05f);
+
+    // Apply reverse voltage to the LED for reverse charging the LED´s capacity
+    // Set anode (PC10) low and cathode (PC11) high
+    anode_pin = 0;
+    cathode_pin = 1;
+    wait(0.01f);
+
+    // Setup anode (PC10) as input
+    anode_pin.input();
+
+    // Wait until the input voltage raises above the Schmitt trigger level
+    // This time is strongly ambient light depending
+    while (anode_pin.read() == 0)
+    {
+    }
+  }
+}