Differential Train Detector

Introduction
I have used a number of methods of detecting a train's position in a layout. Each method has its advantages and disadvantage. I recently read a description of a differential train detector that made use of a comparator and two phototransistors. My thanks to the author of the article, Geoff Bunza.

While I am sure that the circuit described in Model Railroad Hobbyist works well I prefer to use an inexpensive microcontroller rather than a comparator for this circuit.

What follows is a description of my experiments with a differential train detector.

Circuit Description
In the circuit the brightness detected by two phototransistors (Q1 and Q2) is read by the microcontroller's analog to digital channels and compared. Since one phototransistor is mounted outside of the railroad's rails and the other between the rails a passing train causes a shadow to fall on one of the phototransistors. The change in brightness is noted and the LED is illuminated until the two phototransistors again see the same brightness level.

Schematic
The schematic shows the two phototransistors, Q1 and Q2. Their collectors go to +5 volts and their emitters go to ground through a 10K resistor. The four pin header to the far left is for programming.

Software for PIC12F683
The program is written in PIC Basic Pro. The main loop of the program which reads the phototransistors is highlighted.

ansel = 0 'all inputs digital
cmcon0 = 7
Include "modedefs.bas"
define ADC_BITS 8
define ADC_CLOCK 3
define ADC_SAMPLEUS 50
ADCON0=0 'analog
SerialOut var gpio.0 'pin 7
PhotoT2 con 2 'var gpio.1 'pin 6
PhotoT1 con 3 'var gpio.4 'pin 5 - use HPWM 2, ...
DarkDetect var gpio.5 'pin 2 LED
PTRead1 var word
PTRead2 var word
Trigger var word
gpio = %00011110
serout SerialOut,n9600,[10,13,10,13,"Dual PhotoTransistor Train Detector - 4-11-2016",10,13]
pause 1000

TOP:
adcin PhotoT2, PTRead2
adcin PhotoT1, PTRead1
serout serialout,n9600,["PTR2= ",#PTRead2," PTR1= ",#PTRead1," tr= ",#trigger, " diff ",#abs(PTRead2-PTRead1), 10,13]
' change variable based on max reading (perhaps max / 10)
Trigger = (PTread2+ptread1 )/2
Trigger = Trigger / 5
if abs(PTRead2 - PTRead1 ) > Trigger then
     high darkdetect
     pause 500
   else
     low darkdetect
endif
'pause 100
goto top:

Arduino Version
The code and circuit shown here will work with an Arduino UNO and others. You should note that the schematics and code are very similar.

// Test of differential train detector on Ardudino
// Connect emitter of phototransistor to ground through a 10K resistor
// Connect the emitter of phototransistor to Pin A0 or A1
// Connect collector of phototransistor to +5 volts

// Connect LED to pin 10 through a current limiting resistor


const int PhotoT1 = 0; // phototransistor - pin A0
const int PhotoT2 = 1; // phototransistor - pin A1
const int DarkDetect = 10; // LED
int PTRead1;
int PTRead2;
int Trigger;


void setup() {
  pinMode(DarkDetect, OUTPUT);
  Serial.begin (115200);
  Serial.println("Differential Light Test v0.1");
}

void loop() {
  PTRead1 = analogRead(PhotoT1);
  PTRead2 = analogRead(PhotoT2);
  Trigger = (PTRead1 + PTRead2) / 2;
  Trigger = Trigger / 5;
  Serial.print("Trigger ");
  Serial.print(Trigger);
  Serial.print(" PTRead1 ");
  Serial.print(PTRead1);
  Serial.print(" PTRead2 ");
  Serial.println(PTRead2);

  if (abs(PTRead2 - PTRead1) > Trigger) {
    digitalWrite(DarkDetect, HIGH);
  }
  else {
    digitalWrite(DarkDetect, LOW);
  }
  delay(500);
}