The circuit

The sensor circuit can either be build on a piece of veroboard or on a milled PCB.

The components needed:

  • 10kΩ resistor (x3)
  • 68Ω resistor (Please note, that this resistor replaces the 120&Omega resistor on the schematic).
  • 10kΩ trimresistor (The blue resistor with the screw terminal)
  • 100 nF capacitor
  • 15 pF capacitor
  • TCRT5000 lightsensor
  • 8 pin IC socket
  • LM293 operation comparator

The schematic of the sensor circuit. If You chose to build the circuit on veroboard, remember to be very precise regarding connection between the different components. Please note the adjustments on the schematic. Sensor Schematic

The PCB handed out is outlined in the images below. Remember to face the PCB right before start soldering.

PCB from component side Backside of the PCB
Front side of the PCB Backside of the PCB

Be careful to mount the components right and in the right places. The result should look like the picture below. Components mounted on the PCB

How to mount the components on the PCB is outlined below. Note that blue lines means buttom layer.

Components placement The PCB
Component placement Components placed on PCB

Note that four vias, connections between top and buttom layer, has to be added to the PCB as indicated below. Vias

Test the built PCB by using the microscope and multimeter before attaching power to the circuit. After attaching power, measure whether output turns high or low when the sensor is placed below either a black or white surface.