Basic Line Sensor assembly

This is the line sensor circuit board top:

and bottom:


  1. Solder in emitter load resistors R3, R4, and R5 – all 68 Ὠ

    Then the detector load resistors R6, R7. R10 and R11.The value of these detector load resistors depends on the photo transistors being used as detectors. If using the BPW85C phototransistor then use four 4.7K Ὠ resistors for R6, R7, R10 and R11
  2. If emitters are to be pulsed solder in resistors R1 390 Ὠ and R2 10K Ὠ that provide the biasing for the pulse transistor. If the emitters are to be left on all the time, omit these two resistors but put in a wire link on the PCB where it says LNK1 BYPASS
  3. If you want to use the two indicator LEDs, solder in resistors R8 and R9. Before you confirm these values and solder them in, check that the resistance is suitable for the LEDs that you use for LED4 and LED5. Do this by putting one resistor in series with an LED with 5V across it to check that it lights up. If using a high efficiency LED you may be able to use a larger resister of up to 1KὨ
  4. Solder in Capacitor C1 100µF making sure that it is oriented correctly. The longer lead should be the positive side and the negative side should be marked on the body of the capacitor. The PCB shows a + next to the positive lead.
  5. Solder in the two indicator LEDs LED4 and LED5. Make sure these are correctly oriented. The long lead on the LED should go to the hole marked A (Anode) and the shorter lead (and the side with the flat on the LED) should go to the hole marked K (cathode). Make the LEDs stand a few mm above the board by slipping a thin piece of paper between the legs of the LED between the LED and the PCB.

  6. If emitters are to be pulsed, add in transistor Q1. Correct orientation is essential. The transistor has a flat side which should be placed where the word Q1 is printed on the PCB. This will ensue that the emitter, base and collector go correctly into the 3 holes marked e, b and c
  7. Next we need to add the sensor LEDs and phototransistors. These are all mounted on the other side of the PCB to the other components. There are 3 LEDs and 4 phototransistors.
    The values of these parts depend on whether you are using Infra-red invisible light or visible light. See Tools and Materials page for further details.

    The 3 LEDs are soldered in with their long leads (+ve) towards the front of the board marked A. Leave sufficient length on the leads so that the ones at the sides of the board can be bent out by 30 degrees to see the side markers. Once they are soldered in, put 5v and Gnd on the relevant connector pins and 5v on D12 to check that they all light up.

    Next put in the 4 phototransistors. If using the BPW85C these also go with their long leads towards the front of the board marked E (emitter). If using another phototransistor check from the datasheet which lead is the emitter and orient them accordingly. Make sure that there is enough length on the side ones to bend them out by 30 degrees to match the LEDs.

  8. Ten of the 11 holes of connector J1 on the sensor board now need to be attached to a 10 core multi-way wire or by 10 connector leads to the sensor socket on the main board. You do not use the 3.3v connection at the end marked J1. These will plug in via whatever connectors you use into the group of 10 adjacent pins marked from 5v through to A3 on the main PCB. The easiest solution is to use 5 Female to Female Dupont connector leads cut in half with the cut end soldered into the sensor board so that the 10 female ends plug into the Pin header on the board.
  9. The sensor board is fixed to the main PCB with two M3 screws with a 10mm spacer between the main PCB and the sensor board, with the nuts on top. These M3 screws go through the front set of two holes in the main PCB from the bottom.