Line Sensor Mini Wide
By Chris Balmforth, UKMARS
These are the instructions for the UKMARS half size mini wide line follower board. This board places the side marker sensors at the optimum point for picking up side markers on the half size track, and moves the line sensors back closer to the centre of rotation of the robot. These changes require the sensors to operate through slots in the main pcb so this board will only work with main board v1.3 or later. It also gives the user the choice of fitting four line sensors instead of two, allowing the robot to correct for larger errors before losing the line altogether. This requires a total of five emitters instead of three, so the emitter firing circuit uses two transistors and a larger electrolytic capacitor than the standard half size sensor board. This board is designed to plug directly onto the v1.3 UKMARS main board.
This is the top and bottom of the mini wide line sensor circuit board:
Start by adding resistors R7, R9 390 ohms and R8, R10 10K ohms to the top of the board. These provide the bias for the base of transistors Q1 and Q2 which switch the emitter LEDs on and off.
Next add resistors R1 33 ohms and R2, R3, R4, R5 and R6 – all 68 ohms. R1 limits the current through the emitters if they are inadvertently left switched on and the remainder are the current limit resistors for the five emitter LEDs. Leave out R2 and R4 if you are using two line sensors instead of four.
Now add resistors R11, R12, R13, R14, R15 and R16 – all 2.2K ohms. These are the load resistors for the six phototransistors. Leave out R12 and R15 if you are using two line sensors instead of four.
Now add R26 and R27 – both 1K ohms. These are the current limiting resistors for the two indicator LEDs
Now put in the two emitter switching transistors Q1 and Q2 ensuring that they have the flat side facing forwards as shown.
Next add the 2 indicator LEDs, D6 and D7. Almost any LED will work and they can be either 3 or 5mm but it is best to have two of different colours. Ensure that the long lead of the LED (the positive lead) goes into the side of the hole marked A (for anode). Note that both LEDs have the +ve lead facing forwards.
The final item on this side of the board is the 470 microfarad capacitor C1. It stands upright and is polarised so must be put in the right way round with the positive lead going in next to the + sign.. The side of the capacitor will normally have a lighter band with one or more minus signs on one side, so the other side is the positive lead. There is a choice of holes for the leads depending on the specific capacitor chosen.
Now turn the board over and put in the emitter LEDs D1, D2, D3, D4 and D5 if using four line sensors, or just D1, D3, and D5 if using two line sensors. Make sure you put them in the right way round with the long lead at the forward side of the board in the A (for Anode) hole. A short length of heat shrink tubing applied as shown will cut down light leakage to the sensor.
Now put in the six BPW85 phototransistors Q3, Q4, Q5, Q6, Q7 and Q8 next to the emitter LEDs if using four line sensors. Leave out Q4 and Q7 if you are using just two line sensors. Note that on this device the long lead is the emitter which is the negative lead. Make sure you put them in the right way round with the long lead at the forward side of the board in the E (for Emitter) hole. Note that if you use a different phototransistor the emitter, and collector lead positions may be the other way round. Check the device datasheet and insert the Emitter lead in the hole marked E, and the Collector lead in the hole marked C. Again a short length of heat shrink tubing applied as shown will cut down light leakage from the adjacent emitter LEDs.
Finally add the 14 way socket (or header if you soldered the socket to the main pcb) to the underside of the board, making sure that it is seated well down onto the board. Solder one end then the other end then check the seating before soldering the rest of the contacts.
Turn over the completed board. It is ready to plug in and should look like this:
3mm nylon bolts and spacers can be used to hold the board securely onto the main PCB, using the 2 holes near the side sensor slots. The spacer length should be around 11mm and adjusted with nuts or washers for a good fit.
When seen from below the phototransistors and emitter LEDs should be visible through the slots in the main board.
Your sensor board is now ready to test – please remember that the trigger on pin 12 needs to be set high to switch on the emitter LEDs and that about 50-100 microseconds delay should be given after switching on the LEDs for them to get to full brightness before reading the input from the phototransistors. After taking the readings you should switch off the emitter LEDs again to reduce the battery load.