E5 Lab 1
Soldering your first PCB

(Printed Circuit Board)


In this lab you will be constructing a small circuit that will hold sensors for your robot.  The sensors include four light sensors, and two contact sensors.  By the end of the lab you should have an understanding of how the sensor circuitry works, and you should know how to solder. 

Background - the circuit schematic

The circuit that you will be constructing holds six sensors that will interface to the robot that you will be building throughout the semester.  Their are four light sensors, and two touch sensors.  All of the sensors feed into a connector that will later be tied into your robot.

Optical Sensor

The optical sensors are made from Cadmium Sulfide, or CdS.  The cell consists of a thin line of CdS that goes between two terminals.  As light hits the Cadmium Sulfide electrons are dislodged within the material and become free to carry current, thereby lowering the material's resistance.  The CdS cell is put into a circuit in series with a 5 volt source and a resistor as shown.   The circuit may look a little funny because there is not a complete loop, but this is OK.  The drawing uses a convention that has a 5 volt source implicitly in the circuit between the nod labeled "+5V" and the node labeled "Ground."

The current (I) through the circuit is voltage (V=5 volts) divided by the total resistance, Rtotal=Rresistor+RCdS.

The voltage across the CdS cell is this current times the resistance of the CdS cell.

So as light hits the CdS cell, its resistance decreases, as does the voltage across the cell.  By monitoring this voltage we can measure the light hitting the sensor.

This circuit is repeated 4 times on the circuit you will be constructing, once for each photo-sensor.

Touch Sensor

The touch sensor is a little easier to understand.  Its schematic is shown, and consists of a switch and a resistor.  Normally the switch is closed, so the output voltage is zero volts (Ground).  When the switch opens up, current ceases to flow, so the voltage across the resistor goes to zero.  Since the voltage on both sides of the resistor is the same, the output voltage becomes 5 volts.  

There are two of these circuits on your robot, to detect collisions with objects on either side. 

Completed Schematic

A completed schematic is shown below.  There are four light sensor circuits, and two touch sensors (a different circuit symbol is used for the touch switches, but it is functionally equivalent to the one shown above).  All of the signals, including +5 volts and ground, then go to a connector (labeled DIP14HEADER) which will feed the signals to the computer on your robot.  This diagram doesn't show resistors connected to the switches because these are actually on the board that holds the computer.

Construction - soldering your circuit

Your going to be constructing this circuit using a printed circuit board, or PCB.  The PCB is a thin insulating board onto which a circuit is printed.  You then connect your components to the circuit with solder.  Solder is a combination of metals, usually 60% lead/40% tin, that is melted with a soldering iron to connect electrical components.   The solder melts at about 190C (370F).  It  has a core of flux which is seen boiling away as the solder is applied -- the flux is a chemical that improves the quality of the connection being made.  If you haven't soldered before, please read my page that describes soldering (and perhaps check out some of the links at the bottom of that page).  Solder contains lead, so try to avoid breathing the fumes, and wash your hands when you are done.  If you are experienced in soldering, let somebody else in the group (with less experience) do the soldering while you try to help them learn the proper technique.

The diagram shows the layout of the printed circuit board.  The green line represent traces (copper lines that are printed on the board, then covered with solder to prevent oxidation) on the top side of the board -- called the component side.  The red lines represent traces on the bottom side -- called the solder side.  On this layout you can see where the switches (S1 and S2), the CdS cells, the resistors and the 14 pin connector are placed.

To start the construction place the 14 pin socket and four 1000 ohm resistors on the board with the component side (with most of the writing) facing up, as shown.  Solder all the components, and cut the leads of the resistors close to the board.  There should be a completed board to which you can compare yours board.

Next, screw two switches to the solder side of the board with four 3/4" #4-40 screws.  The 3/4" corresponds to the length of the screw, #4 refers to the diameter, and 40 refers to the size of the thread (40 per inch).  Next feed a wire up through one of the holes under one of the three terminals on on one of the  switches, and hook it onto the terminal.  Solder both ends of this wire.  Repeat with the other terminals until all six terminals are soldered.  Attach the bumper as shown by placing wires through the holes, and twisting the wires around the leaves of the switches (thin bus wire works well).  Don't attach too firmly; the switches should have a little play, but the should be in no danger of falling off.

The last step is to attach the photocells.  Take several photo-cells and measure their resistance, which should decrease as more light falls upon them.  Go through them, one at a time, until you find four that have similar resistance when point in the same direction (so that the same amount of light falls on each).  You won't be able to get a really close match, but shoot for the same value within about 20-30% -- the match isn't critical, or even totally necessary.  Slip a 7/8" length of shrink wrap tubing on on leg.  Carefully insert the leads of the photocell from the component side, until the shrink wrap butts up against the PCB.  Hold the photocell there, and solder.  Hold the photocell by the top part, the leads will get very hot as you solder them (metal conducts heat very well).  Repeat until all 4 are attached.  When finished, cut the excess leads off from the back of the PCB.

Attach the bumper loosely to the two switches using buss wire.  The completed board (attached to the robot) is shown .  A larger photo of the whole robot (as seen from below), is also available.



Comments or Questions?
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Erik Cheever
Engineering Department
Swarthmore College