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Building Circuits

Why do it?

You may wish to design and build some small electronic circuits to act as sensors for your robot.  You can try out your ideas using breadboard, as you may have done in other laboratory work.  However, you cannot use breadboard on your robot - it is too big and heavy, and the connections are not secure enough to be reliable on a robot.  So you will have to build your circuits by soldering the components onto small circuit boards.

Strip-board

You will be building circuits on strip-board.  This is a thin sheet of insulating board, with parallel copper strips or tracks on one side, and lots of holes.

To build a circuit, you place components on the plain side of the board, with their leads passing through the holes.  You then solder the component leads to the copper tracks.

Each copper track becomes a node in your circuit - all component leads soldered to the same track are connected together.  There is a special tool for cutting tracks if you need to break one to form two separate nodes.

It is important to plan ahead when building a circuit on strip-board.  Decide in advance where each component will go, and to which tracks it will connect.

Resistors are normally mounted parallel to the board, at right angles to the tracks, as shown above.  Bend the leads carefully, using a pliers to grip the lead between the resistor and the bend point.

If you are short of space, it is possible to mount a resistor perpendicular to the board.  One lead must be bent through 180 degrees - bend it around the pliers to avoid breaking it.

Heat problems

Semiconductor components, such as LEDs (light-emitting diodes) and light sensors, are easily damaged by heat - some can only withstand soldering temperatures for less than 2 seconds.

It is best to mount these components some distance from the board, so that you are not soldering too close to the body of the component.  Someone should also grip the component lead with the pliers, between the component and the soldering point, while you are soldering.  The pliers acts as a heat sink, and absorbs some of the heat that flows along the component lead.

Wire

There are two types of wire available in the lab:

Solid-core wire has one solid conductor with coloured plastic insulation.  It is easy to solder onto a circuit board, just like a resistor.  

Use it for joining tracks on your circuit board, where it will not be moved.  It will break very quickly if it is bent repeatedly.

Stranded wire has a conductor made from many thin strands of wire, surrounded by coloured plastic insulation.  This makes it more flexible.

Use it for connections between your circuit board and the Handyboard, or anywhere where it will be subject to movement.


To remove the insulation from these wires, use the wire-stripper in the tool-kit.  Squeeze it gently around the wire - you want to cut through the plastic insulation, but not the conductor inside.  

Then rotate the wire through 90 degrees and squeeze again, to cut the other sides of the insulation.  While still squeezing gently, pull the wire-stripper towards the end of the wire, to pull off the piece of insulation.

The wire-stripper has an adjustable stop, which can be set so that it cuts just deep enough to cut through the insulation without damaging the conductor.  If yours is not adjusted correctly, you can use the pliers and screwdriver to re-adjust it.

To make it easier to solder stranded wire onto strip-board, you should tin the end first:  

Remove the insulation and twist the strands together lightly to form them into a neat bundle.  Heat the bare wire strands with the tip of the soldering iron, from below.  Then melt a small amount of solder onto the top of the wire strands - it will flow into the spaces between the strands.  Do not apply too much solder, or the wire end will not fit into the hole in the strip-board!

Use the different colours of wire in some sensible way - red for positive, black for negative or 0 V, other colours for your signals.

Joining wires

Most of your wires will probably be soldered to a circuit board, or to a plug for the Handyboard.  If you need to join wires together, without a circuit board, you should twist them first, to get some mechanical strength, then apply a small amount of solder.  

Cover the joint with heat-shrink tube to insulate it.

Heat-shrink tube

This is plastic tube that shrinks to a much smaller diameter when it is heated.  You can use it to insulate your connections, and it will also provide some mechanical strength.  You can also use it (in a larger size) to fit over an LED or a sensor, to act as a shield.  See examples on the soldering page.

Connecting to the Handyboard

To connect your circuit to a port on the Handyboard, use flexible (stranded) wires - the solid-core wire will break if it is bent repeatedly.  One end of each wire will connect to your circuit.  The other ends will connect to a 3-pin plug to suit the Handyboard ports.  You can make this plug from the strip of pins provided - see the instructions.

Practice circuit

This is the circuit that you must build to develop your soldering skills.  It uses four resistors and two flexible (stranded) wires, so you will have to make 10 solder joints in total.  Plan the layout on the circuit board before you start to build it.  The nodes in the circuit are marked to make this easier.

Circuit diagram

Testing

Connect the red and black wires from your circuit to a dc power supply in the laboratory.  Set the power supply to provide exactly 15 V between the two wires.  If in doubt, check the voltage using the digital multi-meter.

Now use the digital multi-meter and probes to measure the voltage across resistor R4 - between nodes c and d.  It should be 3.0 V, or something very close to this.  If it is not, there is something wrong with your circuit - check it!  If you cannot find the problem, ask for help.

When your circuit is working properly, ask one of the staff to inspect it.  You will not be allowed to work with the optical components unless your circuit-building skills are reasonable!

 

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