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 which tracks it will connect
to.
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
which 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° 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 0V, 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 which 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 which 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.
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 15V 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 3.0V, 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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