Prototype Circuits
Introduction

Learning about electronics demands a hands-on approach. Your understanding will develop much more quickly if you put theory into practice by handling and using electronic components. The practical work supporting Introducing Circuits explains how to start building circuits in temporary, or prototype, form.

Prototype board
Look at the diagram below which shows a
prototype board
Prototype board is used for building temporary circuits. Connections are made by pushing components and wire links into the holes in the prototype board.
prototype board

 Figure 1. Prototype board.

Prototype board is used for building temporary circuits without soldering. Components and wire links are pushed into the holes. Inside the prototype board, metal channels with springy contacts make connections. The metal channels are arranged in rows:

 Figure 2. Connecting channels.

Some prototype boards have an easily removed base which lets you see this arrangement. On each side of the board, there are two long channels running from top to bottom. There is a gap in the centre of the board, and on either side of the gap there are short channels, each corresponding to a group of five holes.

Connecting a power supply
 Before you can use the prototype board, you need to connect a power supply.
 The 0 V connection is made to one of the strips on the left-hand side of the board.
 Connecting leads with black insulation are used for the 0 V connection.
 The positive end of the power supply is connected to the right-hand side of the prototype board.
 Leads with red insulation are used for the positive connection.

A power supply
voltage
Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.
voltage
of +6 V to +9 V is suitable for the circuits you are about to test. You can use a laboratory power supply, a regulated '
battery
A battery consists of two or more cells. The cells may be connected in series or in parallel.
battery
eliminator' supply (as used with a personal stereo), or 4 AA size cells in a battery holder. Check the directions of the connecting channels by moving the mouse over the diagram:

 Figure 3. Connecting a power supply.
Connecting a lamp
Connect a lamp to the prototype board, like this:
 Figure 4. Connecting a lamp.

The lamp is connected directly across the power supply and should shine with normal brightness. If it does not, check that

• the power supply is switched on;

• the circuit is complete;

• the wires from the lamp are inserted exactly as shown;

• the lamp filament is undamaged;

• the lamp is screwed into its holder, making proper contact with the holder terminals.

This is like the torch
circuit
A circuit is a closed conducting path.
circuit
in Introducing Circuits. Unless there is a continuous conducting path,
current
Current I is a flow of charged particles, usually electrons.
current
will not flow and the lamp will not light.

Change the circuit as follows:

 Figure 5. Connecting a lamp using link wires.

Now you are using link wires to make connections from the power supply rails to the rows of holes in the centre of the prototype board. When the wires are in the same horizontal row, the springy contacts inside the board make an electrical connection between them.

Lamps in series
Build a new circuit exactly as shown in Fig.6:

 Figure 6. Two lamps in series.

This circuit will not work. The lamps are supposed to be connected one after another in
series
Components are connected in series when they are joined end to end in a circuit, so that the same current flows through each.
series
, but will not light up because the circuit is incomplete. Look at the prototype board layout carefully. Where have mistakes been made? Experiment, moving the lamp connections or adding new links, until both lamps light up.

Complete the following statements to summarize what changes must be made to the circuit in Fig.6 to make both lamps light up.

• The connection from the top lamp and the connection from the bottom lamp must be in the same horizontal row.

At the bottom of the prototype board, the in the centre must be , either by moving the link wire, or by inserting a new link.
 Check your circuit against the diagram shown in Fig.7 below:

 Figure 7. Two lamps in series: corrected circuit.

Once you have got the circuit in Fig.6 to work, what do you notice about the brightness of the lamps?
Has the current flowing through the lamps increased?
Unscrew one of the lamps from its socket. What happens to the other lamp?
Complete the following statement explaining the behaviour of the lamps in Fig.6.

• When two lamps are connected in series, the resistance in the circuit is . This makes it difficult for current to flow and the lamps shine brightly.

In a series circuit, the current flowing at all points is . Because they are connected in series, removing one of the lamps the circuit.
Lamps in parallel
Now build a different circuit, this time with the lamps in
parallel
Components are connected in parallel when they are joined side by side in a circuit, so that they provide alternative pathways for current flow.
parallel
:

 Figure 8. Two lamps in parallel.

What do you notice about the brightness of the lamps in Fig.8?
How does the current through an individual lamp compare with the current flowing in the single lamp in Fig. 5?
Has the current provided by the power supply increased or decreased?
Unscrew one of the lamps from its socket. What happens to the other lamp?
Complete the following statement explaining the behaviour of the lamps in a parallel circuit.

• With the lamps connected in parallel, the current is flowing through . Each lamp draws its normal current, so the total current in the circuit has . The total resistance in the circuit has .

Both lamps ought to be just as bright as a single lamp, but you might find that they are slightly less bright, depending on the type of power supply used.

In the series circuit, there was only for the current so removing one of the lamps broke the circuit. If one lamp is removed from the parallel circuit, current can still flow through the other pathway, so the other lamp remains lit.
Combining series and parallel circuits
 Figure 9. Prototype board and lamps.

Your final prototype for this session includes a combination of series and parallel circuits. When the circuit is correctly assembled, all three lamps should light:

 Figure 10. Series and parallel in combination.
Are the three lamps equally bright?
How does the brightness of each lamp compare with normal brightness?
•  Lamp A Much less bright than normal Slightly less bright than normal Lamps B and C Much less bright than normal Slightly less bright than normal
What happens when the individual lamps are unscrewed?
•  Lamp A lamp A is in a series part of the circuit so the circuit is broken Lamp A is dimmer, lamp B is brighter All the lamps go out Lamp A is dimmer, lamp C is brighter Lamp B lamp B is in a parallel part of the circuit: current continues to flow through lamp C Lamp A is dimmer, lamp B is brighter All the lamps go out Lamp A is dimmer, lamp C is brighter Lamp C lamp C is in a parallel part of the circuit: current continues to flow through lamp B Lamp A is dimmer, lamp B is brighter All the lamps go out Lamp A is dimmer, lamp C is brighter

Experiment with the circuit and interpret your observations in terms of the overall
resistance
Resistance R limits current flow.
resistance
in the circuit and the currents flowing in each lamp.

Complete the following statements summarizing the behaviour of the lamps in Fig.10.

• Lamp A is in a part of the circuit. When it is unscrewed, the circuit is incomplete and the other lamps go out as well.

Lamps B and C are in a part of the circuit. Unscrewing lamp B increases the total in the circuit. This means that less will flow through lamp A.

However, is no longer shared between lamps B and C, so all the current flowing through A flows through C. Lamps A and C are connected in series and are now equally illuminated. (This is identical to the series circuit tested previously.)
Techniques
The joining wires which you use to connect up your circuit must be made from insulated single core wire, described in catalogues as '1/0.6 mm single core equipment wire'. To make the wire links, you need to remove about 5 mm of insulation from each end. This is done using wire strippers:
 Figure 11. Miniature wire stripper.

Adjust the position of the screw, tightening the locking nut on the other side, so that the jaws of the stripper do not close completely. The jaws grip and cut into the outer insulation without penetrating into the wire core.

Automatic strippers, shown below, are quick and easy to use and work well provided you are careful with them:

 Figure 12. Automatic wire stripper.

A pair of cutting blades in the handle lets you cut the wires to length. Place the end of the wire in the jaws against the adjustable stop and close the handles to remove the insulation. Cut links 50 mm long and keep them for reuse.

Summary

Parts list:

 Quantity Item 1 prototype board (breadboard) 1 +6 V to +9 V power supply 3 6 V/0.06 A lamps with screw-in holders link wires wire stripper pliers side cutters

Well done!
Try again!