Introduction

In this practical, you will make your own capacitor and use it to control the frequency of an astable circuit, producing pulses. By monitoring frequency changes, you can find out what happens to the size of the capacitor when you change the area of overlap of the plates or change the space between them.

Capacitors store electric charge. A capacitor consists of two plates of a conducting material separated by a space filled by an
insulator
An insulator is a material which prevents the flow of current. Most non-metals are insulators.
insulator
. The insulating layer is called the dielectric of the capacitor:

 Figure 1. Structure of a capacitor.

Capacitance is measured in farads, where 1 farad stores 1 coulomb of electric charge when the
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
across the capacitor is 1 volt. The farad is a very large unit of capacitance. Practical measurement units are much smaller:

Things about capacitors which can be changed include:

• the area of overlap of the plates

• the distance between the plates

• the material used as an insulator

You can change all of these in the experiments which follow.

Making the capacitor plates
This is easy. All you need is some aluminium kitchen foil and two clear plastic A4 pockets.

 Figure 2. Making capacitor plates.

Cut the foil to size and slide one sheet inside each of the pockets. The plastic is going to be the insulator between the plates of your capacitor.

Building an astable
Fig.3 below shows you how to build a
circuit
A circuit is a closed conducting path.
circuit
in which a capacitor controls the frequency of a pulse generator, or
astable
An astable is a subsystem which generates pulses.
astable
,
circuit
A circuit is a closed conducting path.
circuit
.

 Figure 3. Using a capacitor to control the frequency of an astable.

Follow the diagram carefully. The 4060 integrated circuit has a notch at its head end and must be inserted correctly. Check that you have connected all the link wires. Have you inserted the 47 µF capacitor the right way round? Don't switch on the power supply just yet.

The test capacitor consists of the two sheets of aluminium foil. Leave the foil inside the plastic pockets and make connections directly to the foil using crocodile clip leads. The crocodile clips must not touch each other.

 The astable is made using a CMOS integrated circuit, the 4060. At this stage it is not essential for you to understand anything at all about the internal function of the 4060. You are simply using the device to help you experiment with and understand capacitors. Read the next section but don't be confused if it doesn't seem to make sense!
Here is the pin connection diagram for the 4060:
 Figure 4. Pin connection diagram for 4060 CMOS.
The 4060 contains an astable section followed by a binary counter:
 Figure 5. Block diagram of 4060 CMOS.
To get the astable section to produce pulses, you need to add two resistors and a capacitor:
 Figure 6. Astable circuit using 4060 CMOS.
 In Fig.6, R1 is the 390 kΩ resistor, RT is the 47 kΩ timing resistor, and CT is the capacitor made up of the two sheets of aluminium foil.

If you are unsure about how to use an oscilloscope, look at the unit Using an Oscilloscope or ask your teacher for assistance. VOLTS/DIV should be set to 2 V per division and TIME/DIV to 1 ms per division. Be ready to adjust TIME/DIV to get a clearer picture of your signal.

Set the power supply voltage to 9 V using a
multimeter
A multimeter can be set up to work as a voltmeter, as an ammeter, or as an ohmmeter. These functions are selected by rotating the central control knob to the appropriate position.
multimeter
to measure the voltage. Connect the power supply to your circuit, checking that the positive connection goes to the top right corner of the
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
.

The initial frequency of the pulses can be divided by up to 214 times. Use the oscilloscope to monitor the counter outputs. You will find the fastest pulses at pin 7 of the 4060, with slower pulses at the other counter outputs. When you move the oscilloscope lead/link wire to pin 5, the frequency of the pulses is halved compared with the frequency at pin 7, and is halved again if you move the wire to pin 4. The slowest pulses can be observed at the final output of the counter, pin 3.

Connect the oscilloscope to the counter output at pin 7 (as shown in Fig.3.)
What happens to the frequency of the pulses on the oscilloscope when the area of overlap of the capacitor plates is increased?
What happens to the frequency of the pulses when the plates are pressed closer together?
It is nice to see the pulses on the oscilloscope screen, but it is more fun to hear the changes in the frequency of the pulses by adding an audio amplifier stage to your circuit. This doesn't involve any changes to you existing circuit. Just add the extra components and links:

 Figure 7. Connecting circuit to loudspeaker.

The audio amplifier integrated circuit is an LM386. The circuit diagram for the amplifier stage is:

 Figure 8. Amplifier circuit.

Again, you don't need to understand the details about the LM386 in order to be able to use it.

 Once the circuit is complete, the sound produced will be quite loud.
 The frequency should change when the area of overlap of the capacitor plates is changed, or when they are pressed closer together.
 A lower frequency means that the capacitor is taking longer to fill up and empty.
 In other words, lower frequencies indicate an increase in the size of the capacitor.

The effect of changing the insulator is more difficult to investigate. If you have time, try taking each sheet of foil out of its plastic pocket and putting it into an A4 paper envelope instead. Find out how this affects the size of the capacitor. For the experiment to be meaningful, the thickness of the paper of the envelope and the thickness of the plastic sheet used to make the folders needs to be the same.

Insulators used in making non-polarized capacitors include polyester, polythene, and ceramic. A thin layer of aluminium oxide forms the insulating layer in most polarized capacitors.

Summary

The step-through animation below summarizes the prototype boards constructed in this unit.

 Figure 9. Make your own capacitor.

Parts list:

 Quantity Item 1 switched range multimeter 1 oscilloscope 1 prototype board (breadboard) 1 +9 V d.c. power supply 1 10 Ω 0.5 W carbon film resistor 1 10 kΩ 0.5 W carbon film resistor 1 47 kΩ 0.5 W carbon film resistor 1 390 kΩ 0.5 W carbon film resistor 1 47 nF metallized polyester capacitor, 0.4 in lead pitch 1 100 nF metallized polyester capacitor, 0.4 in lead pitch 1 47 μF 25 V radial electrolytic capacitor 1 220 μF 25 V radial electrolytic capacitor 1 4060 CMOS binary counter/divider oscillator integrated circuit 1 LM386 audio amplifier integrated circuit 1 8 Ω/300 mW miniature loudspeaker 2 A4 plastic pockets 2 A4 paper envelopes aluminium foil scissors red and black 4 mm leads crocodile clips link wires wire stripper pliers side cutters

Well done!
Try again!