Class practical

A CRO (cathode ray oscilloscope) can be used to measure potential differences, and to see how they vary. This makes a good introduction to the oscilloscope. But, today, one would use a digital voltmeter for any serious measurements.

Apparatus and Materials

For each student group

• 1.5 V cells, connected in series, 3
• Low voltage supply
• Voltmeter, 0–5 V
• Leads, 4 mm, 2

• Oscilloscope

Health & Safety and Technical Notes

It may be advisable to set up the students' oscilloscopes with the gain at 1 V/div, the time-base off, and the AC-DC switch on DC. Adjust the focus and brightness controls to give a clear spot at the centre of the screen. (Too bright a spot will damage the screen.)

Procedure

1. Connect the input of the oscilloscope across one of the cells, as shown. Adjust the gain so that the deflection is one division on the graticule. Connect the voltmeter across the cell, in parallel with the oscilloscope.
2. Now connect across two cells, and across three. How does the deflection change?
3. Reverse the leads to the oscilloscope to show the deflection across one, two and three cells in the opposite direction. The CRO acts as a voltmeter.
4. Disconnect the cells. Set the output of the power supply to 2 V, and connect it to the input of the oscilloscope. Then change the output first to 4 V and then to 6 V.
5. Switch on the time-base (1 ms/div) to show the trace spread out. Does the power supply provide a steady voltage?

Teaching Notes

• A CRO is a good voltmeter with an almost infinite input resistance (or impedance, the term used when dealing with alternating potential differences). However, the digital voltmeter is often simpler to use and adequate unless you need to see the waveform.
• When the power supply is connected to the CRO, students might be disappointed. Unless it is a well-smoothed supply (and few school supplies are) then the trace will not be a straight line. The trace might be very bumpy, showing that the power supply has fully rectified potential difference, but no attempt has been made to smooth it. Other supplies might displace the trace from the zero of the y-axis, but then there will be a sine-wave ripple on the top of it. The supply has been partially smoothed.

This experiment was safety-tested in October 2006

A video showing how to use an oscilloscope:

Class practicals

This experiment enables students to become familiar with the various controls on the CRO (cathode ray oscilloscope).

Apparatus and Materials

For each student group

• Power supply, low-voltage, AC
• Leads, 4 mm, 2

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

Procedure

1. As a demonstration, the oscilloscope is set with the gain at volt/cm switched to 1, the time-base control to 1 ms/cm and the AC-DC switch to AC. The AC terminals of the low-voltage power unit (2 volt AC) or the 2-volt terminals of the transformer are connected to the Input and Earth terminals of the oscilloscope. The variable control on the time-base is adjusted until four or five cycles of the waveform appear on the screen. The pattern traced on the screen should remain fixed in position.
2. As a class experiment, students connect the 2-volt AC supply in the same way to the input terminals on the class oscilloscope. The gain should be set at 1 V/cm, the time-base on range 2 ms/cm and the AC-DC switch on DC. Students should adjust the variable control on the time-base to see the pattern opening up and closing.

Teaching Notes

• It may be best to show this experiment first as a demonstration and then let the students achieve the same on their oscilloscopes.
• This experiment could be combined with the making of model transformers. The secondary coil can be connected to the y-terminals, and the trace noted as each extra coil is added to the secondary.

This experiment was safety-tested in March 2007

• A video showing how to use an oscilloscope:

Demonstration

The CRO (cathode ray oscilloscope) can show sinusoidal, square and triangular waveforms.

Apparatus and Materials

• Signal generator, with selectable waveform output
• Loudspeaker
• Leads, 4 mm, 4

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

Procedure

1. Set the oscilloscope with the volt/cm control at 1, the time base control at 1 ms/cm and the AC-DC switch to AC.
2. Set the signal generator to 2 V,50 Hz, sinusoidal output. Connect the high-impedance output terminals to the input of the CRO.
3. Show the effects of varying the output voltage of the signal generator (amplitude changes) and the frequency (period changes).
4. Switch between the different output waveforms (sinusoidal, square and triangular) to show these.
5. Connect a loudspeaker to the low-impedance output of the signal generator. Repeat steps b to d above so that students can hear the effects and relate these to the traces on the screen.
6. If your signal generator can provide a low frequency output (less than 1 Hz), show this on the CRO also. Set the time-base to its slowest setting, and watch the spot as it moves slowly up and down.

Teaching Notes

• It is useful if students can appreciate that sinusoidal waveforms are not the only shape which an alternating voltage can take. Square waveforms are at the heart of modern digital electronics.
• You could also show the Sweep output of one CRO on the screen of another. This is a saw-tooth waveform, used to move the spot at a steady rate left to right across the screen, returning very rapidly to the left at the end of each trace.

This experiment was safety-tested in January 2007

• A video showing how to use an oscilloscope:

• A video showing how to use a signal generator:

Class practical

The separation along the x-axis between two pulses on a CRO (cathode ray oscilloscope) screen is a measure of the time interval between the pulses.

Apparatus and Materials

For each student group

• Microphone
• Leads, 4 mm, 2

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

Procedure

1. Set the time base on the CRO to 1 ms/cm (1 ms/div) and the fine control at the slowest sweep speed. The gain (sensitivity) should be turned up to maximum.
2. Attach the microphone to the input terminals of the oscilloscope. Give two short whistles (or claps) close to the microphone to see successive pulses a short interval apart. Vary the time interval between your two whistles and note the effect.

3. Set the time base on the CRO to 10 ms/cm. Direct the microphone towards a wall, a few metres distant. Clap next to the microphone. The initial pulse and the reflected pulse will show up on the trace. Estimate the time between the pulses (you will need to know the setting of the time-base}, and measure the distance the signal travelled to the wall and back. You can now calculate the speed of sound.

Teaching Notes

• Note that the CRO time-base will need to be set to its calibrated position if measurements of time intervals are to be made.

This experiment was safety-tested in October 2006

• A video showing how to use an oscilloscope:

Class practical

A CRO can show waveforms from tuning forks, musical instruments and the human voice.

Apparatus and Materials

For each student group

• Microphone
• Leads, 4 mm, 2
• Tuning fork

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

Procedure

1. Connect the microphone to the input terminals of the oscilloscope.
2. Set the time-base to 10 ms/cm (10 ms/div).
3. Set the tuning fork vibrating by banging it gently on a knee or resilient pad. Touch its base onto the microphone.
4. Adjust the fine time-base control and the sensitivity control to give a good display.

Teaching Notes

• Using the base of the tuning fork like this often gives extra frequencies. The ideal arrangement is to hold the microphone near a vibrating tuning fork (preferably on a sounding board or box), but there may not be sufficient amplification.
• Once the microphone is introduced, students will want to make their own sounds either by singing, shouting or playing a musical instrument.
• The pulse shapes of pure notes from a tuning fork, sine waves, can be compared with the wave shapes for musical instruments.
• The frequency of the note can be measured from the screen by using the time-base setting.
• You can also show waveforms using a loudspeaker connected to an appropriately-set datalogger.

This experiment was safety-tested in October 2006

• A video showing how to use an oscilloscope:

Demonstration

The CRO (cathode ray oscilloscope) gives a visual display of the rectifying action of a diode.

Apparatus and Materials

• Diode (see technical note)
• Power supply, low-voltage, AC
• Crocodile clips, 2
• Leads, 4 mm, 3

• Oscilloscope

Health & Safety and Technical Notes

A cathode-ray tube requires voltages classified as hazardous live. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

Read our standard health & safety guidance

1N4001 diodes are very cheap and suitable.

Procedure

1. Set the power supply to 12 V AC.

2. Connect the power supply, as shown, to the oscilloscope to display the waveform. The gain control should be set to 2 V/cm and the time-base to 10 ms/cm (10 ms/div).

3. Switch off the power supply. Connect the diode into the circuit as shown above. Switch on and observe the waveform.
4. If you have a double beam CRO, then you can show the original signal as well as the rectified one.
5. Repeat with the diode reversed.

Teaching Notes

• One way round, the diode will remove the positive peaks of the trace. The other way round, the negative troughs will be removed.
• Full-wave rectification by a diode bridge, made from 4 diodes as shown below, could also be shown.

• Think about the way to arrange the four diodes so that in the course of a cycle of the AC supply current will go through in each half cycle and make humps in the same direction. The sketch does not show you which way each diode must point. You need to decide that.
• If you are using a real CRO (rather than a datalogger), you could mention that the CRO itself is using an electron beam to display the changing voltage at its input.
• You could discuss the miniaturization that is possible by building integrated circuits onto a wafer of semiconductor. Students may have heard of Moore’s law, in which Intel’s co-founder Gordon Moore proposed the trend that the number of components on an integrated circuit would approximately double every two years. It has held from 1972 to at least 2006.

This experiment was safety-tested in January 2007

• A video showing how to use an oscilloscope: