In this experiment, you’ll learn how to use an electronic musical keyboard as a source of variable-frequency AC voltage signals. You need not purchase an expensive keyboard for this—but one with at least a few dozen “voice” selections (piano, flute, harp, etc.) would be good. The “mono” plug will be plugged into the headphone jack of the musical keyboard, so get a plug that’s the correct size for the keyboard.
The “impedance matching transformer” is a small-size transformer easily obtained from an electronics supply store. One may be scavenged from a small, junk radio: it connects between the speaker and the circuit board (amplifier), so is easily identifiable by location.
The primary winding is rated in ohms of impedance (1000 Ω), and is usually center-tapped. The secondary winding is 8 Ω and not center-tapped. These impedance figures are not the same as DC resistance, so don’t expect to read 1000 Ω and 8 Ω with your ohmmeter—however, the 1000 Ω winding will read more resistance than the 8 Ω winding, because it has more turns.
If such a transformer cannot be obtained for the experiment, a regular 120V/6V step-down power transformer works fairly well, too.
Lessons In Electric Circuits, Volume 2, chapter 1: “Basic AC Theory”
Lessons In Electric Circuits, Volume 2, chapter 7: “Mixed-Frequency AC Signals”
Normally, a student of electronics in a school would have access to a device called a signal generator, or function generator used to make variable-frequency voltage waveforms to power AC circuits. An inexpensive electronic keyboard is a cheaper alternative to a regular signal generator and provides features that most signal generators cannot match, such as producing mixed-frequency waves.
To “tap in” to the AC voltage produced by the keyboard, you’ll need to insert a plug into the headphone jack (sometimes just labeled “phone” on the keyboard) complete with two wires for connection to circuits of your own design. When you insert the plug into the jack, the normal speaker built into the keyboard will be disconnected (assuming the keyboard is equipped with one), and the signal that used to power that speaker will be available at the plug wires. In this particular experiment, I recommend using the keyboard to power the 8 Ω side of an audio “output” transformer to step up voltage to a higher level. If using a power transformer instead of an audio output transformer, connect the keyboard to the low-voltage winding so that it operates as a step-up device. Keyboards produce very low voltage signals, so there is no shock hazard in this experiment.
Using an inexpensive Yamaha keyboard, I have found that the “panflute” voice setting produces the truest sine-wave waveform. This waveform, or something close to it (flute, for example), is recommended to start experimenting with since it is relatively free of harmonics (many waveforms mixed together, of integer-multiple frequency). Being composed of just one frequency, it is a less complex waveform for your multimeter to measure. Make sure the keyboard is set to a mode where the note will be sustained as any key is held down—otherwise, the amplitude (voltage) of the waveform will be constantly changing (high when the key is first pressed, then decaying rapidly to zero).
Using an AC voltmeter, read the voltage direct from the headphone plug. Then, read the voltage as stepped up by the transformer, noting the step ratio. If your multimeter has a “frequency” function, use it to measure the frequency of the waveform produced by the keyboard. Try different notes on the keyboard and record their frequencies. Do you notice a pattern in frequency as you activate different notes, especially keys that are similar to each other (notice the 12-key black-and-white pattern repeated on the keyboard from left to right)? If you don’t mind making marks on your keyboard, write the frequencies in Hertz in black ink on the white keys, near the tops where fingers are less likely to rub the numbers off.
Ideally, there should be no change in signal amplitude (voltage) as different frequencies (notes on the keyboard) are tried. If you adjust the volume up and down, you should discover that changes in amplitude should have little or no impact on frequency measurement. Amplitude and frequency are two completely independent aspects of an AC signal.
Try connecting the keyboard output to a 10 kΩ load resistance (through the headphone plug), and measure AC current with your multimeter. If your multimeter has a frequency function, you can measure the frequency of this current as well. It should be the same as for the voltage for any given note (keyboard key).
by Robert Keim
In Partnership with Rohde & Schwarz
by Gary Elinoff