In this experiment, you’ll learn how to use an electronic musical keyboard as a source of variable-frequency AC voltage signals, as illustrated in Figure 1.
You need not purchase an expensive keyboard for this, but one with at least a few dozen voice selections (piano, flute, harp, etc.) is preferred. 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. However, 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.
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 also be scavenged from a small junk radio; it connects between the speaker and the circuit board (amplifier), so it 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 120 V/6 V step-down power transformer works fairly well, too.
Step 1: Connect two wires to the mono headphone plug, as illustrated in Figure 1. If you purchase a mono plug with a cable, you can split out the two wires from the cable.
Step 2: Connect these two wires to the 8 Ω side of an audio output transformer to step up the voltage to a higher level, as illustrated in Figure 2.
If using a power transformer instead of an audio output transformer, connect the mono plug wires to the low-voltage winding so that the transformer operates as a step-up device. Keyboards produce very low voltage signals, so there is no shock hazard in this experiment.
Step 3: To tap into the AC voltage produced by the keyboard, insert the plug into the headphone jack (sometimes just labeled phone on the keyboard).
Step 4: 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 is used to power that speaker will be available at the plug wires.
Step 5: 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).
Step 6: Using an AC voltmeter, read the voltage directly from the headphone plug. Then, read the voltage as stepped up by the transformer, noting the step ratio.
Step 7: 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)? Ideally, there should be no change in signal amplitude (voltage) as different frequencies (notes on the keyboard) are tried.
If you don’t mind making marks on your keyboard, write the frequencies in Hertz in black ink on the white keys near the top, where fingers are less likely to rub the numbers off.
Step 8: Adjust the volume up and down and measure the frequencies. 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.
Step 9: Try connecting the keyboard output to a 10 kΩ load resistance (through the headphone plug), and measure the 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 the voltage for any given note (keyboard key).
Learn more about the fundamentals behind this project in the resources below.
In Partnership with Würth Elektronik
In Partnership with Infineon Technologies
In Partnership with Infineon Technologies
In Partnership with Eaton Electronic Components
In Partnership with Microchip, Menlo Micro & Endries International