Musical keyboard as a signal generator

  
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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`
Musical keyboard as a signal generator - schematic

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 ©), and is usually center-tapped. The secondary winding is 8 © and not center-tapped. These impedance figures are not the same as DC resistance, so don`t expect to read 1000 © and 8 © with your ohmmeter - however, the 1000 © winding will read more resistance than the 8 © winding, because it has more turns. 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 in to 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...



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