Actual electronic organs are pretty sophisticated devices they comprise tone generators for the basic notes with dividers to provide a range of octaves.
In addition 'voicing‘ is provided so that the organ can instantly be converted to give out sounds for a wide range of instruments.
Such a circuit is of course very complex and costly to build and although kits are available even these are out- side the range of most constructors.
Great fun, however, can be had using very simple electronic circuits
The electronic organ shown here covers over 12 octaves including sharps and flats and although only one note can be played at a time, nearly all tunes can be played and this is largely up to skill of the user.
Advantage of an Electronic Organ
The advantage of this circuit over so many others of basically similar design that have appeared elsewhere is that there is no complex setting up procedure as far as getting the notes right is concerned.
Fixed resistors are used and it is the value of these that determines the actual notes although there is preset to get those notes to the right part of the scale.
Tone Generator
The circuit comprises two distinct sections, the tone generator and the vibrato unit. The tone generator comprises TrZ, a unijunction transistor type 2N2646 together with C5, R5, VR1 and the tone resistors, R6 plus those resistors labelled Ra ~ Rn.
The emitter of the unijunction goes to test probe which can select the correct notes and the frequency of the note depends on the total resistance in that chain.
VR1 is a skeleton preset which alters the whole range. The output is taken from this oscillator via C6 to the volume control which should be connected to an external amplifier.
This circuit by itself will produce the correct notes and is a true electronic organ but the notes lack character and are not particularly pleasant. Character can he added by mixing with this note a low frequency a.f. signal known as vibrato.
This works at a frequency which will lie in the range of 10Hz to 30Hz and it raises and lowers the level of the output very slightly at this frequency.
This stage which comprises the components around Tr1 can be built separately at a later stage but it is strongly recommended that it is fitted.
The Oscillator
The oscillator is a phase shift type and the output is taken tram the collector via C4 to feed directly to the emitter of the uni-Junction. In this way the low frequency note is not heard when the probe is not actually selecting a note.
Phase shift oscillators can sometimes be troublesome to get working but if trouble is experienced a small change in the value of R3, here shown as 1.8 ohms, will usually cure the problem. It is important that the resistors specified in the components list for the tone notes are used.
They may at first appear as slightly unusual values; they lie in what is known as the E24 series and such values can usually be bought at the larger component stockists.
The range of the organ can be extended by adding further resistors but these will have to be found by experiment.
C5 in the circuit has a direct effect on the note and a good quality component should be used here. it is possible to alter the vibrato frequency to suit personal tastes by altering the values of R1 and in fact a variable resistor could be used in its place.
It is also possible to alter the level of vibrato applied to the unijunction by experimenting with C4 but if too high a value component is used this will affect the operation of the tone oscillator. SW2 is included so that the vibrato can be removed if required.
Battery Consumption
The current consumption of the circuit is not high and a PP3 battery will be sufficient. The simple amplifier described elsewhere in this book can be used with this circuit. This uses a PP9 battery and if built as a single unit, only this battery will be necessary.
Construction can take any form but note that the probe can not be just any piece of metal for if a poor contact is made even {or a traction of a second, the organ sounds horrible. Various probes can be tried until one that makes a nice clean note instantly is found.
Parts List
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