The simple metronomes described so far only provide a
variable rate beat and as such are useful for only simple musical lessons. Most
of the music is composed on so many beats per bar scale. Audibly one out of a
number of beats, selected by the operator sounds louder than the others to put
emphasis on that particular beat. The complete diagram that uses five
transistors and two diodes is given in Figure 4. Unijunction transistor UJT Q1 is wired as a conventional
RC Relaxation Oscillator whose basic repetition rate is determined by the
components R1,R2 and C1. every time C1 discharges through the base emitter
junction of Q1, a positive going pulse is produced by R4 in the base B1 of
Q1.these basic beats are coupled through C4 to the direct coupled amplifier
consisting of Q3,Q4 and Q5. In addition to providing basic pulses the
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Accented Beat
Metronome
circuit diagram
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positive
going section of the pulse available at the base B1 of Q1, also charges
capacitor C3 through diode D1. diode D2 shorts any negative going portion of
the pulse. Capacitor C3 therefore accumulates a charge over a number of cycles
and eventually reaches the firing voltage of Q2, whereupon a reinforcing pulse
is produced, the charge required to fire Q2 depends on the voltage at which the
bias across the junction goes forward which is controlled by reducing the
potential on B2, so that the junvtion potential must also drop. R5 was chosen
so that in the prototype at maximum resistance it lowered the junction
potential so far that the unit triggered on every input pulse while at minimum.
The preset R6 limited accentuation to no higher then one in six. This range
would encompass any musical scale in use today. These accentuation pulses are
fed to the base of first audio amplifier transistor Q3 where it is mixed with
the main pulses coming through C4.
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