The classical mechanical metronome is a spring powered
clockwork mechanism mounted in a pyramidal box and controlled by an adjustable
inverted pendulum. Its disadvantage is that is must be rewound the clectronic
version bescribed here has none of these drawbacks. If its rate control is
adjusted to give one sound click every second then by counting the number of
seconds a very simple timer is at your disposal. The circuit diagram and parts
list are given in figure i. Transistors Q2 and Q1 are wired to simulate the
function of a unijunction transistor.
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| circuit diagram |
This pair has high impedance unless it is
riggered when its impedance becomes low. On initial switch-on capacitor c1
charges through the emitter of q2 and Q1 in the regenerative switch turns on as
Q2 goes into conduction crrrent is then conducted through C2 to the base of Q3
which also turns on, switching Q1 on; C1 increases , which is in series with
the speaker producing a loud click. When the charge on C1 increases
the emitter of Q2 becomes more positive,
turns off the regenerative switch comprising of Q1 and Q2 and stops the flow of
current to Q3. When this occurs C2 must discharge through R1 and the combination
of R9 with Q3 . As the charge on C1 lessens, the cmitter of Q2 becomes less
positive, the regenerative switch is triggered into conduction and Q3 receives
another pulse which is heard in the loudspeaker through the switching action of
Q4. Potentiometer R3 controls the discharging rate of capacitor C1 and hence
the number of beats per minute. It should be calibrated using a stop watch, if
desired . Since the circuit draws 25 mA
current typically. No special power supply is
required: any 9 volts source would do.
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