Presented here is a circuit that alters a tune/melody to a different pitch, level without making the tune lose its basic features. The signal characteristics can be varied at different instants of time. The melody generated by melody generator IC can be delayed and its pitch can be varied at different time instants. This can produce different musical effects from the same melody.
The word ‘modulation’ is used to mean alteration of the melody by creating delay and other effects.
Description (IC name)
Phase Locked Loop (for VCO)
UM 66(or any melody IC)
10K ohm(or any)
The circuit modifies the melody generated by the melody generator IC UM66. Voltage Controlled oscillators are used to generate oscillations when a constant voltage is applied. Phase locked loop ICs (CD 4046) (IC1 and IC2 in the circuit) are used for VCOs. PLL chips have VCO and Phase comparator. The voltage that is applied to VCOs can be varied by potentiometers. In the circuit two potentiometers 5K ohm and 10K ohm were used to vary the input voltage to VCO. These voltages are given to pins 9 of both IC1 and IC2. The VCOs outputs at pins 4 of CD4046 are time-multiplexed by a multiplexer IC CD 4051 (IC4 in circuit diagram). The selection signal for multiplexer is generated by 555 timer (IC 3) that is configured as an astable multivibrator. The duration of each high and low pulse from 555 timer is for 4.7msec. The duty cycle is made sure to be 50% by introducing a small-signal diode IN4148 between pins 7 and 6 of IC3. The multiplexer (IC4) gives the multiplexed oscillatory output at pin 3. This output is given to base terminal of the npn transistor through a 40K ohm resistor. Different values of resistors were used and tested, but a proper tone was obtained for 40K ohm. The collector terminal is given as input to melody generator UM66 (other melody generator ICs can be used). The output of melody generator is given to a 4 ohm speaker (SP) with the other terminal of speaker connected to 5 volts supply.
As the positions of both the potentiometers are varied, the same melody with many different variations in its tone, level, pitch, delay effects can be heard. Similarly other melodies can be varied by just placing that particular melody IC at the position of UM66 in the above circuit.
In case the melody generator IC gets over heated then use an npn transistor (BC547) to drive the speaker. Connect the output of melody IC to base of transistor through a resistor, ground the emitter, and connect speaker across collector and +5 Volts.