Elektor_Mag_-_January-February_2021

54 January & February 2021 http://www.elektormagazine.com

idea: Elektor Labs

Wandering mono audio

Before digital signal processing entered

the scene, musicians had to use ingenious

electromechanical contraptions to create

certain sound effects. Take the Hammond

organ, the electromechanical precursor of the

synthesiser (somewhat succinctly expressed),

an instrument that musicians like Walter

Wanderley used to create delightful sounds.

There was, however, one drawback: such an

organ was mono. This resulted in a rather

feeble sound eminating when on stage in front

of an audience.

Fortunately there was Donald Leslie who

developed a particularly clever solution to

this problem. He came up with the idea that

a rotating loudspeaker could ’swing’ the mono

sound around the room, creating a much

wider sound image. Due to the moving parts,

a rotating loudspeaker is difficult to achieve

electrically. The Leslie loudspeaker solved this

by mounting a rotating cylinder with a sound

hole horizontally above the loudspeaker —

more or less as sketched in Figure 7.

Here we treat you to a (very, very simple)

electronic equivalent that may not match

the quality of an original (and very expen-

sive) Leslie speaker, but it remains interesting

enough for your own experiments. Figure 8

shows the schematic.

discharged. Transistor T1 turns off again due

to the discharge of capacitor C1 and thus

switches on transistor T2. The process is then

repeated endlessly.

The oscillation frequency of the pulse oscil-

lator in Figure 4 can be changed propor-

tionally by adjusting the capacitance of

capacitor C1. Potentiometer R5 permits

a frequency range greater than 1:10 to be

covered. LED1 provides a visual feedback

of the frequency tuning. The LED is at its

brightest at the beginning of the range and

is least bright at the end of the range. The

oscillator operates over a frequency range

of 3 to 30 Hz. Using the values shown in

the circuit, the current through LED1 varies

between 20 and 2 mA at a supply voltage of

9 V. When the supply voltage is varied from

5 to 15 V, the oscillation frequency changes

by no more than 10%.

The pulse oscillators in Figure 5 and

Figure 6 also operate at a nominal supply

voltage of 9 V across the frequency range

0.8 to 10 kHz and 0.35 to 2.8 kHz respectively.

The oscillator in Figure 5 is controlled by

changing the ratio of the resistive voltage

divider arms (resistors R4, R5 and R6, the

right half the bridge circuit). The opera-

tion mode of the oscillator in Figure 6 is

controlled by resistor chain R2, R3 and R4

that regulate the discharge/charge processes

in the left half of the bridge circuit.

idea: Andrey M. Shustov (Russia)

and Michael A. Shustov (Germany)

Three shades of Schmitt trigger
oscillator
Schmitt triggers or asymmetrical emitter-cou-
pled triggers can be used to construct simple
wide-range pulse oscillators. Transforming a
Schmitt trigger device into a oscillator is quite
simple. The output of the trigger is connected
back to the input by a resistor and a capacitor
is connected between the input of the trigger
and the common supply rail.

Figures 4, 5 and 6 show practical circuits
of pulse oscillators, based on the modified
Schmitt trigger. A bridge circuit consisting of a
resistive divider, where the emitter of the input
transistor is connected to the mid-point of a
resistive divider, and a time-setting RC circuit,
formed by the additional hookup elements,
all act to convert a trigger into a repetitive
pulse oscillator.

The p-n transition of the transistor lies
between the diagonally opposite pair of
connections of the bridge. When capac-
itor C1 is discharged, transistor T1 blocks
and transistor T2 conducts. The voltage
across then capacitor rises again. Once it
exceeds the voltage of the mid-point of the
resistor-divider’s mid-point by several volts,
input transistor T1 conducts again, turning
T2 off. This de-energises the resistor-di-
vider and the time-setting capacitor C1 is

Figure 4: Basic circuit of a pulse oscillator based
on a modified Schmitt trigger.

Figure 5: Second variant of a pulse oscillator

based on a modified Schmitt trigger.

Figure 6: And finally a third variant; here the

pulse duration can be controlled.