Circuits

VCOs are the most basic circuits in our experimental music instruments.

An VCO - voltage controlled oscillator - is a circuit where we can change the oscillator frequency modifiying the voltage that we apply in one of the inputs. This is the same that saying that we can control de frequency of the generated sound through a simple device that is able to change this voltage.

There are lots of devices that can modify voltage at a circuit point. We can usepotenciometers (variable resistors) or get this variation from the physical world using sensors. For example, with a LDR - Light Dependent Resistor - you can use a light to make an instrument that change its frequency with the variation of darkness. This will be our interface with the instrument.

In the heart of our VCO there´s a 555 timer in aestable mode. Aestable mode seems that the signal is generated continously since the power supply is plugged. The 555 timer is one of the simplest Integrated circuits and more used in the history of electronics. It has only 8 pins and it´s quite easy to understand. See more info at 555 at wikipedia

The signal that we are going to get is a digital signal (like a rectangle), it only has two values: 0 and 5 volts. Also, this signal is periodic that seems that after a period of time the waveform is the same. The frequency of the sound depends on the time that the signal is on the highest level during a period. A curiosity in this signals is that its spectrum does not contain even harmonics (ideally). It only contains even-numbered harmonics of the main frequency (3rd, 5th, etc). In order to define a specific range of frequencies we have to connect some other resistors and condensers at other points of the circuit.

We can change the frequency of these timer modifiying the voltage at pin 5.

In our case we have included a button at the output of the circuit, until you don´t press it you will not have sound. This is a way to have short time notes and not a continuum.

VCO schematics and board" (cleft):emi)))project

FX Module (Special Effects Module)

In this circuit we use two 555 timers to generate sounds that are a little more complex that in a simple VCO circuit.

The strategy is connect two 555 in cascade. The first one generates a signal with less frequency than the second one. The whole effect is a sound where you can find interesant transients and chirps.

This circuit could seem complex but if you look into the schematic you will see that it is not more than two VCO connected. You can imagine the way they work: the level of the first signal controls the frequency of the second one.

FX Module schematics and board" (cleft):emi)))project

Beat Oscillator

This circuit is basically the same one as the VCO.

When the oscillating frequency is so small, less than 10 beats per second we loose the illusion of continous sound and we perceive a beat.

We can modify the beat rate and we can transform our VCO in a drum machine or in an exact variable metronome. To modify this rate you can different kind of sensors and potenciometers.

Beat Oscillator" (cleft):emi)))project

Open VCO

When you want to use a versatile input VCO this is your circuit.

You can solder lots of sensors to the basic sensors boards at connect them directly to the input of this circuit.

Also you have an open header at pin 5 of the 555 timer to include new variation of the circuit.

Open VCO" (cleft):emi)))project

Mic Preamplifier

A Microphone is a sensor that responds to sound.

For example, when you shout close to a microphone your energy is converted to electricity (i.e current). Usually this current is so small that we have to amplify it.

Designing a sound amplifier is a quite dificult task due to the special properties of the sound. Amplifiers don´t have a linear response in all the sound spectrum and they also add some noise to the signal that we want to amplify. In a small circuit like the one that we propose you are not going to have a very good quality of sound but you can amplify the mic current up to 100 times.

One of the main properties of an amplifier is the gain. Gain (in voltage) is the rate between the amplitude of the signal that we obtain at the output and the amplitude of the signal that we have connected at the input. It is a way to know how powerful is our amplifier.

Mic Preamp" (cleft):emi)))project

Sensor Interface

Sensors are devices able to convert things that happen in the physical world to electricity. Examples: - an NTC variable resistor changes its resistance due to the temperature of the enviroment - a Tilt sensor is able to map inclination - a LDR is a Light Dependent resistor to measure the quantity of light

We can use sensors in our circuits to control some sound properties. Also, we can simulate buttons and switches (for example with a tilt sensor).

We have designed this small circuit for resistive sensors. This kind of sensors need a resistor to be connected to the power supply. Without this resistor we could burn our sensor. It´s usual to use 1K resistors or similar.

Sensor Interface" (cleft):emi)))project

Hall Sensor Interface

Hall sensor is a specific device that can measure magnetic fields in its enviroment.

A Hall Sensor has 3 pins, two for ground and Vcc and a third for the output.

Hall Sensor Interface" (cleft):emi)))project

Speaker Circuit

This is a quite simple circuit to connect your circuits to the output.

We have included a trimmer betweeen the input and the speaker to control the final volume. A trimmer is a small variable resistor but smaller than a potenciometer. We can adjust the value turning a small screw that it´s over the trimmer.

We suggest you to use 8 ohms speakers.

Note: There´s a mistake in input/output labeling at the circuits. The output labeled as "Speaker" is in fact the input. Input labeled as "S" is actually the output where you have to solder your speaker.

Speaker circuit" (cleft):emi)))project

Mixer

With this circuit you can mix two signals and get the output.

Because of we are working under a range of [0,5] volts you have to adjust the gain of each channel to avoid clipping (if you want so). In fact, we are mixing two signals in the same range so the final range could be double [0,10] volts. You can adjust these gains through two potenciometers at the input.

Mixer" (cleft):emi)))project

Toy Organ-IC

This is an experiment about how to build a small organ with less than 1 euro.

We have added a set of capacitors to our VCO. When you push a button you are adding a capacitor to the circuit and also you are modifying the oscillator frequency. You can press two button at the same time and you will adding two capacitors in parallel (so you are adding their values).

Organ-IC" (cleft):emi)))project

Tone generator

We are going to use a 556 timer to build a simple chirps generator.

A 556 it´s only two 555 timers in the same IC.

Tone generator" (cleft):emi)))project

Voice Controlled VCO

This is an experiment about how to control a VCO with your voice.

At the heart of the circuit you can find two parts: - A VCO that to generate sound - A schmitt trigger

The VCO is working as usually. It creates a digital waveform controlled by an external voltage.

The schmitt trigger is a circuit that offers only two possible values for the output. It is a trigger because until the input has a value greater than a specified threshold the output is on low value. You can specify the threshold and also the output values.

When you speak throgh a microphone you add a voltage to the circuit. These voltage is not enough to be greater than the threshold so we need to include a preamplifier (see circuit). We connect the amplifier output to the schmitt trigger. When we shout the voltage will be greater than the threshold and the VCO will begin to work. If you get into sound the voltage will not be greater than te threshold and the VCO will not work.

VCO-IC-e" (cleft):emi)))project