For my final project, I made a drone synthesizer. It produces 6 separate continuous, droning frequencies that can be controlled using potentiometers to change each frequency. Along with being able to change the frequency of each drone sound, a mute switch was added to duck the volume of all drones for added performance applications. A master volume knob was also added to the output if it gets too loud.
The bulk of this synthesizer is a simple oscillator circuit that only uses 2 resistors, 1 capacitor, 1 potentiometer, and 1 transistor. I replicated this same circuit 6 times to create the different droning oscillators.
Typically, an oscillator is built using an op-amp circuit that produces a square or triangle wave. So how can a transistor be used to create an oscillator? First, it is important to understand how a transistor works. A standard BJT (bipolar junction transistor) is composed of 3 semiconductive materials stacked on top of each other, corresponding to the 3 parts of a transistor: the collector, base, and emitter. Transistors can either be n-type meaning the semiconductor contains extra electrons, or p-type which has electrons removed. In this synth I made, I used an NPN trasistor, which means that the collector is n-type, base is p-type, and emitter is n-type. This type of transistor is designed to pass electrons from from the emitter to the collector, with the collector holding all the electrons that are going to be passed on to the next part of the circiut.
The way that these transistors are able to create oscillators is by a mode the trasistor enters called reverse avalanche. The oscillator circuit works by taking 18V of DC power (from two 9V batteries in series), passes it through a current limiting resistor, then into the capacitor and transistor in parallel. The capacitor charges up with electrons, and once it reaches a certain point the transistor enters avalanche breakdown mode. The transistor, in this breakdown mode, experiences negative resistance, which means the higher the current, the lower the resistance. The capacitor then discharges the electrons to ground, and the voltage decreases, bringing the transistor out of reverse avalanche mode. This is rapidly repeated, causing a frequency to be output from this rapid charging/discharging of electrons to ground. Transistors are commonly used as an amplifying component, similar to how op-amps are used in typical oscillators, so they amplify the frequency of the charging and discharging of electrons to create an amplified audio signal.
For the components I used for each oscillator, there is a 1k resistor connected from power to a 10k potentiometer that will control the frequency by increasing or decreasing resistance. This then moves to the capacitor and transistor in parallel. I varied my capacitor values between 1uF and 10uF because they each produce either higher or lower frequencies, so this added some variety to the output. The transistor used was a 2N3904 NPN transistor, which requires 18V of power to oscillate (different transistors will require different voltages to work this way). With the oscillators, I have a 100k resistor on the output of each one to limit how loud the oscillators are; this is simply there to allow all of the oscillators to work nicely together. I sent all 6 outputs to a single wire, then that was connected to the auido out. For this, I considered using a summing amplifier to sum the output signals of all 6 oscillators, but when I simply connected all of them directly to the audio output, the total volume wasn’t too out of hand and the oscillators didn’t interefere with each other in a way that affected how they sounded.
For the implemented mute switch, 9V of power is sent to the circuit. A button is wired to 2 large 100kΩ resistors, then to a large 100uF capacitor. These values were selected randomly, but I knew that I needed large resistor and capacitor values to make the muting work. This first part of the circuit is then sent to a vactrol (LED pointed at a photocell), which will duck the volume once the LED is lit (when the button is pushed). This will increase the resistance of the current flow, meaning the signal experiences more resistance, which will lower the volume.
Along with the mute switch, I implemented a master volume knob by putting a 10k potentiometer on the output of the oscillators.