This is about CoyuyoC, a drone synthesizer based on the versatile, long-lived, and often-cited-in-my-blog (losaparatos.blogspot.com) NE555. But this time, we won't use its traditional square wave output but an alternative output where the waveform varies between a triangular wave, a ramp wave, and what we can call a "shark fin" wave. Let's see how this works:

The magic of the 555 lies in controlling the charge and discharge of a capacitor. In this configuration (astable), the capacitor will constantly charge and discharge, generating a wave, an oscillation, a voltage that will continuously vary between a minimum and a maximum. The 555 was designed to generate a square wave, as square as possible, to serve as the heart and metronome of early digital circuits, where a 1 (one) represents a maximum voltage value and a 0 (zero) represents the minimum. That square wave output, which generated zeros and ones at a constant rhythm, is the output from pin 3, but that's not the output we'll use today. If you look closely, connected to pin 2 is the famous capacitor. The integrated circuit manages its charge, detecting when it "fills up" (when it reaches the threshold) and immediately enabling a discharge path. The signal we have there on pin 2 (which in this configuration is connected to pin 6) takes the form of an ascending curve and a descending curve, resembling a fang or a shark fin. That will be the signal we'll use as the output, but not before "buffering" it, meaning passing it through a non-amplifying transistor (the input/output ratio of this stage is 1:1) to isolate the 555 from whatever we connect to the output.

The word "CoyuyoC" is simply an attempt at a palindrome based on coyuyo, the name given here in northern Argentina to the Quesada Gigas, a species of giant cicada that produces a very distinctive and omnipresent sound during warm summer afternoons. They generate this sound through two resonant membranes used by males to attract females. In this competition, hundreds can sound at once, creating a ubiquitous drone sound. The sound starts as isolated pulses and then transitions into a constant tone once the two resonant membranes "synchronize."

Mimicking this, CoyuyoC features two oscillators: one low-frequency oscillator (LFO) and a higher-frequency voltage-controlled oscillator (VCO). These oscillators can be controlled independently via two potentiometers or connected so that the LFO controls the VCO. The LFO and VCO can be coupled through a resistor or a photoresistor (5 mm in this case). But let's take a closer look at the circuit:

The input voltage can range from 9 to 15 volts. Oscillator 1, the LFO, generates low, pulsating frequencies, from 0.2 Hz up to 600 Hz, while Oscillator 2 (VCO) comfortably reaches up to 5 kHz. Through the Modulation On/Off switch, we can decide whether the oscillators will operate independently or if the LFO will modulate the VCO with gentle oscillations. Remember that the audio output (AUDIO_OUT) is two-channel (note that I avoid calling it stereo, as that's a different thing), and the oscillators can each output through their own channel, creating beautiful binaural or pseudo-binaural emissions. With the Direct/Optical Modulation switch, we can choose whether the modulation will be direct, through a fixed 1 kOhm resistor, or through a photoresistor (LDR, Light-Dependent Resistor), allowing us to regulate the modulating signal level from the LFO to the VCO using light. Additionally, by using intermittent lights, we can re-modulate the signal coming from the LFO, generating wonderful effects, especially when the LFO frequency is low.

One of the most unique aspects of CoyuyoC is that the potentiometers control the ascending and descending ramps of the oscillator, and depending on their position, the signal will change in both shape and frequency. Thus, each potentiometer controls:

• R9 > LFO ascending ramp
• R1 > LFO descending ramp
• R10 > VCO ascending ramp
• R5 > VCO descending ramp...