Many of the discarded electronic devices we find in the trash contain circuit boards with components that can be salvaged and repurposed into a simple synthesizer.

Extracting components from obsolete equipment is straightforward. Heat the solder joints on the circuit board using a soldering iron or a hot-air station. Then remove the molten solder with a desoldering pump or extract the components directly. Useful parts include resistors, diodes, capacitors, transistors, potentiometers, and audio jacks.

There is one component that must be purchased, but it is very inexpensive, typically costing around one dollar: a Hex Schmitt Trigger IC. It can be found at electronics suppliers under part numbers such as 40106, 4584, or 74C14. Prefixes and suffixes generally do not matter—for example, an HF40106BP will work perfectly. However, if the middle part of the designation changes, the device may no longer operate at the required voltage. For instance, a 74HC14 is not suitable. I also recommend using an IC socket.

The circuit we will build is a polyphonic oscillator originally described in Nicolas Collins' book Handmade Electronic Music.

To simplify assembly, I designed a PCB that only requires soldering the components into place, although the circuit can also be built on a breadboard.

The IC is the component connected to the footprint with seven holes on each side. Capacitors are installed in positions C1, C2, and so on. Diodes are placed in D1 and D2. Potentiometers or resistors go in positions R1 through R4. Output Jack 1 is connected to -R and -L, while the second output jack is connected to O and -. The 9V battery connects to V+ and GND.

Some component values that work particularly well are:

• 1N914 or 1N4148 diodes
• Linear potentiometers from 10kΩ to 100kΩ, photoresistors, or even fixed resistors
• 0.1 µF ceramic capacitors or 8.2 µF electrolytic capacitors. For electrolytic capacitors, orient the positive lead toward the IC in C2, or toward the outer edge of the PCB in C4.

Larger capacitors produce lower-frequency sounds reminiscent of metronome clicks, while smaller capacitors generate higher-pitched tones that can extend into ultrasonic frequencies.

The circuit is divided into two sections. The section without diodes sends signals to the left and right channels of a stereo jack. The two channels may have different output levels, which can be corrected using the amplifier's balance control or by adding 10kΩ resistors to each output. The diode section combines the audio signals using ring modulation and does not require volume balancing.

A bit more about sound generation with the Hex Schmitt Trigger. The IC contains six inverters. Each inverter turns a logic 1 into a logic 0, or a logic 0 into a logic 1. When 9V is applied to the input, the inverter outputs 0V. That output is fed back to the input through a resistor, causing the process to repeat continuously and generate a square wave. The oscillation frequency depends on the resistor and capacitor values that determine the charging and discharging rate of the circuit.

There is still plenty of room for improvement in the PCB design—which, incidentally, was my first PCB project. In a future revision I plan to add 3 mm mounting holes in the corners for enclosure installation, replace the square solder pads with headers or jumper connections, and improve the overall component layout.

Demo https://www.youtube.com/shorts/wHtayY4GwAU

For 3d printed parts go to https://www.pcbway.com/project/shareproject/Simple_Synth_5a25d138.html

For a related project, see the Atari Punk Console, which uses two 555 timer ICs:

https://en.wikipedia.org/wiki/Atari_Punk_Console