This project was spawned from reading Nic Collins excellent "Handmade Electronic Music". I've been an avid circuit bender and electronics enthusiast for years, and had these parts laying around for nearly a decade. Finally, last Christmas eve, I got to work, and after a few calculations and some tinkering, built this thing in a dorm room. Literally, with a soldering iron and finger nail clipper to cut wires. The idea came from a friend who writes code for video games, and wanted something that sounded like "weird, electronic UFO noises". This circuit sounds reminiscent of a theremin.

I built mine with 2 sets of gated oscillators. The chips can be configured in many ways--4 oscillators in a row, 4 single oscillators.... The trick is that the final outputs cannot just be tied together or the device will not function. The outputs need to be decoupled either with resistors or diodes (which have pronounced differences in sound).

It would be a good idea to breadboard and tweak the circuit before soldering it up.

I selected the values so that the the two halves oscillated with overlapping frequencies. With careful tuning, the gating can create interesting rhythms with the tone being modified by shading the photoresistors or shining a light on them. It's great fun (and super annoying!) fun!

Step 1: An Oscillator Circuit Based on the NAND Logic Function

One of the original home-built
synthesizer projects, and perhaps still the best known, is the so called “Atari Punk Console”. This device was the brain child of Forest Mims III, and got its name from the lo-fidelity, cheesy video game sound that it produced. (see this excellent Instructable https://www.instructables.com/id/How-to-make-the-A... ). The device is based on a pair of 555 timer IC chips, or a 556 dual timer chip. It's a so called “gated oscillator” where the output of timer circuit turns on and off the output of the other. It gives rise to a strange “stepped tone” that is a great deal of fun to play with.

Presented here is a weird electronic noise generating device that is actually somewhat musical. It is based on the 4093 Quad NAND gate devices presented in Nic Collins' excellent “Homemade Electronic Music”. Unlike a basic “oscillator” circuit, where the frequency is controlled by the time constant of a RC circuit (a resistor/capacitor circuit), a NAND gate actually has 2 inputs. The oscillators can be wired to run independently, or more interestingly, they can be wired such that the output of one gates the output of the next.

As stated, the frequency (ie, the tone) produced is a function of a resistance and a capacitance. An RC- circuit charges and discharges based on an exponential function. Small resistors and capacitors can charge and discharge quickly (ie, they allow the chip to change state rapidly and produce high pitches), while large capacitors and resistors produce low frequencies. Large enough values actually produce a kind of metronome “tock tock tock” sound.

The easiest and cheapest way to control the tone of such a device is by picking a capacitor value, and then installing a variable resistance of some kind to adjust the tone. The cheapest, although musically most difficult to play, is a cheap photo resistor. When light is shining on them, the resistance goes down. In the shade, the resistance goes up. Another easy to implement strategy is a potentiometer.

It would be a good idea to breadboard the device—the sonic response can be varied by easily plugging in different capacitors and resistors, and then transferred to perf-board.

The device presented here is actually a pair of gated oscillators, with the outputs tied together through resistors of diodes (switchable). The diode coupling produces an interesting “ring modulator” kind of sound. I designed this one so that the first stage would act to set up a kind of stable “beat”. By using a 10uF capacitor, and a 1Meg potentiometer, frequencies are quite low. This turns on and off the second stage which uses a 0.1uF capacitor, and a photoresistor. There is no reason that all 4 cannot be wired together, or that pots or photo resistors cannot be used for each stage. (this is why bread boarding is a good idea—TONS of design freedom with easily switchable parts).

The device outputs about ½ Watt. It can power a small speaker (an old radio speaker should work). It can also power an amplifier, mixer, or other audio inputs. It would be a good idea to install a volume pot after the coupling diodes or resistors.

Construction should follow good practices. Keep leads short, make nice clean solder joints.... (there are great Instructables regarding perf-board construction)

Step 2: Parts

It is doubtful that the 4093 IC chip will be available at a local electronics store. At this writing, Jameco sells them for ~$0.45 a piece. Get a tube of 10 while you're at it. The other parts should be commonly available. Photoresistors are normally available at Radio Shack, or cheap from Jameco or other suppliers. Assorted parts such as 9V battery connectors, switches, etc, are easily sourced locally, or purchased online (which, in my experience, is vastly cheaper). Locally they are typically sold as a "grab bag", but when purchased through a supplier, they come in several values. I prefer the 16k variety. Something close to that will work.

I built this one on perf-board, using some wire-wrap and jumping other connectors with bits of insulated wire. It's a small circuit, so feel free to lay it out with room to work and solder. This attempt (the prototype) is quite sloppy. It was fabricated from scraps and recycled parts, and built in about 3 hours in a dorm room! I did not follow my own advice, and neglected to bread board it. This meant soldering and de-soldering parts in. Do yourself a favor, and test the stages on a bread board.

This version does not have a volume control, on/off switch, volume control, etc. For an actual "instrument", it would be advisable to install these things, as well as an output jack. Start at maybe 50k for the volume pot. This will compensate for the diode coupled circuit being significantly louder than the resistor coupled circuit.

Step 3: Construction Tips

Solder the 14 pin IC socket to the perf board. Note that the socket has a notch in it, which denotes where the #1 pin starts. IC chips are numbered from upper left, counter clockwise. I like to start with the power to the chip. Note here that the entire IC chip requires 9V (to pin 14) and ground (pin 7).

Also, as in the drawing, the first oscillator is set at 9V. Then the feedback resistors and capacitors can be soldered into place. For this, it is a good idea to start with the final package, to make sure the leads to the pots and photoresistors are long enough to allow mounting.

It is a good idea to test each oscillator as it is completed. If you power up a full circuit and it "doesn't work", that is a lot of wire to chase down a problem in. Testing after each step will allow tweaking and repairs as needed. Do a good job on the solder. Nice clean, hot joints. Clip off excess wire and keep things neat and direct.

All 4 of the NAND gates are equivalent. I laid mine out with the "right 2" and "left 2", and the mounted the coupling diodes and resistors underneath

Addendum: It dawned on me this morning, a word should be said about trouble shooting. If the device "doesn't work", check the obvious first. Is +9V supplied to pin 14? Is pin 7 grounded? Is the chip installed right side up? If this checks out, check the capacitors. Electrolytics are polarized. Make sure the (-) side is towards ground. Another perf-board problem is solder-bridges. If too much solder, or tiny strand of wire bridges 2 pads, the circuit can be shorted. Check the gaps between all the pads. An X-acto knife is good for cleaning up small bridges, or they can be de-soldered with a braid. As far as "tweaking" the components, adjust the values as follows. If you turn on the device, but notice that the frequency is so high that only bats can hear it (you might hear a very high pitch whining), increase the capacitor value or the resistor (this was why a series resistor was placed with the photoresistor. Their value was too low to work properly, and would go into ultra-sonic mode in bright light. The series resistor limits the high end). The capacitor can also be increased in value. For ceramic disks, remember that capacitance ADDS in PARALLEL. If a 0.1uF is producing too high of a frequency, try adding an additional 0.1uF in parallel with it.

Again--do yourself a favor and bread board the circuit. This allows for changing the response with easily plugging in new or additional parts. Take careful notes, draw up the diagram, and if possible, keep the bread board version intact. Then build an exact copy on the perf board. Having a working device to examine makes building the hard-wired version much easier.

Finally--to mount the photo resistors, I have had luck with drilling a hole in a plastic project box just a hair bigger. I install the photoresistor from the inside of the box, and then apply a tiny drop of super glue which locks it in place. You will have to remove it with a drill should it require repairs, but it works great.

Step 4: Finished Circuit

While not actually "finished" (the circuit is a mess of wires, alligator clips....), the circuit is up and running. Here are 2 sound files with the outputs set to diode and resistor coupled. They were recorded to laptop, using a pillow over the output speaker to reduce the volume.

It really should be mounted in a cool project box, or something more secure. This one is asking to get broken!

Enjoy--it's a fun and weird sounding circuit. Using a free program such as Audacity allows for modifying the sound with echo, delay, flanging, stereo....