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What’s Under The Hood

Going Deep Inside the Slugophone: And How to Build Your Own

Analog tone generator…or inter-species communication device?

Jules demonstrates the Slugophone.     Photo: Bonnie Hulkower

Recently featured at the World Maker Faire in New York, the Slugophone is a last-century-style electronic organ with a twist: a fun interactive music maker, it can be used to “play” almost anything — an unwitting friend, a bouquet of flowers… even an insect!

The Slugophone is an easy project to construct, and great for learning (or demonstrating) basic circuit principles. The total cost of the project is probably $20-50, depending on what parts you already have and how fancy you want to get with the case, etc.

Education or entertainment?

With different settings of the selector switch, the Slugophone functions as:

1) A square wave audio oscillator, with continuous or keyed output;

2) A light-controlled Theremin, which produces eerie tones when you wave your  hand over a photocell, or;

3) A versatile device with external leads (probes, grid, etc) able to convert any changing resistance into a series of audio tones.

And here’s where it gets interesting. At the maker faire, we had lots of fun letting our “test subjects” play each other: each one holds a probe, and depending on how and where they touch, a different sound is generated. We also put an insect into a chamber with a circuit board on the floor, acting as a conductive grid. As it moved around, its body’s contact with the grid created a variable tone — a whole different type of insect song.

At the heart of the Slugophone is the venerable 555 IC timer chip: easily obtainable anywhere, it’s one of the most popular chips ever made. Add a few resistors, capacitors, switches and assorted doodads, and the circuit’s ready to go. You can get as fancy as you want to with the case (like our steampunk-inspired console)…but first, the basics.

slug organ-1The Square-Wave Generator

Engineers call this circuit an astable multivibrator, but you can just think of it as the tone generator. (If you google “555 tone generator,” you will find multiple examples similar to this one.) Here’s how it works: The output of the chip switches rapidly from high to low, emitting a series of pulses (a square wave) that the speaker changes into audible sound. The resistors and capacitor control how frequently the chip fires its pulses, and thus the tone it makes. Changing the resistor or capacitor values will change the tone, which can range from a slow metronome to an ultrasonic squeal.

Mixing metaphors

Think of the resistor as a beer tap, and the capacitor as a mug. As soon as the mug is completely full, the charge (I mean beer) it holds gets poured into your circuit, making it happy (I mean functional.) If you have an oversize mug (a high-value capacitor), and your tap is open only a trickle (a high-value resistor), you will be waiting a long time for the mug to fill; conversely, with a small glass and a wide-open keg, the glass fills and dumps constantly. Since the tone produced by the Slugophone is ultimately controlled by the how often the capacitor dumps its charge, choosing the right component values is critical.

Fine-tuning the circuit

In the example circuit above, a potentiometer (variable resistor) controls the charge rate of the capacitor, producing different tones.  If you just want a basic synthesizer and you have your choice of components, any number of combinations will work: I found a 500K pot, paired with a .0047uF capacitor that’s bypassed with a 3.3M resistor, gave a wide range of tones.

slug photocell circuit-1But sometimes you don’t have so many choices. For example, in the Theremin circuit, the variable resistor is a CdS photocell, which has a resistance of a few K when it’s dark, and next to nothing in bright light. Since those values can’t be changed, the capacitor must be selected to produce the widest tonal range from those resistance values. With my photocell, a relatively large capacitor of .66uF gave the best results.

slug probes circuit-1

The probe circuit may also pose a problem: depending what you’re measuring, the resistance could be large or small. I chose a compromise value for this capacitor of .01uF, since most of the things I planned to probe (like skin, plants, pets, etc) would have high resistance. Here’s what the circuits would look like in each of those cases. But wait – don’t build it yet!


cap sel sw There are two more modifications that make the circuit more versatile, and they both use switches. The first is a quick way of changing the capacitor from the one built into the circuit to another one you want to try – without having to unsolder it! This is simply a double-throw switch that bypasses the built-in component for the one you want to substitute. Both capacitors are connected to ground, so only one wire needs to be switched. The test capacitor is held in place by a spring-clip arrangement you can see on the top right of the Slugophone’s case.

slug main selector swThe second switch is a bit more complicated: It makes it possible for the Slugophone to change easily from one function to another. The one I used is a four-pole, three-position rotary switch that I found in my junk box. The nice thing about it is, it accomplishes everything with just a turn of the knob: besides sending both leads of the input resistor to the chip, it also pairs the chosen resistor with the appropriate capacitor. And to top it off, it sends current to a LED indicator light telling you which mode (organ, Theremin, or probe) the Slugphone is in. Here’s a schematic, which incorporates all three resistor-capacitor combinations, plus the test-capacitor switch and the LED indicator circuit. (If you can’t get a hold of this kind of switch, don’t worry: you can simply use a jack to hook up to your inputs, one at a time — but you may have to change the capacitor manually.)

Speak Up

slug pushpullYou probably noticed a couple of transistors hooked together by their bases at the output of pin 3, just after a resistor and before the speaker: That’s a class B push-pull amplifier. It’s needed because the output of the chip by itself isn’t strong enough to drive the speakers. Each transistor in this circuit amplifies half of the waveform, either positive or negative. After you put them together, an electrolytic capacitor (100 uF) removes stray DC voltage to leave a clearer signal. Now you’ll get plenty of sound power from the 8-ohm speaker — so much that, for the sake of decorum, a volume control (200-ohm potentiometer) has been added.

Bells and Whistles

Of course, every electronic circuit needs power, and most need to be turned off once in a while. For that, we need a 9-volt battery, a battery clip and an on-off switch. You can use a fancy lighted one if you want, but be sure it’s designed for the right voltage.

slug keysw-1By the way, did you notice that I slipped an extra switch into the drawing? Switch 4 goes on the wire between the potentiometer and the switch pole returning to pin 7 of the 555 chip. It’s actually two switches arranged in parallel, and it makes it possible to play separate notes, piano style, on the Slugphone. When the single-throw switch is closed, current always flows and a tone is constantly produced. When it’s open, notes are played by pressing on (closing) the momentary contacts of the normally-open switch. (But before you schedule a concert, keep in mind that it plays only one at a time, row-row-row-your-boat style.)

Putting it all together

Here’s a schematic diagram of the entire Slugphone circuit. It may look complicated, but it’s really just all of the pieces added together.

slugophone schematic

And that, in a very big nutshell, is what’s inside the slugophone. The great thing about this circuit is that it’s extremely versatile, and it doesn’t have a lot of critical parts. If this is your first build, take it a piece at a time, build it on a breadboard, and get each part working before you put it all together. Then go out, find a slug, and have a good time!


About Chris Losee

Chris Losee is a farmer, writer and maker living in the mid-Hudson Valley region of New York State. He's also one-third of the Grommet Laboratories team: a father-daughters collaboration exploring the interface between electronics, sound and vision, and living things. The lab’s "Slugophone" project -- an insect-operated synthesizer -- was featured at World Maker Faire 2011. They followed this up in 2012 with the Musiquarium -- an aquatic soundscape mediated by fish -- which won two blue ribbon Editors Choice awards. Chris, Sam & Jules are intrigued by home-made electronic gadgets, biomorphic sculpture and beam robotics. The laboratory's recently-acquired raspberry pi is expected to help further their investigations into physical computing.

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