Introduction background

Every synthesizer starts with an oscillator. It’s the part that generates the tones that are then shaped by filters and amplifiers. It’s analogous to the string of a stringed instrument, or the reed of a reed instrument. Different oscillators produce different wave shapes—the one we’re going to make produces square waves, or rather, something close to a square wave. 
 

We’ll use a venerable integrated-circuit chip called a 555 timer, whose invention dates back to 1971. It could well be the most popular chip ever made. It’s a bit of a Swiss Army knife for timing applications, and a Web search will turn up hundreds of different circuit configurations using it. The circuit we’ll build is technically an astable multivibrator, meaning it’s a circuit which has two states, neither of which is stable—the circuit will flip back and forth, or oscillate between them. It is this sudden flipping between states that gives the square-wave oscillator its characteristic wave shape, in contrast to other oscillators which have waveforms which vary continuously, producing shapes such as sine waves and triangle waves. 
 

While a single oscillator is not the most expressive instrument, you can compare the output to early electronic instruments such as the ondes Martentot and the theremin. Simple oscillators were used in electronic music of the 1950s and ’60s—for example, in Hugh LeCaine’s oscillator bank of 1959. They have also found use in pop music—the lead singer of the ’60s band Silver Apples, played a laboratory oscillator on stage, and in dub reggae oscillators can be heard through layers of delay and reverb (these are known as ‘dub sirens’). Of course, if the sound of a single oscillator is not enough, you can always build several, and combine them for a more complex sound, as LeCaine did.
 

We’ll build the oscillator without a fixed input device—you can experiment with a variety of input devices: knobs (potentiometers), various kinds of sensors, liquids, pencil lines, or even your own body. By varying the resistance of the input device you vary the amount of resistance that completes the circuit, and in so doing you vary the frequency—or pitch—of the oscillator. If you decide you like some particular device, you can solder it in and make it a permanent part of your tone generator (as long as it’s not your body). 
 

Set Up materials and tools

  • a.

    1 small piece of circuit board.

  • b.

    1 1k-ohm quarter-watt resistor (coded with three bars: brown, black, and red).

  • c.

    1 150-ohm quarter-watt resistor (coded with three bars: brown, green, and brown).

  • d.

    1 0.1 µF capacitor (usually marked 104).

  • e.

    1 0.01 µF capacitor (usually marked 103).

  • f.

    1 battery holder for a 9-volt battery.

  • g.

    1 555 timer chip. The chip you want should be an NE555 or an LM555, or be compatible with those, and be in an 8pin DIP package.

  • h.

    1 8-ohm speaker.

  • i.

    1 alligator-clip test lead.

  • j.

    1 100k-ohm potentiometer. Audio taper (type “a”) is best, but linear (type “b”) will work just fine.

  • k.

    1 9-volt battery.

  • l.

    Insulated hookup wire, 22 gauge or smaller.

  • m.

    Rosin-core electronics solder (consider using lead-free variety).

 

  • n.

    Soldering iron, with a small tip, 40 watts or less.

  • o.

    Wire strippers.

  • p.

    Small wire cutters.

  • q.

    Needle-nose pliers.

  • r.

    1 set of helping hands (not pictured).

  • s.

    1 indelible-ink marker (not pictured).

  • t.

    Safety glasses or goggles (not pictured). Please wear them!

 
 

Make It project specifications

Time Required: An afternoon. 
Complexity: Easy. 
Cost: Ten dollars. 
 
 

What you need to know before beginning:
 
1. How to solder
a. Here is a good collection of soldering resources on the website of Limor Fried, an engineer and artist who makes and sells electronic kits.
b. Watch a video tutorial here.
 
 
3. How to work with a circuit board
a. Circuit-board techniques, as used in this project, can be explored here.
b. An excellent book for electronics beginners is Make: Electronics, from O’Reilly Media.
 
4. Information on the 555 timer
 
 

1 prepare the 555 timer chip

  • a.

    Identify pin 1 of your chip. The key to identifying the pins is finding pin 1, which is to the left of the circular indentation at the uppermost corner of the chip. The remaining pins are numbered 2 through 8 in a counter-clockwise direction around the chip.

 

  • b.

    Check the pins of your chip. When the chip is manufactured, the pins are not often at right angles to the chip, which prevents the chip from fitting into the holes of the circuit board.

 

  • c.

    Straighten the pins of the chip if they are not at right angles to the chip, straighten the pins of the chip. Do this by slightly bending them on a flat surface, as shown.

 

2 mount the 555 timer chip to the circuit board

  • a.

    Orient the chip on the circuit board, making sure that pin 1 is positioned to the lower left, as shown. 

 

  • b.

    You will solder and make connections on the underside of the board, so make sure all the pins of the chip go through the board, and are not bent underneath the chip. Don’t force them. 

  • c.

    Flip the board upside down and solder the chip in place at all eight pins.

 

  • d.

    Mark pin 1. On the underside of the board, with an indelible marker write 1 to keep the correct orientation when you’re working on the circuit board. Note: Because this is a mirror image of the board top, the remaining pins are numbered 2 through 8 in a clockwise fashion around the chip.

 

3 wire up the circuit

  • a.

    Using a length of hookup wire, solder a connection between pins 6 and 2 of the chip.

  • b.

    Using a length of hookup wire, solder a connection between pins 4 and 8 of the chip.

 

  • c.

    Prepare the 1k-ohm resistor, bending leads as shown.

 

  • d.

    Mount the 1k-ohm resistor as shown.

Note: the resistor can go in either way—there is no positive or negative lead. 
 

  • e.

    Solder a connection between the other lead of the resistor to pin 7 of the chip. 

 

  • f.

    Solder a connection between one lead of the resistor to pin 8 of the chip.

 

  • g.

    Mount the first 0.1 µF capacitor as shown. 

Note: these capacitors, like the resistors, do not have polarity—they can go in either way. 
 

  • h.

    Solder a connection between one lead of the capacitor to pin 1 of the chip. 

 

  • i.

    Solder a connection between the other lead of the capacitor to pin 2 of the chip.

 
  • j.

    Mount the second 0.01 µF capacitor as shown. 

 

  • k.

    Solder a connection between one lead of the second capacitor and pin 5 of the chip.

 

  • l.

    Using a length of hookup wire, solder a connection between the other lead of the second capacitor and pin 1 of the chip. 

 

  • m.

    Mount the 150-ohm resistor as shown.

 
  • n.

    Solder a connection between one lead of the resistor to pin 4 of the chip.

 

4 mount the loudspeaker to the circuit

  • a.

    Cut a 3-inch piece of hookup wire and solder it to pin 3 of the chip.

  • b.

    Cut a 3-inch piece of hookup wire and solder it to the free end of the 150-ohm resistor, as shown. These will be your loudspeaker leads.

 

  • c.

    Solder your speaker leads directly to your loudspeaker, as shown. 

Note: you don’t need to worry about polarity, although the speaker will be marked to indicate the positive terminal, with a + sign, or a red terminal. 
 

  • d.

    Cut the alligator-clip lead in half, and strip the cut ends as shown. 

 

  • e.

    “Tin” the leads with a small amount of solder, to keep them from fraying.

 

  • f.

    Solder the stripped end of one alligator clip to pin 6 of the chip, and the other clip to pin 7 of the chip, as shown. 

 

  • g.

    Solder the red lead from the battery clip to pin 8 of the chip.

 

  • h.

    Solder the black lead from the battery clip to pin 1 of the chip, as shown. 

5 test your circuit

  • a.

    Connect one alligator-clip lead to the middle terminal of the 100k-ohm potentiometer and another to one of the outer terminals, as shown. Leave the other terminal free. Set the potentiometer to about mid-point. 

 

  • b.

    Connect the 9-volt battery. You should hear a tone from the speaker. 

 
 

How It Works resistor-capacitor combination

Often, when we need some sort of timing function in an electronic circuit, we use a combination of a resistor and a capacitor. A capacitor is like a bucket for electrons. Just as with a real bucket, we can fill it at one rate and empty it at the same rate, or empty it faster or slower than we filled it. The rate of filling the capacitor with electrons is determined by the resistor which we connect to it. The product of the value of the resistor in ohms, and that of the capacitor in farads is called the time constant, and is given in seconds. As a rule of thumb, a given resistor-capacitor combination will charge up to the applied voltage in five time constants. In our circuit, the timing of the waveform is controlled by the 1k-ohm resistor, the resistance connected between the alligator-clip leads, and the 0.1μF capacitor. The 555 timer “looks at” the voltage on the capacitor (via pins 2 and 6) and begins to charge the capacitor through both the 1k resistor and the variable resistor. At this time, the waveform on the output pin goes high. When the voltage on the capacitor reaches two thirds of the supply voltage, the output goes low, and the capacitor begins to be discharged by pin 7, this time only through the variable resistor. When the voltage falls to one third of the supply voltage, the cycle repeats.
 
 
 

Help! troubleshooting

If there is no sound, immediately disconnect the battery!
 
1. Make sure you are using a fresh battery.
 
2. Check if the chip is hot. Be careful. Touch it quickly, as if you were testing an iron. If it’s hot, three things are possible:
 a. The power to the chip is wired backwards.
 b. The output is short-circuited.
 c. You did not correctly identify pin 1, and therefore wired the chip backwards.
 
3. If there is no sound, and the chip is not hot:
 a. Check that all connections have been made to the correct pins, and that there are no solder “bridges” between adjacent pins. If any solder connections look suspicious, reheat them with the soldering iron to remelt the solder. Make sure that the power is not connected to the circuit while soldering.
 b. Check that you have not mixed up the resistors and capacitors. It’s possible that your circuit may “sort of” work if you’ve done this.
 c. If you’ve made a mistake, you can often correct it using de-soldering braid, available where you buy electronics supplies. 
 

Use It various resistors

Besides the potentiometer, you can connect any resistive element between the two alligator-clip leads to produce different frequencies. Lower resistances produce higher frequencies, and vice-versa. Some things to try:
 
Various fixed resistors.
 
Food or liquids. Please do not consume them, being sure to discard them after they have been in contact with the leads.
 
Your own body. Try holding the leads with dry and wet fingers and notice the difference. There are no hazardous voltages involved, although you may feel a tingle.
 
Soil or wood. You can use long nails as moisture sensors, either in the ground or hammered into a board.
 

Use It pencil drawings

Use a very soft pencil, such as a 2B or softer, and for best results draw very heavy lines. Clip one lead to a spot on the drawing, and move the other lead around. See if you can draw a properly tuned keyboard. 
 

Use It more ideas

Light-dependent resistors, also known as cadmium sulphide photocells. You can make a surprisingly expressive light-theremin out of your oscillator. Try exposing the sensor to small flashing lights, such as bicycle lights, to produce rhythms. 
 
Flexible-bend sensors: For sources, search the Web for “flex sensor,” “bend sensor,” or “stretch sensor,” using your favourite Web search engine.
 
Try connecting more than one of these devices at a time, and see how they interact.
 
Try various speakers. Pretty much any speaker you can find can be used.
 
You could try placing the speaker in various resonant objects, or adding things that modify the sound. Look up the ondes Martenot to see the various kinds of speakers that it used, or read about David Tudor’s piece Rainforest for some inspiration.
 

Reference tone generator circuit diagram

To view the circuit diagram in detail, please download the pdf above.

Reference tone generator circuit schematic

To view the circuit schematic in detail, please download the pdf above.
 

Images by: Adam Coish. Schematics and diagrams by: Rob Cruickshank.