Light Theremin (Simple)

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Is it possible to create a musical instrument that can be played without direct contact?

Big Ideas: 
  • A photoresistor changes its resistance based on how much light it sees
  • A speaker can be run by sending an alternating current through it
  • An inverter can be used to produce a square wave

Purpose:

To create a Light Theremin, a musical instrument that can be played by using your hand to control how much light the instrument sees. Different amounts of light lead to different pitches, and with practice simple songs can be played.

A brief theoretical discussion of the principles behind the circuit is also included at the end of this article, although the discussion is aimed at middle/high school students with a basic understanding of circuit components such as resistors and capacitors.

 

Introduction:

What is a Theremin? Put simply, it is an electronic instrument that does not need physical contact from the musician: to play it, you simply need to move your hands closer and further away to control pitch and volume. The Theremin was created by Leon Theremin in the early twentieth century, who invented it while doing research into proximity sensors. It has since evolved into a musical instrument in its own right, and has been used in a wide variety of situations.

The Theremin in this article is different than the one created by Theremin – he used two antennas which act as capacitors when the player’s hands pass over them to change the pitch and volume, while this activity uses light sensors instead. Nevertheless, the final result will behave similarly and produce an equally electronic sound.

This activity was originally created for a WWEST workshop aimed at children Grades 4-6, and constructing the circuit is simple enough that children in this age group can create it by following the below instructions. A more advanced version of this Theremin aimed at upper year middle school and high school students can also be found on the C21 website (http://c21.phas.ubc.ca/article/light-theremin-advanced), which has more features and a better pitch range.

 

Materials:

All of the materials can be found at a local hobby electronics shop, or ordered online through a shop like Digikey or All Electronics. The part numbers of the components used in the sample circuit are included in the parentheses where relevant, although any equivalent parts may be used.

You will need:

  • Breadboard 
  • 10 kΩ Potentiometer
  • 560 nF Capacitor (see note below)
  • Photoresistor 
  • Hex Inverter type 7404 
  • 9V Battery OR 4 AA Batteries
  • Voltage Regulator type UA7805C – Not needed if you use x4 AA batteries or have a power source capable of producing 5 V
  • Small speaker or earphones (and/or jack for computer microphone input)
  • Assorted wires/jumper cables

 

Note that you may need a different value for the capacitor depending on what photoresistor you have or what the ambient light conditions of your room are. If you find that your sound is very high pitched or does not have much range try different capacitors in the 300-600 nF range. The photoresistor used in the sample has a resistance of 2.7 kΩ in ambient light.

 

Instructions:

The first three steps can be skipped if you are using four AA batteries connected in series or have a source capable of producing 5V – just put the positive end of the batteries directly into the upper-left corner of the hex inverter chip. The voltage regulator is used only because the chip needs 5V and the simplest way to get this value is to convert a 9V battery into 5V.

  1. Put a jumper wire between the upper “+” power line and the fourth column of the upper block of the breadboard (which contains f-g-h-i-j), and another wire between the lower “-” line and the fifth column of that same block (above picture, left). As f-g-h-i-j are all connected along a column (as well as a-b-c-d-e, and everything along the “+” and “-” lines), the exact location along the column is not important
  2. Put the voltage regulator in as shown in the above-right picture. The left pin should go in column four, the middle in column five and the right in column six. Ensure the orientation is the same as the picture
  3. Connect a wire between the sixth column of the breadboard (where the right pin of the regulator is) to the 10th column (above, left)
  4. Put in the hex inverter chip as indicated in the above-right picture. The top left pin should go to the 10th column of the breadboard, and the chip should be put along the middle gap. Note the orientation: the indent should go on the left and the circle should go on the right, so the text is right-side up when viewed from the same orientation as the pictures
  5. Put a jumper cable between the bottom-right pin’s column and the lower “-” rail (above, left)
  6. Connect columns 14 and 15 in the upper block using a jumper cable (above, right)
  7. Put the capacitor between the 13th and 16th columns of the upper block (above, left)
  8. Put the potentiometer so that its left pin is in the 16th column (just below where you placed the capacitor), middle pin is in the 17th column and right pin in the 18th column (above, right) 
  9. Put the photoresistor between the 13th and 14th pins (above, left)
  10. Put one of the speaker wires to column 17 (directly above the middle pin of the potentiometer) and the other to the bottom “-” line (above, right)
  11. Finally, connect the red wire from the battery to the upper “+” line and the black wire to the lower “-” line. The circuit is now complete!
  12. The volume can be adjusted by turning the dial on the potentiometer back and forth, and the pitch can be adjusted by moving your hand over the sensor. Try shining a flashlight on the sensor to see what happens!

 

Explanation:

A speaker produces sound when a current is sent back and forth through it: this alternating current creates a magnetic field which causes the cone of the speaker to wobble back and forth producing the sound vibrations you hear. In order for the Theremin to work, then, an alternating current (in this case, a square wave) must be produced by the circuit. This section will explain how such a current is produced.

The circuit diagram for the Light Theremin is shown below:

The Theremin circuit can be split into three parts: the voltage regulator, the square wave generator and the volume control.

Voltage Regulator:

This part of the circuit includes the battery and the voltage regulator, and connects directly to the hex inverter. The hex inverter chip can only be powered by 5V – generally, connecting a 9V battery directly to the chip would cause it to burn out. A voltage regulator is used to turn the voltage produced by a 9V battery into one more suitable for the inverter. Since most people do not have access to a supply that can produce 5V (which is common in computers and computer power supplies), the simplest way is to use a regulator such as the one in this project.

Square Wave Generator:

This is the core of the circuit, and contains the hex inverter, the capacitor and the photoresistor. A NOT inverter (represented as a triangle with a circle on one end in the above diagram) essentially takes a voltage and flips it around: if the input is 0V it produces 5V, and if the input is 5V it produces 0V. If the input is somewhere in the middle, such as 3.4V, the inverter will round to the nearest boundary (in the ideal case, at least – most inverters have uneven thresholds and may round 2.3V to 5V for example).

Looking at the circuit, we can see there are two inverters, and that there is a photoresistor and capacitor connecting their outputs with the first inverter’s input. If we assume the input of the left inverter starts as low (~0V), the left inverter’s output is 5V and the right inverter’s output is 0V. This creates a voltage difference across the resister, causing current to flow.

This current charges the capacitor, which opposes the flow of the current by raising the voltage at the input to the left inverter. Once the voltage reaches the inverter’s threshold, the outputs flip and the process repeats (above picture). Since the output of the second inverter is either high or low, this repeating process generates a signal which alternates between high voltage and low voltage creating a square wave. The speed at which the process repeats (i.e. the frequency of the wave) is dependent on the resistance of the photoresistor and the capacitance of the capacitor. Since variable capacitors are rare (and ones that are controlled by light non-existent), a photoresistor which changes its resistance depending on the amount of light is used to control the pitch.

Volume Control:

This part of the circuit includes the potentiometer and the speaker. The speaker’s volume is dependent on how much current passes through it. The inverter will always output a constant voltage, and the resistance of the speaker is also constant. Hence, to control the volume one can put a potentiometer between the two: this will adjust the total resistance and thus the amount of current that flows.

Comments

In the speaker it is the coil

In the speaker it is the coil mechanically connected to membrane which moves, not the magnet. You should give the type of inverter uses, not the sellers part number. It is ...7404.

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