A theremin is a musical instrument that is played without actually touching the instrument. The original theremin used radio frequency interference caused by the movement of the player's hand to change the pitch of the instrument. The optical theremin depends on the intensity of light that falls on a photoresistor also controlled by the movement of the player's hand.
The 555 Timer IC was introduced by a company called Signetics (later bought out by Philips) in 1972 and was designed by Hans R. Camenzind in 1971. The 555 chip has 25 transistors, 15 resistors and 2 diodes in an 8 pin DIP (Dual In-line Package) and looks like a square bug with eight legs. It has a notch at the top and Pin 1 is in the top left corner. (source: http://en.wikipedia.org/wiki/555_timer_IC)
(Source: http://en.wikipedia.org/wiki/File:Signetics_NE555N.JPG)
(Source: http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php)
The following are the pin outs for the 555 Timer IC that are used in this build:
(Designed with Online Circuit Simulator--if you would like to design your own circuits go to http://www.falstad.com/circuit/)
Pin 1 is ground. It is connected to the negative side of your battery or power along with any other components in your circuit connected to ground.
Pin 2 is the Trigger pin. It will be connected to ground through a 0.01uf capacitor. The charging and discharging of this capacitor switches on pins 3 and 7.
Pin 3 is the Output pin. In this circuit it outputs a square wave signal.
Pin 4 is the Reset pin. This pin is connected to the positive side of the battery. Connecting up pin 4 is optional and the Optical Theremin will work without it.
Pin 5 is the Control pin. It is not used in this circuit. See http://en.wikipedia.org/wiki/555_timer_IC or http://www.markallen.com/teaching/ucsd/147a/lectures/lecture4/5.php for more info on this pin.
Pin 6 is the Threshold pin. The 0.01uf capacitor will charge up and when it reaches about 2/3 Vcc (voltage from the battery), this is detected by the Threshold pin. This will end the timing interval and send 0v to the Output pin 3 (switches it off).
Pin 7 is the Discharge pin. This pin is also switched off by the Threshold pin 6. When pin 7 is switched off it cuts the power to the 0.01uf capacitor which causes it to discharge. Pin 7 also controls timing. Pin 7 is connected to the 10K ohm resistor and the 100K ohm resistor. Changing the value of the 100K ohm resistor changes the timing of pin 7 and thus changes the frequency of the square wave output by pin 3.
Pin 8 is connected to the positive side of your battery or power along with any other components in your circuit connected to positive
The 555 chip is in astable mode which means that Pin 3 is sending a continuous stream of pulses between 9 volts and 0 volts called a square wave signal.
In the following circuit I have modified the standard 555 square wave generator by replacing the 100k ohm resistor with a Light Dependant Resistor (LDR) or photoresistor. I have also added a piezoelectric speaker and an on/off switch.
Parts needed for the build:
555 Timer IC (I used a KIA555p, but the NE555 will do just fine)
Small Breadboard
Solderless breadboard jumper wire kit
SPST switch
Piezoelectric speaker
Light Dependant Resistor (LDR) or photoresistor (mine was quite small such that it can fit inside a plastic ballpoint pen tip)
10K ohm resistor
0.01uf capacitor
9 volt breadboard battery clip
9 volt battery
When you switch on the circuit:
Step 1. The 0.01uf capacitor charges up.
Step 2. When the charge in the capacitor reaches 2/3 Voltage, this is detected by pin 6, the Threshold pin.
Step 3. The Threshold pin 6 switches off the Output pin 3.
Step 4. The Threshold pin 6 switches off pin 7, the Discharge pin.
Step 5. When the Discharge pin 7 is switched off this cuts the power to the 0.01uf capacitor which causes it to discharge.
Step 6. When the discharging capacitor reaches 1/3 Vcc (voltage from the battery), this is detected by the Trigger pin 2.
Step 7. The Trigger pin 2 sends 9 volts to pin 3 the Output pin.
Step 8. The Trigger pin 2 sends 9 volts to pin 7 the discharge pin which causes the 0.01uf capacitor to charge up.
Step 9. Go back to Step 1.
This process repeats creating the square wave signal and you hear that signal from the speaker as a tone.
Move your hand so that it casts a shadow on the Light Dependant Resistor (LDR) and listen to the pitch as it changes. Since the photoresistor (LDR) is connected to pin 7, the amount of light that falls on the photoresistor changes the resistance, and this controls the timing of how often the 0.01uf capcitor charges and discharges. You'll notice when more light that falls on the photoresistor this makes the pitch higher. Less light makes the pitch lower.
My components are small enough to fit inside a zinc tablet box:
My photoresistor LDR is small enough to fit inside a plastic ball point pen tip.
The plastic ball point pen tip only allows a small amount of light to fall onto the LDR and this gives you a lot more control when playing the Optical Theremin.
The above picture was shot at an angle to show the plastic ballpoint pen tip covering the photoresistor (LDR). You can also see the piezoelectric speaker wedged between the breadboard and the side of the box.
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