555 Timer Hacks: Cable Testers, Magnetic Stirrers, and Lego Grabbers Oh My!


Step 4: Build an Optical Theremin

Picture of Build an Optical Theremin
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 the photoresistor also controlled by the movement of the player's hand.

Parts needed

1 Base Grid (11” x 7.7”) # 6SC BG
1 Eight-Pin IC Socket # 6SC ?U8
1 0.02uF Capacitor # 6SC C1
1 100 ohm Resistor # 6SC R1
1 Variable Resistor #6SC RV
1 Whistle Chip # 6SC WC
1 Photosensitive Resistor # 6SC RP
1 9 Volt Battery Holder # 6SC B5
1 Press Switch # 6SC S2
3 Single Snap Conductor # 6SC 01
8 Conductor with 2-snaps # 6SC 02
2 Conductor with 3-snaps # 6SC 03
1 Conductor with 4-snaps # 6SC 04
1 Conductor with 5-snaps # 6SC 05

Snap Circuits Parts can be ordered separately from

Build the circuit show in the photographs.

Once you build the circuit, switch the B5 block on. You will hear a tone from the Whistle Chip. Move the slider on the Variable Resistor (RV) and you will hear the tone rise and lower in pitch depending on which way you move the slider. Next move your hand so that it casts a shadow on the Photosensitive Resistor (RP) and listen to the pitch as it changes.

As seen in the video in Step One you can replace the 3 snap conductor with the press switch (S2).

Let's see if we can make sense of what is happening. The 555 chip is in astable mode which means that Pin 3 is sending a continuous stream of pulses called a square wave signal to the piezoelectric speaker (WC) that you hear as a tone. The square wave signal is caused by the charging and discharging of the 0.02uf capacitor C1.

When you switch on the power at the B5 block:

Step 1. The 0.02uf capacitor C1 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 0.02uf capacitor which causes it to discharge.

Step 6. When the discharging capacitor reaches 1/3 Vcc, this is detected by the Trigger pin 2.

Step 7. The Trigger pin 2 sends 6 volts to pin 3 the Output pin.

Step 8. The Trigger pin 2 sends 6 volts to pin 7 the dischrage pin which causes the 0.02uf capacitor to charge up.

Step 9. Go back to Step 1.

This process repeats creating the square wave signal (see picture) and you hear that signal from the speaker as a tone.

When you move the slider on the Variable Resistor (RV) this changes the resistance of the circuit. Since the Variable resistor is connected to pin 7, changing the resistance controls the timing of how often the 0.02uf capacitor (C1) charges and discharges. Since the photoresistor (RP) is connected to pin 7 as well, the amount of light that falls on the photoresistor changes the resistance too, and this also controls the timing of how often the capcitor (C1) 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.

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