What it shows
The pitch of a 555 astable depends on its timing resistor. Eight keys, each connecting a different resistor, give eight notes of a musical scale.
The science
In the astable circuit the frequency is f = 1.44 / ((R1 + 2 R2) x C). With R1 = 1 kΩ and C = 100 nF, an R2 of about 27 kΩ gives 262 Hz (middle C), 24 kΩ gives about 294 Hz, and 13 kΩ gives 523 Hz (the next C). Pressing a key places its resistor as R2, so each key plays its own note.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One NE555 with a socket
- Eight push buttons
- Eight 47 kΩ trimmer potentiometers (or fixed resistors, see steps)
- A 1 kΩ resistor, a 100 nF capacitor and a 100 µF capacitor
- A 100 Ω resistor and an 8 Ω small speaker
- A 9 V battery with snap connector and a breadboard
- A phone tuner app
Safety
For this project: Battery power only. Fit the 555 with pin 1 next to the notch. Keep the volume low for your ears.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place the 555 and join pins 1 to ground, and pins 4 and 8 to 9 V.
Connect the 1 kΩ resistor from 9 V to pin 7, and the 100 nF capacitor from pin 2 to ground.
Join pins 2 and 6 together to make the capacitor timing node.
Wire the eight buttons in a row, each button joined to one trimmer, and the trimmer to pin 7 as R2.
Join the other side of every button to the pin 2/6 node so each button connects its own trimmer between pin 7 and the node.
Connect pin 3 through the 100 Ω resistor and the 100 µF capacitor (positive towards the 555) to the speaker, and the speaker to ground.
Press the first key and turn its trimmer to about 27 kΩ; check the pitch against a phone tuner app, aiming for C at 262 Hz.
Tune the other keys in turn to D, E, F, G, A, B and the upper C using the tuner, then play a simple tune.
What you should see
Each key sounds a different note and the eight notes make a rising scale when pressed in order.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What sets the pitch of the 555 oscillator?
The timing resistors and the capacitor.
2Why does a higher resistance give a lower note?
The capacitor takes longer to charge, so the cycle is slower and the frequency is lower.
3What is the frequency of a note called?
The pitch: more cycles per second is a higher pitch.
4Why use trimmers rather than fixed resistors?
Standard resistor values are not exact enough, so each note must be tuned by ear or by an app.
Take it further
Add a ninth key with a switch that adds a second capacitor to move the whole scale an octave lower.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each