What it shows
Running a 555 astable fast enough produces audio frequencies. A potentiometer changes the timing resistance, so turning the knob slides the pitch up and down, demonstrating the link between frequency and pitch.
The science
The 555 charges the 10 nF capacitor through the 1 kΩ resistor and the potentiometer, and discharges it through the potentiometer alone, switching its output each time the capacitor reaches two thirds or one third of the supply. The frequency is 1.44 divided by (R1 + 2 R2) times C. With the pot at 100 kΩ that is about 700 Hz; with the pot near zero it rises above 50 kHz, beyond hearing. The piezo disc bends a little every time pin 3 switches, and that vibration is the sound you hear.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One NE555 timer IC
- A 1 kΩ resistor and a 100 kΩ potentiometer
- A 10 nF ceramic capacitor (marked 103) and a second 10 nF for pin 5
- A passive piezo disc or a small 8 Ω speaker with a 100 Ω series resistor
- A 9 V battery with a snap connector
- A breadboard and jumper wires
Safety
For this project: Battery power only. The sound can be shrill; keep it away from ears and pets.
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 across the breadboard channel, notch at the top; connect pin 1 to ground and pins 8 and 4 to the 9 V rail.
Connect the 1 kΩ resistor from the 9 V rail to pin 7.
Connect the potentiometer middle pin to pin 6 and one end pin to pin 7; leave the third pin free.
Join pin 6 to pin 2 with a jumper and connect the 10 nF timing capacitor from pin 2 to ground.
Connect the second 10 nF capacitor from pin 5 to ground.
Connect the piezo disc from pin 3 to ground (or the speaker through a 100 Ω resistor).
Snap on the battery and turn the knob: the tone slides from a low buzz to a high whistle.
Find the position where you can no longer hear it; ask people of different ages where their limit is and compare.
What you should see
A continuous tone sounds from the disc, and turning the knob moves the pitch smoothly through the audible range.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What property of the sound changes when the frequency changes?
The pitch; higher frequency means a higher note.
2Why can a piezo disc be connected straight to pin 3?
It behaves like a small capacitor and draws very little current.
3Why does the speaker need a series resistor?
An 8 Ω speaker would draw more current than the 555 output can safely supply.
4What is the upper limit of human hearing?
About 20 kHz for children, lower for older adults.
Take it further
Add a second 555 running at 2 Hz to pin 5 of the first and you have a two-tone siren.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
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