What it shows
One 555 timer makes a slow beat of about one per second, and it wobbles the pitch of a second 555 that produces an audible tone. Together they give a rising and falling siren.
The science
Each 555 wired as an astable oscillator has an output frequency of about 1.44 / ((R1 + 2 R2) x C). IC1 with 10 kΩ, 47 kΩ and 10 µF gives about 1.4 Hz. IC2 with 1 kΩ, 10 kΩ and 10 nF gives about 6.9 kHz. The output of IC1 feeds the control voltage pin 5 of IC2 through 10 kΩ, which moves its switching thresholds up and down and so slowly changes the pitch.
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.
- Two NE555 timer ICs with 8-pin sockets
- Resistors: 10 kΩ, 47 kΩ, 1 kΩ, 10 kΩ, 100 Ω
- Capacitors: 10 µF (16 V), 10 nF and 100 µF (16 V)
- An 8 Ω small speaker
- A 9 V battery with snap connector and a switch
- A breadboard and jumper wires
Safety
For this project: Battery power only. Fit each 555 the right way round: pin 1 is next to the notch or dot. A reversed chip gets hot quickly.
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 IC1 and IC2 on the breadboard and connect pin 1 of both to ground and pins 4 and 8 of both to 9 V.
For IC1, connect 10 kΩ from 9 V to pin 7, and 47 kΩ from pin 7 to pins 6 and 2 joined together.
Connect the 10 µF capacitor from pin 2 of IC1 to ground, positive leg to pin 2.
For IC2, connect 1 kΩ from 9 V to pin 7, and 10 kΩ from pin 7 to pins 6 and 2 joined together.
Connect a 10 nF capacitor from pin 2 of IC2 to ground.
Connect pin 3 of IC1 through a 10 kΩ resistor to pin 5 of IC2.
Connect pin 3 of IC2 through the 100 Ω resistor and the 100 µF capacitor (positive towards the IC) to the speaker, other side to ground.
Switch on: you should hear a warbling two-tone siren. Change the 10 µF to 4.7 µF for a faster warble.
What you should see
The speaker makes a siren-like tone that wobbles up and down about once a second.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does the 555 timer do here?
It works as an astable oscillator that switches its output on and off repeatedly.
2Why does IC1 change the pitch of IC2?
Its output moves the control voltage on pin 5, shifting IC2 thresholds.
3What is the 100 µF capacitor for?
It blocks DC from the speaker and passes only the audio signal.
4How can you lower the pitch?
Increase the capacitor or resistors of IC2.
Take it further
Add a push button that powers the siren only while pressed, making a pocket alarm.
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