What it shows
A very slow 555 astable circuit beats like a clock. A 100 kΩ potentiometer sets the speed from about 35 to 160 beats per minute, which is useful for music practice.
The science
The 555 capacitor charges and discharges between one third and two thirds of the supply through R1 and R2. The frequency is about 1.44 / ((R1 + 2 R2) x C). With R1 = 10 kΩ, R2 = 22 kΩ plus the knob, and C = 10 µF, the knob changes the total from 54 kΩ to 254 kΩ, so the beat runs from about 2.7 Hz down to 0.57 Hz: 160 down to 34 beats a minute.
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 with a socket
- A 10 kΩ resistor, a 22 kΩ resistor and a 100 kΩ potentiometer
- A 10 µF 16 V electrolytic capacitor and a 10 nF capacitor
- A 470 Ω resistor and an LED
- An active buzzer, 3 to 12 V
- A 9 V battery with snap connector and a breadboard
Safety
For this project: Battery power only. Fit the 555 the right way round.
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 connect pin 1 to ground, and pins 4 and 8 to 9 V.
Connect the 10 kΩ resistor from 9 V to pin 7.
Connect the 22 kΩ resistor in series with the 100 kΩ potentiometer between pin 7 and pins 2 and 6 joined together.
Connect the 10 µF capacitor from pin 2 to ground, positive leg to pin 2.
Connect the 10 nF capacitor from pin 5 to ground to steady the circuit.
Connect the 470 Ω resistor and the LED in series from pin 3 to ground, long leg towards the resistor.
Connect the buzzer between pin 3 and ground, red lead to pin 3.
Switch on and turn the knob; count beats for 15 seconds, multiply by 4 and write the beats per minute at each knob position.
What you should see
The LED blinks and the buzzer clicks at a steady rate that can be set between about 35 and 160 beats per minute.
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 a metronome do?
It gives a steady beat so a musician can keep time.
2How does the knob change the beat?
It changes the resistance in the timing network, so the capacitor charges more slowly or quickly.
3What is the 10 nF capacitor on pin 5 for?
It filters noise on the control voltage pin.
4How do you convert Hz to beats per minute?
Multiply by 60.
Take it further
Mark a dial with 40, 60, 80, 100, 120 and 160 beats per minute using a phone stopwatch.
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