What it shows
A 555 timer in astable mode charges and discharges a capacitor through two resistors, producing a steady stream of pulses. With 1 kΩ, 470 kΩ and 1 µF the output switches about 1.5 times a second, blinking an LED without any switch being pressed.
The science
The 555 watches the voltage on the 1 µF capacitor. It lets the capacitor charge through the 1 kΩ and 470 kΩ resistors until it reaches two thirds of 9 V, then flips its output low and discharges the capacitor through the 470 kΩ resistor via pin 7 until it drops to one third, then flips high and charges again. The frequency is 1.44 divided by (R1 + 2 R2) times C, which with these values is about 1.5 Hz. The LED on pin 3 with its 470 Ω resistor blinks at that rate; the 10 nF capacitor on pin 5 just keeps the timing steady.
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, a 470 kΩ resistor and a 470 Ω resistor
- A 1 µF electrolytic capacitor and a 10 nF ceramic capacitor
- One red LED
- A 9 V battery with a snap connector
- A breadboard and jumper wires
Safety
For this project: Battery power only. Fit the electrolytic capacitor the right way round; its negative lead is marked with a stripe.
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
9 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.
Push the 555 across the centre channel of the breadboard with its notch or dot at the top; pin 1 is bottom left of the notch.
Connect pin 1 to the ground rail and pin 8 to the 9 V rail; also connect pin 4 to the 9 V rail.
Connect the 1 kΩ resistor from the 9 V rail to pin 7, and the 470 kΩ resistor from pin 7 to pin 6.
Join pin 6 to pin 2 with a jumper.
Connect the 1 µF capacitor from pin 2 to ground with its stripe (negative) lead on the ground side.
Connect the 10 nF capacitor from pin 5 to ground.
Connect the LED long leg through the 470 Ω resistor to pin 3 and the short leg to ground.
Snap on the battery: the LED blinks about one and a half times a second.
Swap the 1 µF for 10 µF: the blink slows to about once every seven seconds. Record both rates with a stopwatch.
What you should see
The LED blinks steadily at about 1.5 Hz, and changing the capacitor changes the rate as the formula predicts.
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 astable mean?
The circuit has no stable state and keeps switching between high and low by itself.
2What two voltages does the 555 compare the capacitor with?
One third and two thirds of the supply voltage.
3Why is the LED only on for slightly more than half the time?
The capacitor charges through both resistors but discharges through only one, so the high time is slightly longer.
4What happens if pin 4 is left unconnected?
The reset pin can pick up noise and stop the oscillation; it should be tied to the supply.
Take it further
Replace the 470 kΩ resistor with a 1 MΩ potentiometer and make a blink rate you can adjust with a knob.
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
Find out more about electronics and robotics
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