What it shows
Two transistors, each switching the other through a capacitor, make an astable multivibrator: a circuit with no stable state that swaps over by itself. The two LEDs flash alternately about once a second.
The science
When one transistor is on, its collector is near 0 V and its LED is lit. The capacitor joined to that collector pulls the other transistor base down, keeping it off, while that capacitor slowly charges through its 47 kΩ resistor. When the base reaches about 0.7 V the second transistor turns on, and the first is forced off through the other capacitor. The cycle repeats. Each half-cycle lasts about 0.7 x R x C, which is about 0.7 s for 47 kΩ and 22 µF.
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 BC547 transistors
- Two 47 kΩ resistors (base) and two 470 Ω resistors (LED)
- Two 22 µF 16 V electrolytic capacitors
- Two LEDs of different colours
- A 9 V battery with snap connector
- A breadboard and jumper wires
Safety
For this project: Battery power only. Electrolytic capacitors have a marked negative leg: fit them the right way round or they can heat up and leak.
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 both BC547 transistors on the breadboard with the flat faces pointing the same way, and join both emitters to the ground rail.
Connect a 470 Ω resistor from the 9 V rail to the long leg of each LED, and the short leg of each LED to the collector of its transistor.
Connect a 47 kΩ resistor from the 9 V rail to the base of each transistor.
Connect the positive leg of the first capacitor to the collector of transistor 1 and its negative leg to the base of transistor 2.
Connect the positive leg of the second capacitor to the collector of transistor 2 and its negative leg to the base of transistor 1.
Recheck each LED, capacitor and transistor direction, then snap on the 9 V battery.
Watch the LEDs alternate. If both stay lit, one capacitor is reversed or a base resistor is touching the wrong row.
Replace both 22 µF capacitors with 100 µF ones and time ten flashes; record how the speed changes.
What you should see
The two LEDs flash alternately; larger capacitors give slower flashing, which matches the 0.7 x R x C estimate.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is this called an astable circuit?
It has no stable state, so it keeps switching between two quasi-stable states on its own.
2What sets the flash speed?
The base resistors and the capacitors: bigger values give a slower flash.
3Why does each LED need a resistor?
To limit the collector current to a safe value, about 14 mA from 9 V.
4What would you change to flash faster?
Use smaller capacitors, such as 4.7 µF, or smaller base resistors.
Take it further
Use unequal capacitors, 10 µF and 47 µF, and observe that one LED stays lit longer than the other.
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